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    SubjectRe: [PATCH v18 15/15] Documentation: prctl/seccomp_filter
    On Thu, Apr 12, 2012 at 5:48 PM, Will Drewry <wad@chromium.org> wrote:
    > Documents how system call filtering using Berkeley Packet
    > Filter programs works and how it may be used.
    > Includes an example for x86 and a semi-generic
    > example using a macro-based code generator.

    Hi guys, that is quite the To/CC list. I didn't have the heart to add
    yet another address (linux-next) to it...

    Can you have a look at this link? It appears to be breaking
    the i386 allmodconfig builds in linux-next.

    http://kisskb.ellerman.id.au/kisskb/buildresult/6123842/

    "samples/seccomp/bpf-direct.c:47:2: error: #error Unsupported platform"

    Thanks,
    Paul.
    --

    >
    > Acked-by: Eric Paris <eparis@redhat.com>
    > Signed-off-by: Will Drewry <wad@chromium.org>
    >
    > v18: - added acked by
    >     - update no new privs numbers
    > v17: - remove @compat note and add Pitfalls section for arch checking
    >       (keescook@chromium.org)
    > v16: -
    > v15: -
    > v14: - rebase/nochanges
    > v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
    > v12: - comment on the ptrace_event use
    >     - update arch support comment
    >     - note the behavior of SECCOMP_RET_DATA when there are multiple filters
    >       (keescook@chromium.org)
    >     - lots of samples/ clean up incl 64-bit bpf-direct support
    >       (markus@chromium.org)
    >     - rebase to linux-next
    > v11: - overhaul return value language, updates (keescook@chromium.org)
    >     - comment on do_exit(SIGSYS)
    > v10: - update for SIGSYS
    >     - update for new seccomp_data layout
    >     - update for ptrace option use
    > v9: - updated bpf-direct.c for SIGILL
    > v8: - add PR_SET_NO_NEW_PRIVS to the samples.
    > v7: - updated for all the new stuff in v7: TRAP, TRACE
    >    - only talk about PR_SET_SECCOMP now
    >    - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
    >    - adds dropper.c: a simple system call disabler
    > v6: - tweak the language to note the requirement of
    >      PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
    > v5: - update sample to use system call arguments
    >    - adds a "fancy" example using a macro-based generator
    >    - cleaned up bpf in the sample
    >    - update docs to mention arguments
    >    - fix prctl value (eparis@redhat.com)
    >    - language cleanup (rdunlap@xenotime.net)
    > v4: - update for no_new_privs use
    >    - minor tweaks
    > v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
    >    - document use of tentative always-unprivileged
    >    - guard sample compilation for i386 and x86_64
    > v2: - move code to samples (corbet@lwn.net)
    > ---
    >  Documentation/prctl/seccomp_filter.txt |  163 ++++++++++++++++++++++
    >  samples/Makefile                       |    2 +-
    >  samples/seccomp/Makefile               |   38 +++++
    >  samples/seccomp/bpf-direct.c           |  176 +++++++++++++++++++++++
    >  samples/seccomp/bpf-fancy.c            |  102 ++++++++++++++
    >  samples/seccomp/bpf-helper.c           |   89 ++++++++++++
    >  samples/seccomp/bpf-helper.h           |  238 ++++++++++++++++++++++++++++++++
    >  samples/seccomp/dropper.c              |   68 +++++++++
    >  8 files changed, 875 insertions(+), 1 deletions(-)
    >  create mode 100644 Documentation/prctl/seccomp_filter.txt
    >  create mode 100644 samples/seccomp/Makefile
    >  create mode 100644 samples/seccomp/bpf-direct.c
    >  create mode 100644 samples/seccomp/bpf-fancy.c
    >  create mode 100644 samples/seccomp/bpf-helper.c
    >  create mode 100644 samples/seccomp/bpf-helper.h
    >  create mode 100644 samples/seccomp/dropper.c
    >
    > diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
    > new file mode 100644
    > index 0000000..597c3c5
    > --- /dev/null
    > +++ b/Documentation/prctl/seccomp_filter.txt
    > @@ -0,0 +1,163 @@
    > +               SECure COMPuting with filters
    > +               =============================
    > +
    > +Introduction
    > +------------
    > +
    > +A large number of system calls are exposed to every userland process
    > +with many of them going unused for the entire lifetime of the process.
