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    SubjectRe: [PATCH v5 3/5] x86: Split syscall_trace_enter into two phases
    Hi,

    On Fri, Sep 05, 2014 at 03:13:54PM -0700, Andy Lutomirski wrote:
    > This splits syscall_trace_enter into syscall_trace_enter_phase1 and
    > syscall_trace_enter_phase2. Only phase 2 has full pt_regs, and only
    > phase 2 is permitted to modify any of pt_regs except for orig_ax.

    This breaks ptrace, see below.

    > The intent is that phase 1 can be called from the syscall fast path.
    >
    > In this implementation, phase1 can handle any combination of
    > TIF_NOHZ (RCU context tracking), TIF_SECCOMP, and TIF_SYSCALL_AUDIT,
    > unless seccomp requests a ptrace event, in which case phase2 is
    > forced.
    >
    > In principle, this could yield a big speedup for TIF_NOHZ as well as
    > for TIF_SECCOMP if syscall exit work were similarly split up.
    >
    > Signed-off-by: Andy Lutomirski <luto@amacapital.net>
    > ---
    > arch/x86/include/asm/ptrace.h | 5 ++
    > arch/x86/kernel/ptrace.c | 157 +++++++++++++++++++++++++++++++++++-------
    > 2 files changed, 138 insertions(+), 24 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
    > index 6205f0c434db..86fc2bb82287 100644
    > --- a/arch/x86/include/asm/ptrace.h
    > +++ b/arch/x86/include/asm/ptrace.h
    > @@ -75,6 +75,11 @@ convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs);
    > extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
    > int error_code, int si_code);
    >
    > +
    > +extern unsigned long syscall_trace_enter_phase1(struct pt_regs *, u32 arch);
    > +extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch,
    > + unsigned long phase1_result);
    > +
    > extern long syscall_trace_enter(struct pt_regs *);
    > extern void syscall_trace_leave(struct pt_regs *);
    >
    > diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
    > index bbf338a04a5d..29576c244699 100644
    > --- a/arch/x86/kernel/ptrace.c
    > +++ b/arch/x86/kernel/ptrace.c
    > @@ -1441,20 +1441,126 @@ void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
    > force_sig_info(SIGTRAP, &info, tsk);
    > }
    >
    > +static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
    > +{
    > +#ifdef CONFIG_X86_64
    > + if (arch == AUDIT_ARCH_X86_64) {
    > + audit_syscall_entry(arch, regs->orig_ax, regs->di,
    > + regs->si, regs->dx, regs->r10);
    > + } else
    > +#endif
    > + {
    > + audit_syscall_entry(arch, regs->orig_ax, regs->bx,
    > + regs->cx, regs->dx, regs->si);
    > + }
    > +}
    > +
    > /*
    > - * We must return the syscall number to actually look up in the table.
    > - * This can be -1L to skip running any syscall at all.
    > + * We can return 0 to resume the syscall or anything else to go to phase
    > + * 2. If we resume the syscall, we need to put something appropriate in
    > + * regs->orig_ax.
    > + *
    > + * NB: We don't have full pt_regs here, but regs->orig_ax and regs->ax
    > + * are fully functional.
    > + *
    > + * For phase 2's benefit, our return value is:
    > + * 0: resume the syscall
    > + * 1: go to phase 2; no seccomp phase 2 needed
    > + * anything else: go to phase 2; pass return value to seccomp
    > */
    > -long syscall_trace_enter(struct pt_regs *regs)
    > +unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
    > {
    > - long ret = 0;
    > + unsigned long ret = 0;
    > + u32 work;
    > +
    > + BUG_ON(regs != task_pt_regs(current));
    > +
    > + work = ACCESS_ONCE(current_thread_info()->flags) &
    > + _TIF_WORK_SYSCALL_ENTRY;
    >
    > /*
    > * If TIF_NOHZ is set, we are required to call user_exit() before
    > * doing anything that could touch RCU.
    > */
    > - if (test_thread_flag(TIF_NOHZ))
    > + if (work & _TIF_NOHZ) {
    > user_exit();
    > + work &= ~TIF_NOHZ;
    > + }
    > +
    > +#ifdef CONFIG_SECCOMP
    > + /*
    > + * Do seccomp first -- it should minimize exposure of other
    > + * code, and keeping seccomp fast is probably more valuable
    > + * than the rest of this.
    > + */
    > + if (work & _TIF_SECCOMP) {
    > + struct seccomp_data sd;
    > +
    > + sd.arch = arch;
    > + sd.nr = regs->orig_ax;
    > + sd.instruction_pointer = regs->ip;
    > +#ifdef CONFIG_X86_64
    > + if (arch == AUDIT_ARCH_X86_64) {
    > + sd.args[0] = regs->di;
    > + sd.args[1] = regs->si;
    > + sd.args[2] = regs->dx;
    > + sd.args[3] = regs->r10;
    > + sd.args[4] = regs->r8;
    > + sd.args[5] = regs->r9;
    > + } else
    > +#endif
    > + {
    > + sd.args[0] = regs->bx;
    > + sd.args[1] = regs->cx;
    > + sd.args[2] = regs->dx;
    > + sd.args[3] = regs->si;
    > + sd.args[4] = regs->di;
    > + sd.args[5] = regs->bp;
    > + }
    > +
    > + BUILD_BUG_ON(SECCOMP_PHASE1_OK != 0);
    > + BUILD_BUG_ON(SECCOMP_PHASE1_SKIP != 1);
    > +
    > + ret = seccomp_phase1(&sd);
    > + if (ret == SECCOMP_PHASE1_SKIP) {
    > + regs->orig_ax = -1;

    How the tracer is expected to get the correct syscall number after that?


    --
    ldv


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