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Subject[PATCH 2/2] fdmap(2)
From: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@epam.com>

Implement system call for bulk retrieveing of opened descriptors
in binary form.

Some daemons could use it to reliably close file descriptors
before starting. Currently they close everything upto some number
which formally is not reliable. Other natural users are lsof(1) and CRIU
(although lsof does so much in /proc that the effect is thoroughly buried).

Once again, /proc, the only way to learn anything about file descriptors
may not be available.

Sample program:

#include <stdlib.h>
#include <stdio.h>

static inline long sys_fdmap(int pid, int *fd, unsigned int n, int start)
{
register long r10 asm ("r10") = start;
register long r8 asm ("r8") = 0;
long rv;
asm volatile (
"syscall"
: "=a" (rv)
: "0" (334), "D" (pid), "S" (fd), "d" (n), "r" (r10), "r" (r8)
: "rcx", "r11", "cc", "memory"
);
return rv;
}

int main(int argc, char *argv[])
{
int fd[3];
int pid;
unsigned int start;
int n;

pid = 0;
if (argc > 1)
pid = atoi(argv[1]);

start = 0;
while ((n = sys_fdmap(pid, fd, sizeof(fd)/sizeof(fd[0]), start)) > 0) {
unsigned int i;

for (i = 0; i < n; i++) {
printf(" %u", fd[i]);
}
printf("\n");
start = fd[n - 1] + 1;
}

return 0;
}

Signed-off-by: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@epam.com>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

arch/x86/entry/syscalls/syscall_64.tbl | 1
fs/Makefile | 2
fs/fdmap.c | 105 +++++++++++++++++++
include/linux/syscalls.h | 2
tools/testing/selftests/fdmap/.gitignore | 1
tools/testing/selftests/fdmap/Makefile | 7 +
tools/testing/selftests/fdmap/fdmap.c | 112 +++++++++++++++++++++
tools/testing/selftests/fdmap/fdmap.h | 12 ++
tools/testing/selftests/fdmap/fdmap_test.c | 153 +++++++++++++++++++++++++++++
9 files changed, 394 insertions(+), 1 deletion(-)

--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -340,6 +340,7 @@
331 common pkey_free sys_pkey_free
332 common statx sys_statx
333 common pidmap sys_pidmap
+334 common fdmap sys_fdmap

#
# x32-specific system call numbers start at 512 to avoid cache impact
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,7 +11,7 @@ obj-y := open.o read_write.o file_table.o super.o \
attr.o bad_inode.o file.o filesystems.o namespace.o \
seq_file.o xattr.o libfs.o fs-writeback.o \
pnode.o splice.o sync.o utimes.o \
- stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
+ stack.o fs_struct.o statfs.o fs_pin.o nsfs.o fdmap.o

ifeq ($(CONFIG_BLOCK),y)
obj-y += buffer.o block_dev.o direct-io.o mpage.o
--- /dev/null
+++ b/fs/fdmap.c
@@ -0,0 +1,105 @@
+#include <linux/bitops.h>
+#include <linux/fdtable.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/syscalls.h>
+#include <linux/uaccess.h>
+
+/**
+ * fdmap - get opened file descriptors of a process
+ * @pid: the pid of the target process
+ * @fds: allocated userspace buffer
+ * @count: buffer size (in descriptors)
+ * @start_fd: first descriptor to search from (inclusive)
+ * @flags: reserved for future functionality, must be zero
+ *
+ * If @pid is zero then it's current process.
+ * Return: number of descriptors written. An error code otherwise.
+ */
+SYSCALL_DEFINE5(fdmap, pid_t, pid, int __user *, fds, unsigned int, count,
+ int, start_fd, int, flags)
+{
+ struct task_struct *task;
+ struct files_struct *files;
+ unsigned long search_mask;
+ unsigned int user_index, offset;
+ int masksize;
+
+ if (start_fd < 0 || flags != 0)
+ return -EINVAL;
+
+ if (!pid) {
+ files = get_files_struct(current);
+ } else {
+ rcu_read_lock();
+ task = find_task_by_vpid(pid);
+ if (!task) {
+ rcu_read_unlock();
+ return -ESRCH;
+ }
+ if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
+ rcu_read_unlock();
+ return -EACCES;
+ }
+ files = get_files_struct(task);
+ rcu_read_unlock();
+ }
+ if (!files)
+ return 0;
+
+ offset = start_fd / BITS_PER_LONG;
+ search_mask = ULONG_MAX << (start_fd % BITS_PER_LONG);
+ user_index = 0;
+#define FDS_BUF_SIZE (1024/sizeof(unsigned long))
+ masksize = FDS_BUF_SIZE;
+ while (user_index < count && masksize == FDS_BUF_SIZE) {
+ unsigned long open_fds[FDS_BUF_SIZE];
+ struct fdtable *fdt;
+ unsigned int i;
+
+ /*
+ * fdt->max_fds can grow, get it every time
+ * before copying part into internal buffer.
+ */
+ rcu_read_lock();
+ fdt = files_fdtable(files);
+ masksize = fdt->max_fds / 8 - offset * sizeof(long);
+ if (masksize < 0) {
+ rcu_read_unlock();
+ break;
+ }
+ masksize = min(masksize, (int)sizeof(open_fds));
+ memcpy(open_fds, fdt->open_fds + offset, masksize);
+ rcu_read_unlock();
+
+ open_fds[0] &= search_mask;
+ search_mask = ULONG_MAX;
+ masksize = (masksize + sizeof(long) - 1) / sizeof(long);
+ start_fd = offset * BITS_PER_LONG;
+ /*
+ * for_each_set_bit_from() can re-read first word
+ * multiple times which is not optimal.
+ */
+ for (i = 0; i < masksize; i++) {
+ unsigned long mask = open_fds[i];
+
+ while (mask) {
+ unsigned int real_fd = start_fd + __ffs(mask);
+
+ if (put_user(real_fd, fds + user_index)) {
+ put_files_struct(files);
+ return -EFAULT;
+ }
+ if (++user_index >= count)
+ goto out;
+ mask &= mask - 1;
+ }
+ start_fd += BITS_PER_LONG;
+ }
+ offset += FDS_BUF_SIZE;
+ }
+out:
+ put_files_struct(files);
+
+ return user_index;
+}
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -922,6 +922,8 @@ asmlinkage long sys_pkey_alloc(unsigned long flags, unsigned long init_val);
asmlinkage long sys_pkey_free(int pkey);
asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);
+asmlinkage long sys_fdmap(pid_t pid, int __user *fds, unsigned int count,
+ int start_fd, int flags);