    > +As system calls change and mature, bugs are found and eradicated.  A
    > +certain subset of userland applications benefit by having a reduced set
    > +of available system calls.  The resulting set reduces the total kernel
    > +surface exposed to the application.  System call filtering is meant for
    > +use with those applications.
    > +
    > +Seccomp filtering provides a means for a process to specify a filter for
    > +incoming system calls.  The filter is expressed as a Berkeley Packet
    > +Filter (BPF) program, as with socket filters, except that the data
    > +operated on is related to the system call being made: system call
    > +number and the system call arguments.  This allows for expressive
    > +filtering of system calls using a filter program language with a long
    > +history of being exposed to userland and a straightforward data set.
    > +
    > +Additionally, BPF makes it impossible for users of seccomp to fall prey
    > +to time-of-check-time-of-use (TOCTOU) attacks that are common in system
    > +call interposition frameworks.  BPF programs may not dereference
    > +pointers which constrains all filters to solely evaluating the system
    > +call arguments directly.
    > +
    > +What it isn't
    > +-------------
    > +
    > +System call filtering isn't a sandbox.  It provides a clearly defined
    > +mechanism for minimizing the exposed kernel surface.  It is meant to be
    > +a tool for sandbox developers to use.  Beyond that, policy for logical
    > +behavior and information flow should be managed with a combination of
    > +other system hardening techniques and, potentially, an LSM of your
    > +choosing.  Expressive, dynamic filters provide further options down this
    > +path (avoiding pathological sizes or selecting which of the multiplexed
    > +system calls in socketcall() is allowed, for instance) which could be
    > +construed, incorrectly, as a more complete sandboxing solution.
    > +
    > +Usage
    > +-----
    > +
    > +An additional seccomp mode is added and is enabled using the same
    > +prctl(2) call as the strict seccomp.  If the architecture has
    > +CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
    > +
    > +PR_SET_SECCOMP:
    > +       Now takes an additional argument which specifies a new filter
    > +       using a BPF program.
    > +       The BPF program will be executed over struct seccomp_data
    > +       reflecting the system call number, arguments, and other
    > +       metadata.  The BPF program must then return one of the
    > +       acceptable values to inform the kernel which action should be
    > +       taken.
    > +
    > +       Usage:
    > +               prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
    > +
    > +       The 'prog' argument is a pointer to a struct sock_fprog which
    > +       will contain the filter program.  If the program is invalid, the
    > +       call will return -1 and set errno to EINVAL.
    > +
    > +       If fork/clone and execve are allowed by @prog, any child
    > +       processes will be constrained to the same filters and system
    > +       call ABI as the parent.
    > +
    > +       Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
    > +       run with CAP_SYS_ADMIN privileges in its namespace.  If these are not
    > +       true, -EACCES will be returned.  This requirement ensures that filter
    > +       programs cannot be applied to child processes with greater privileges
    > +       than the task that installed them.
    > +
    > +       Additionally, if prctl(2) is allowed by the attached filter,
    > +       additional filters may be layered on which will increase evaluation
    > +       time, but allow for further decreasing the attack surface during
    > +       execution of a process.
    > +
    > +The above call returns 0 on success and non-zero on error.
    > +
    > +Return values
    > +-------------
    > +A seccomp filter may return any of the following values. If multiple
    > +filters exist, the return value for the evaluation of a given system
    > +call will always use the highest precedent value. (For example,
    > +SECCOMP_RET_KILL will always take precedence.)
    > +
    > +In precedence order, they are:
    > +
    > +SECCOMP_RET_KILL:
    > +       Results in the task exiting immediately without executing the
    > +       system call.  The exit status of the task (status & 0x7f) will
    > +       be SIGSYS, not SIGKILL.
    > +
    > +SECCOMP_RET_TRAP:
    > +       Results in the kernel sending a SIGSYS signal to the triggering
    > +       task without executing the system call.  The kernel will
    > +       rollback the register state to just before the system call
    > +       entry such that a signal handler in the task will be able to
    > +       inspect the ucontext_t->uc_mcontext registers and emulate
    > +       system call success or failure upon return from the signal
    > +       handler.
    > +
    > +       The SECCOMP_RET_DATA portion of the return value will be passed
    > +       as si_errno.
    > +
    > +       SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
    > +
    > +SECCOMP_RET_ERRNO:
    > +       Results in the lower 16-bits of the return value being passed
    > +       to userland as the errno without executing the system call.
    > +
    > +SECCOMP_RET_TRACE:
    > +       When returned, this value will cause the kernel to attempt to
    > +       notify a ptrace()-based tracer prior to executing the system
    > +       call.  If there is no tracer present, -ENOSYS is returned to
    > +       userland and the system call is not executed.
    > +
    > +       A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
    > +       using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified
    > +       of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
    > +       the BPF program return value will be available to the tracer
    > +       via PTRACE_GETEVENTMSG.
    > +
    > +SECCOMP_RET_ALLOW:
    > +       Results in the system call being executed.
    > +
    > +If multiple filters exist, the return value for the evaluation of a
    > +given system call will always use the highest precedent value.
    > +
    > +Precedence is only determined using the SECCOMP_RET_ACTION mask.  When
    > +multiple filters return values of the same precedence, only the
    > +SECCOMP_RET_DATA from the most recently installed filter will be
    > +returned.
    > +
    > +Pitfalls
    > +--------
    > +
    > +The biggest pitfall to avoid during use is filtering on system call
    > +number without checking the architecture value.  Why?  On any
    > +architecture that supports multiple system call invocation conventions,
    > +the system call numbers may vary based on the specific invocation.  If
    > +the numbers in the different calling conventions overlap, then checks in
    > +the filters may be abused.  Always check the arch value!
    > +
    > +Example
    > +-------
    > +
    > +The samples/seccomp/ directory contains both an x86-specific example
    > +and a more generic example of a higher level macro interface for BPF
    > +program generation.
    > +
    > +
    > +
    > +Adding architecture support
    > +-----------------------
    > +
    > +See arch/Kconfig for the authoritative requirements.  In general, if an
    > +architecture supports both ptrace_event and seccomp, it will be able to
    > +support seccomp filter with minor fixup: SIGSYS support and seccomp return
    > +value checking.  Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
    > +to its arch-specific Kconfig.
    > diff --git a/samples/Makefile b/samples/Makefile
    > index 2f75851..5ef08bb 100644
    > --- a/samples/Makefile
    > +++ b/samples/Makefile
    > @@ -1,4 +1,4 @@
    >  # Makefile for Linux samples code
    >
    >  obj-$(CONFIG_SAMPLES)  += kobject/ kprobes/ tracepoints/ trace_events/ \
    > -                          hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/
    > +                          hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/
    > diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
    > new file mode 100644
    > index 0000000..e8fe0f5
    > --- /dev/null
    > +++ b/samples/seccomp/Makefile
    > @@ -0,0 +1,38 @@
    > +# kbuild trick to avoid linker error. Can be omitted if a module is built.
    > +obj- := dummy.o
    > +
    > +hostprogs-$(CONFIG_SECCOMP) := bpf-fancy dropper
    > +bpf-fancy-objs := bpf-fancy.o bpf-helper.o
    > +
    > +HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
    > +HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
    > +HOSTCFLAGS_bpf-helper.o += -I$(objtree)/usr/include
    > +HOSTCFLAGS_bpf-helper.o += -idirafter $(objtree)/include
    > +
    > +HOSTCFLAGS_dropper.o += -I$(objtree)/usr/include
    > +HOSTCFLAGS_dropper.o += -idirafter $(objtree)/include
    > +dropper-objs := dropper.o
    > +
    > +# bpf-direct.c is x86-only.
    > +ifeq ($(SRCARCH),x86)
    > +# List of programs to build
    > +hostprogs-$(CONFIG_SECCOMP) += bpf-direct
    > +bpf-direct-objs := bpf-direct.o
    > +endif
    > +
    > +HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
    > +HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
    > +
    > +# Try to match the kernel target.
    > +ifeq ($(CONFIG_64BIT),)
    > +HOSTCFLAGS_bpf-direct.o += -m32
    > +HOSTCFLAGS_dropper.o += -m32
    > +HOSTCFLAGS_bpf-helper.o += -m32
    > +HOSTCFLAGS_bpf-fancy.o += -m32
    > +HOSTLOADLIBES_bpf-direct += -m32
    > +HOSTLOADLIBES_bpf-fancy += -m32
    > +HOSTLOADLIBES_dropper += -m32
    > +endif
    > +
    > +# Tell kbuild to always build the programs
    > +always := $(hostprogs-y)
    > diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c
    > new file mode 100644
    > index 0000000..26f523e
    > --- /dev/null
    > +++ b/samples/seccomp/bpf-direct.c
    > @@ -0,0 +1,176 @@
    > +/*
    > + * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
    > + *
    > + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
    > + * Author: Will Drewry <wad@chromium.org>
    > + *
    > + * The code may be used by anyone for any purpose,
    > + * and can serve as a starting point for developing
    > + * applications using prctl(PR_SET_SECCOMP, 2, ...).