asmlinkage long sys_pidmap(int __user *pids,
unsigned int pids_count,
--- /dev/null
+++ b/tools/testing/selftests/fdmap/.gitignore
@@ -0,0 +1 @@
+fdmap_test
--- /dev/null
+++ b/tools/testing/selftests/fdmap/Makefile
@@ -0,0 +1,7 @@
+TEST_GEN_PROGS := fdmap_test
+CFLAGS += -Wall
+
+include ../lib.mk
+
+$(TEST_GEN_PROGS): fdmap_test.c fdmap.c fdmap.h ../kselftest_harness.h
+ $(CC) $(CFLAGS) $(LDFLAGS) $< fdmap.c -o $@
--- /dev/null
+++ b/tools/testing/selftests/fdmap/fdmap.c
@@ -0,0 +1,112 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <dirent.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/types.h>
+#include "fdmap.h"
+
+#define BUF_SIZE 1024
+
+long fdmap(pid_t pid, int *fds, size_t count, int start_fd, int flags)
+{
+ register int64_t r10 asm("r10") = start_fd;
+ register int64_t r8 asm("r8") = flags;
+ long ret;
+
+ asm volatile (
+ "syscall"
+ : "=a"(ret)
+ : "0" (334),
+ "D" (pid), "S" (fds), "d" (count), "r" (r10), "r" (r8)
+ : "rcx", "r11", "cc", "memory"
+ );
+ return ret;
+}
+
+int fdmap_full(pid_t pid, int **fds, size_t *n)
+{
+ int buf[BUF_SIZE], start_fd = 0;
+ long ret;
+
+ *n = 0;
+ *fds = NULL;
+ for (;;) {
+ int *new_buff;
+
+ ret = fdmap(pid, buf, BUF_SIZE, start_fd, 0);
+ if (ret < 0)
+ break;
+ if (!ret)
+ return 0;
+
+ new_buff = realloc(*fds, (*n + ret) * sizeof(int));
+ if (!new_buff) {
+ ret = -errno;
+ break;
+ }
+ *fds = new_buff;
+ memcpy(*fds + *n, buf, ret * sizeof(int));
+ *n += ret;
+ start_fd = (*fds)[*n - 1] + 1;
+ }
+ free(*fds);
+ *fds = NULL;
+ return -ret;
+}
+
+int fdmap_proc(pid_t pid, int **fds, size_t *n)
+{
+ char fds_path[20];
+ int dir_fd = 0;
+ struct dirent *fd_link;
+ DIR *fds_dir;
+
+ *fds = NULL;
+ *n = 0;
+ if (!pid)
+ strcpy(fds_path, "/proc/self/fd");
+ else
+ sprintf(fds_path, "/proc/%d/fd", pid);
+
+ fds_dir = opendir(fds_path);
+ if (!fds_dir)
+ return errno == ENOENT ? ESRCH : errno;
+ if (!pid)
+ dir_fd = dirfd(fds_dir);
+
+ while ((fd_link = readdir(fds_dir))) {
+ if (fd_link->d_name[0] < '0'
+ || fd_link->d_name[0] > '9')
+ continue;
+ if (*n % BUF_SIZE == 0) {
+ int *new_buff;
+
+ new_buff = realloc(*fds, (*n + BUF_SIZE) * sizeof(int));
+ if (!new_buff) {
+ int ret = errno;
+
+ free(*fds);
+ *fds = NULL;
+ return ret;
+ }
+ *fds = new_buff;
+ }
+ (*fds)[*n] = atoi(fd_link->d_name);
+ *n += 1;
+ }
+ closedir(fds_dir);
+
+ if (!pid) {
+ size_t i;
+
+ for (i = 0; i < *n; i++)