    > + */
    > +#define __USE_GNU 1
    > +#define _GNU_SOURCE 1
    > +
    > +#include <linux/types.h>
    > +#include <linux/filter.h>
    > +#include <linux/seccomp.h>
    > +#include <linux/unistd.h>
    > +#include <signal.h>
    > +#include <stdio.h>
    > +#include <stddef.h>
    > +#include <string.h>
    > +#include <sys/prctl.h>
    > +#include <unistd.h>
    > +
    > +#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
    > +#define syscall_nr (offsetof(struct seccomp_data, nr))
    > +
    > +#if defined(__i386__)
    > +#define REG_RESULT     REG_EAX
    > +#define REG_SYSCALL    REG_EAX
    > +#define REG_ARG0       REG_EBX
    > +#define REG_ARG1       REG_ECX
    > +#define REG_ARG2       REG_EDX
    > +#define REG_ARG3       REG_ESI
    > +#define REG_ARG4       REG_EDI
    > +#define REG_ARG5       REG_EBP
    > +#elif defined(__x86_64__)
    > +#define REG_RESULT     REG_RAX
    > +#define REG_SYSCALL    REG_RAX
    > +#define REG_ARG0       REG_RDI
    > +#define REG_ARG1       REG_RSI
    > +#define REG_ARG2       REG_RDX
    > +#define REG_ARG3       REG_R10
    > +#define REG_ARG4       REG_R8
    > +#define REG_ARG5       REG_R9
    > +#else
    > +#error Unsupported platform
    > +#endif
    > +
    > +#ifndef PR_SET_NO_NEW_PRIVS
    > +#define PR_SET_NO_NEW_PRIVS 38
    > +#endif
    > +
    > +#ifndef SYS_SECCOMP
    > +#define SYS_SECCOMP 1
    > +#endif
    > +
    > +static void emulator(int nr, siginfo_t *info, void *void_context)
    > +{
    > +       ucontext_t *ctx = (ucontext_t *)(void_context);
    > +       int syscall;
    > +       char *buf;
    > +       ssize_t bytes;
    > +       size_t len;
    > +       if (info->si_code != SYS_SECCOMP)
    > +               return;
    > +       if (!ctx)
    > +               return;
    > +       syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
    > +       buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
    > +       len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
    > +
    > +       if (syscall != __NR_write)
    > +               return;
    > +       if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
    > +               return;
    > +       /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
    > +       ctx->uc_mcontext.gregs[REG_RESULT] = -1;
    > +       if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
    > +               bytes = write(STDOUT_FILENO, buf, len);
    > +               ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
    > +       }
    > +       return;
    > +}
    > +
    > +static int install_emulator(void)
    > +{
    > +       struct sigaction act;
    > +       sigset_t mask;
    > +       memset(&act, 0, sizeof(act));
    > +       sigemptyset(&mask);
    > +       sigaddset(&mask, SIGSYS);
    > +
    > +       act.sa_sigaction = &emulator;
    > +       act.sa_flags = SA_SIGINFO;
    > +       if (sigaction(SIGSYS, &act, NULL) < 0) {
    > +               perror("sigaction");
    > +               return -1;
    > +       }
    > +       if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
    > +               perror("sigprocmask");
    > +               return -1;
    > +       }
    > +       return 0;
    > +}
    > +
    > +static int install_filter(void)
    > +{
    > +       struct sock_filter filter[] = {
    > +               /* Grab the system call number */
    > +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
    > +               /* Jump table for the allowed syscalls */
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +#ifdef __NR_sigreturn
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +#endif
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
    > +
    > +               /* Check that read is only using stdin. */
    > +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
    > +
    > +               /* Check that write is only using stdout */
    > +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
    > +               /* Trap attempts to write to stderr */
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
    > +
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
    > +       };
    > +       struct sock_fprog prog = {
    > +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
    > +               .filter = filter,
    > +       };
    > +
    > +       if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
    > +               perror("prctl(NO_NEW_PRIVS)");
    > +               return 1;
    > +       }
    > +
    > +
    > +       if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
    > +               perror("prctl");
    > +               return 1;
    > +       }
    > +       return 0;
    > +}
    > +
    > +#define payload(_c) (_c), sizeof((_c))
    > +int main(int argc, char **argv)
    > +{
    > +       char buf[4096];
    > +       ssize_t bytes = 0;
    > +       if (install_emulator())
    > +               return 1;
    > +       if (install_filter())
    > +               return 1;
    > +       syscall(__NR_write, STDOUT_FILENO,
    > +               payload("OHAI! WHAT IS YOUR NAME? "));
    > +       bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
    > +       syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
    > +       syscall(__NR_write, STDOUT_FILENO, buf, bytes);
    > +       syscall(__NR_write, STDERR_FILENO,
    > +               payload("Error message going to STDERR\n"));
    > +       return 0;
    > +}
    > diff --git a/samples/seccomp/bpf-fancy.c b/samples/seccomp/bpf-fancy.c
    > new file mode 100644
    > index 0000000..8eb483a
    > --- /dev/null
    > +++ b/samples/seccomp/bpf-fancy.c
    > @@ -0,0 +1,102 @@
    > +/*
    > + * Seccomp BPF example using a macro-based generator.
    > + *
    > + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
    > + * Author: Will Drewry <wad@chromium.org>
    > + *
    > + * The code may be used by anyone for any purpose,
    > + * and can serve as a starting point for developing
    > + * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
    > + */
    > +
    > +#include <linux/filter.h>
    > +#include <linux/seccomp.h>
    > +#include <linux/unistd.h>
    > +#include <stdio.h>
    > +#include <string.h>
    > +#include <sys/prctl.h>
    > +#include <unistd.h>
    > +
    > +#include "bpf-helper.h"
    > +
    > +#ifndef PR_SET_NO_NEW_PRIVS
    > +#define PR_SET_NO_NEW_PRIVS 38
    > +#endif
    > +
    > +int main(int argc, char **argv)
    > +{
    > +       struct bpf_labels l;
    > +       static const char msg1[] = "Please type something: ";
    > +       static const char msg2[] = "You typed: ";
    > +       char buf[256];
    > +       struct sock_filter filter[] = {
    > +               /* TODO: LOAD_SYSCALL_NR(arch) and enforce an arch */
    > +               LOAD_SYSCALL_NR,
    > +               SYSCALL(__NR_exit, ALLOW),
    > +               SYSCALL(__NR_exit_group, ALLOW),
    > +               SYSCALL(__NR_write, JUMP(&l, write_fd)),
    > +               SYSCALL(__NR_read, JUMP(&l, read)),
    > +               DENY,  /* Don't passthrough into a label */
    > +
    > +               LABEL(&l, read),
    > +               ARG(0),
    > +               JNE(STDIN_FILENO, DENY),
    > +               ARG(1),
    > +               JNE((unsigned long)buf, DENY),
    > +               ARG(2),
    > +               JGE(sizeof(buf), DENY),
    > +               ALLOW,
    > +
    > +               LABEL(&l, write_fd),
    > +               ARG(0),
    > +               JEQ(STDOUT_FILENO, JUMP(&l, write_buf)),
    > +               JEQ(STDERR_FILENO, JUMP(&l, write_buf)),
    > +               DENY,
    > +
    > +               LABEL(&l, write_buf),
    > +               ARG(1),
    > +               JEQ((unsigned long)msg1, JUMP(&l, msg1_len)),
    > +               JEQ((unsigned long)msg2, JUMP(&l, msg2_len)),
    > +               JEQ((unsigned long)buf, JUMP(&l, buf_len)),
    > +               DENY,
    > +
    > +               LABEL(&l, msg1_len),
    > +               ARG(2),
    > +               JLT(sizeof(msg1), ALLOW),
    > +               DENY,
    > +
    > +               LABEL(&l, msg2_len),
    > +               ARG(2),
    > +               JLT(sizeof(msg2), ALLOW),
    > +               DENY,
    > +
    > +               LABEL(&l, buf_len),
    > +               ARG(2),
    > +               JLT(sizeof(buf), ALLOW),
    > +               DENY,
    > +       };
    > +       struct sock_fprog prog = {
    > +               .filter = filter,
    > +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
    > +       };
    > +       ssize_t bytes;
    > +       bpf_resolve_jumps(&l, filter, sizeof(filter)/sizeof(*filter));
    > +
    > +       if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
    > +               perror("prctl(NO_NEW_PRIVS)");
    > +               return 1;
    > +       }
    > +
    > +       if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
    > +               perror("prctl(SECCOMP)");
    > +               return 1;
    > +       }
    > +       syscall(__NR_write, STDOUT_FILENO, msg1, strlen(msg1));
    > +       bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)-1);
    > +       bytes = (bytes > 0 ? bytes : 0);
    > +       syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2));
    > +       syscall(__NR_write, STDERR_FILENO, buf, bytes);
    > +       /* Now get killed */
    > +       syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)+2);
    > +       return 0;
    > +}
    > diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c
    > new file mode 100644
    > index 0000000..579cfe3
    > --- /dev/null
    > +++ b/samples/seccomp/bpf-helper.c
    > @@ -0,0 +1,89 @@
    > +/*
    > + * Seccomp BPF helper functions
    > + *
    > + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
    > + * Author: Will Drewry <wad@chromium.org>
    > + *
    > + * The code may be used by anyone for any purpose,
    > + * and can serve as a starting point for developing
    > + * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
    > + */
    > +
    > +#include <stdio.h>
    > +#include <string.h>
    > +
    > +#include "bpf-helper.h"
    > +
    > +int bpf_resolve_jumps(struct bpf_labels *labels,
    > +                     struct sock_filter *filter, size_t count)
    > +{
    > +       struct sock_filter *begin = filter;
    > +       __u8 insn = count - 1;
    > +
    > +       if (count < 1)
    > +               return -1;
    > +       /*
    > +       * Walk it once, backwards, to build the label table and do fixups.
    > +       * Since backward jumps are disallowed by BPF, this is easy.
    > +       */
    > +       filter += insn;
    > +       for (; filter >= begin; --insn, --filter) {
    > +               if (filter->code != (BPF_JMP+BPF_JA))
    > +                       continue;
    > +               switch ((filter->jt<<8)|filter->jf) {
    > +               case (JUMP_JT<<8)|JUMP_JF:
    > +                       if (labels->labels[filter->k].location == 0xffffffff) {
    > +                               fprintf(stderr, "Unresolved label: '%s'\n",
    > +                                       labels->labels[filter->k].label);
    > +                               return 1;
    > +                       }
    > +                       filter->k = labels->labels[filter->k].location -
    > +                                   (insn + 1);
    > +                       filter->jt = 0;
    > +                       filter->jf = 0;
    > +                       continue;
    > +               case (LABEL_JT<<8)|LABEL_JF:
    > +                       if (labels->labels[filter->k].location != 0xffffffff) {
    > +                               fprintf(stderr, "Duplicate label use: '%s'\n",
    > +                                       labels->labels[filter->k].label);
    > +                               return 1;
    > +                       }
    > +                       labels->labels[filter->k].location = insn;
    > +                       filter->k = 0; /* fall through */
    > +                       filter->jt = 0;
    > +                       filter->jf = 0;
    > +                       continue;
    > +               }
    > +       }
    > +       return 0;
    > +}
    > +
    > +/* Simple lookup table for labels. */
    > +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
    > +{
    > +       struct __bpf_label *begin = labels->labels, *end;
    > +       int id;
    > +       if (labels->count == 0) {
    > +               begin->label = label;
    > +               begin->location = 0xffffffff;
    > +               labels->count++;
    > +               return 0;
    > +       }
    > +       end = begin + labels->count;
    > +       for (id = 0; begin < end; ++begin, ++id) {
    > +               if (!strcmp(label, begin->label))
    > +                       return id;
    > +       }
    > +       begin->label = label;
    > +       begin->location = 0xffffffff;
    > +       labels->count++;
    > +       return id;
    > +}
    > +
    > +void seccomp_bpf_print(struct sock_filter *filter, size_t count)
    > +{
    > +       struct sock_filter *end = filter + count;
    > +       for ( ; filter < end; ++filter)
    > +               printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
    > +                       filter->code, filter->jt, filter->jf, filter->k);
    > +}
    > diff --git a/samples/seccomp/bpf-helper.h b/samples/seccomp/bpf-helper.h
    > new file mode 100644
    > index 0000000..643279d
    > --- /dev/null
    > +++ b/samples/seccomp/bpf-helper.h
    > @@ -0,0 +1,238 @@
    > +/*
    > + * Example wrapper around BPF macros.
    > + *
    > + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
    > + * Author: Will Drewry <wad@chromium.org>
    > + *
    > + * The code may be used by anyone for any purpose,
    > + * and can serve as a starting point for developing
    > + * applications using prctl(PR_SET_SECCOMP, 2, ...).
    > + *
    > + * No guarantees are provided with respect to the correctness
    > + * or functionality of this code.
    > + */
    > +#ifndef __BPF_HELPER_H__
    > +#define __BPF_HELPER_H__
    > +
    > +#include <asm/bitsperlong.h>   /* for __BITS_PER_LONG */
    > +#include <endian.h>
    > +#include <linux/filter.h>
    > +#include <linux/seccomp.h>     /* for seccomp_data */
    > +#include <linux/types.h>
    > +#include <linux/unistd.h>
    > +#include <stddef.h>
    > +
    > +#define BPF_LABELS_MAX 256
    > +struct bpf_labels {
    > +       int count;
    > +       struct __bpf_label {
    > +               const char *label;
    > +               __u32 location;
    > +       } labels[BPF_LABELS_MAX];
    > +};
    > +
    > +int bpf_resolve_jumps(struct bpf_labels *labels,
    > +                     struct sock_filter *filter, size_t count);
    > +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label);
    > +void seccomp_bpf_print(struct sock_filter *filter, size_t count);
    > +
    > +#define JUMP_JT 0xff
    > +#define JUMP_JF 0xff
    > +#define LABEL_JT 0xfe
    > +#define LABEL_JF 0xfe
    > +
    > +#define ALLOW \
    > +       BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
    > +#define DENY \
    > +       BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
    > +#define JUMP(labels, label) \
    > +       BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
    > +                JUMP_JT, JUMP_JF)
    > +#define LABEL(labels, label) \
    > +       BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
    > +                LABEL_JT, LABEL_JF)
    > +#define SYSCALL(nr, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), \
    > +       jt
    > +
    > +/* Lame, but just an example */
    > +#define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label)
    > +
    > +#define EXPAND(...) __VA_ARGS__
    > +/* Map all width-sensitive operations */
    > +#if __BITS_PER_LONG == 32
    > +
    > +#define JEQ(x, jt) JEQ32(x, EXPAND(jt))
    > +#define JNE(x, jt) JNE32(x, EXPAND(jt))
    > +#define JGT(x, jt) JGT32(x, EXPAND(jt))
    > +#define JLT(x, jt) JLT32(x, EXPAND(jt))
    > +#define JGE(x, jt) JGE32(x, EXPAND(jt))
    > +#define JLE(x, jt) JLE32(x, EXPAND(jt))
    > +#define JA(x, jt) JA32(x, EXPAND(jt))
    > +#define ARG(i) ARG_32(i)
    > +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
    > +
    > +#elif __BITS_PER_LONG == 64
    > +
    > +/* Ensure that we load the logically correct offset. */
    > +#if __BYTE_ORDER == __LITTLE_ENDIAN
    > +#define ENDIAN(_lo, _hi) _lo, _hi
    > +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
    > +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
    > +#elif __BYTE_ORDER == __BIG_ENDIAN
    > +#define ENDIAN(_lo, _hi) _hi, _lo
    > +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
    > +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
    > +#else
    > +#error "Unknown endianness"
    > +#endif
    > +
    > +union arg64 {
    > +       struct {
    > +               __u32 ENDIAN(lo32, hi32);
    > +       };
    > +       __u64 u64;
    > +};
    > +
    > +#define JEQ(x, jt) \
    > +       JEQ64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +#define JGT(x, jt) \
    > +       JGT64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +#define JGE(x, jt) \
    > +       JGE64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +#define JNE(x, jt) \
    > +       JNE64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +#define JLT(x, jt) \
    > +       JLT64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +#define JLE(x, jt) \
    > +       JLE64(((union arg64){.u64 = (x)}).lo32, \
    > +             ((union arg64){.u64 = (x)}).hi32, \
    > +             EXPAND(jt))
    > +
    > +#define JA(x, jt) \
    > +       JA64(((union arg64){.u64 = (x)}).lo32, \
    > +              ((union arg64){.u64 = (x)}).hi32, \
    > +              EXPAND(jt))
    > +#define ARG(i) ARG_64(i)
    > +
    > +#else
    > +#error __BITS_PER_LONG value unusable.
    > +#endif
    > +
    > +/* Loads the arg into A */
    > +#define ARG_32(idx) \
    > +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx))
    > +
    > +/* Loads hi into A and lo in X */
    > +#define ARG_64(idx) \
    > +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)), \
    > +       BPF_STMT(BPF_ST, 0), /* lo -> M[0] */ \
    > +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, HI_ARG(idx)), \
    > +       BPF_STMT(BPF_ST, 1) /* hi -> M[1] */
    > +
    > +#define JEQ32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 0, 1), \
    > +       jt
    > +
    > +#define JNE32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 1, 0), \
    > +       jt
    > +
    > +/* Checks the lo, then swaps to check the hi. A=lo,X=hi */
    > +#define JEQ64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 0, 2), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JNE64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 5, 0), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 2, 0), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JA32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (value), 0, 1), \
    > +       jt
    > +
    > +#define JA64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (hi), 3, 0), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (lo), 0, 2), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JGE32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 0, 1), \
    > +       jt
    > +
    > +#define JLT32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 1, 0), \
    > +       jt
    > +
    > +/* Shortcut checking if hi > arg.hi. */
    > +#define JGE64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 0, 2), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JLT64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JGT32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 0, 1), \
    > +       jt
    > +
    > +#define JLE32(value, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 1, 0), \
    > +       jt
    > +
    > +/* Check hi > args.hi first, then do the GE checking */
    > +#define JGT64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 0, 2), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define JLE64(lo, hi, jt) \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 6, 0), \
    > +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 3), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
    > +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
    > +       jt, \
    > +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
    > +
    > +#define LOAD_SYSCALL_NR \
    > +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
    > +                offsetof(struct seccomp_data, nr))
    > +
    > +#endif  /* __BPF_HELPER_H__ */
    > diff --git a/samples/seccomp/dropper.c b/samples/seccomp/dropper.c
    > new file mode 100644
    > index 0000000..c69c347
    > --- /dev/null
    > +++ b/samples/seccomp/dropper.c
    > @@ -0,0 +1,68 @@
    > +/*
    > + * Naive system call dropper built on seccomp_filter.
    > + *
    > + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
    > + * Author: Will Drewry <wad@chromium.org>
    > + *
    > + * The code may be used by anyone for any purpose,
    > + * and can serve as a starting point for developing
    > + * applications using prctl(PR_SET_SECCOMP, 2, ...).
    > + *
    > + * When run, returns the specified errno for the specified
    > + * system call number against the given architecture.
    > + *
    > + * Run this one as root as PR_SET_NO_NEW_PRIVS is not called.
    > + */
    > +
    > +#include <errno.h>
    > +#include <linux/audit.h>
    > +#include <linux/filter.h>
    > +#include <linux/seccomp.h>
    > +#include <linux/unistd.h>
    > +#include <stdio.h>
    > +#include <stddef.h>
    > +#include <stdlib.h>
    > +#include <sys/prctl.h>
    > +#include <unistd.h>
    > +
    > +static int install_filter(int nr, int arch, int error)
    > +{
    > +       struct sock_filter filter[] = {
    > +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
    > +                        (offsetof(struct seccomp_data, arch))),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
    > +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
    > +                        (offsetof(struct seccomp_data, nr))),
    > +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
    > +               BPF_STMT(BPF_RET+BPF_K,
    > +                        SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
    > +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    > +       };
    > +       struct sock_fprog prog = {
    > +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
    > +               .filter = filter,
    > +       };
    > +       if (prctl(PR_SET_SECCOMP, 2, &prog)) {
    > +               perror("prctl");
    > +               return 1;
    > +       }
    > +       return 0;
    > +}
    > +
    > +int main(int argc, char **argv)
    > +{
    > +       if (argc < 5) {
    > +               fprintf(stderr, "Usage:\n"
    > +                       "dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
    > +                       "Hint:  AUDIT_ARCH_I386: 0x%X\n"
    > +                       "       AUDIT_ARCH_X86_64: 0x%X\n"
    > +                       "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
    > +               return 1;
    > +       }
    > +       if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
    > +                          strtol(argv[3], NULL, 0)))
    > +               return 1;
    > +       execv(argv[4], &argv[4]);
    > +       printf("Failed to execv\n");
    > +       return 255;
    > +}
    > --
    > 1.7.5.4
    >
    > --
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