+ if ((*fds)[i] == dir_fd)
+ break;
+ i++;
+ memmove(*fds + i - 1, *fds + i, (*n - i) * sizeof(int));
+ (*n)--;
+ }
+ return 0;
+}
--- /dev/null
+++ b/tools/testing/selftests/fdmap/fdmap.h
@@ -0,0 +1,12 @@
+#ifndef FDMAP_H
+#define FDMAP_H
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+long fdmap(pid_t pid, int *fds, size_t count, int start_fd, int flags);
+int fdmap_full(pid_t pid, int **fds, size_t *n);
+int fdmap_proc(pid_t pid, int **fds, size_t *n);
+
+#endif
--- /dev/null
+++ b/tools/testing/selftests/fdmap/fdmap_test.c
@@ -0,0 +1,153 @@
+#include <errno.h>
+#include <syscall.h>
+#include <sys/time.h>
+#include <sys/resource.h>
+#include <limits.h>
+#include "../kselftest_harness.h"
+#include "fdmap.h"
+
+TEST(efault) {
+ int ret;
+
+ ret = syscall(334, 0, NULL, 20 * sizeof(int), 0, 0);
+ ASSERT_EQ(-1, ret);
+ ASSERT_EQ(EFAULT, errno);
+}
+
+TEST(big_start_fd) {
+ int fds[1];
+ int ret;
+
+ ret = syscall(334, 0, fds, sizeof(int), INT_MAX, 0);
+ ASSERT_EQ(0, ret);
+}
+
+TEST(einval) {
+ int ret;
+
+ ret = syscall(334, 0, NULL, 0, -1, 0);
+ ASSERT_EQ(-1, ret);
+ ASSERT_EQ(EINVAL, errno);
+
+ ret = syscall(334, 0, NULL, 0, 0, 1);
+ ASSERT_EQ(-1, ret);
+ ASSERT_EQ(EINVAL, errno);
+}
+
+TEST(esrch) {
+ int fds[1], ret;
+ pid_t pid;
+
+ pid = fork();
+ ASSERT_NE(-1, pid);
+ if (!pid)
+ exit(0);
+ waitpid(pid, NULL, 0);
+
+ ret = syscall(334, pid, fds, sizeof(int), 0, 0);
+ ASSERT_EQ(-1, ret);
+ ASSERT_EQ(ESRCH, errno);
+}
+
+TEST(simple) {
+ int *fds1, *fds2;
+ size_t size1, size2, i;
+ int ret1, ret2;
+
+ ret1 = fdmap_full(0, &fds1, &size1);
+ ret2 = fdmap_proc(0, &fds2, &size2);
+ ASSERT_EQ(ret2, ret1);
+ ASSERT_EQ(size2, size1);
+ for (i = 0; i < size1; i++)
+ ASSERT_EQ(fds2[i], fds1[i]);
+ free(fds1);
+ free(fds2);
+}
+
+TEST(init) {
+ int *fds1, *fds2;
+ size_t size1, size2, i;
+ int ret1, ret2;
+
+ ret1 = fdmap_full(1, &fds1, &size1);
+ ret2 = fdmap_proc(1, &fds2, &size2);
+ ASSERT_EQ(ret2, ret1);
+ ASSERT_EQ(size2, size1);
+ for (i = 0; i < size1; i++)
+ ASSERT_EQ(fds2[i], fds1[i]);
+ free(fds1);
+ free(fds2);
+}
+
+TEST(zero) {
+ int *fds, i;
+ size_t size;
+ int ret;
+
+ ret = fdmap_proc(0, &fds, &size);
+ ASSERT_EQ(0, ret);
+ for (i = 0; i < size; i++)
+ close(fds[i]);
+ free(fds);
+ fds = NULL;
+
+ ret = fdmap_full(0, &fds, &size);
+ ASSERT_EQ(0, ret);
+ ASSERT_EQ(0, size);
+}
+
+TEST(more_fds) {
+ int *fds1, *fds2, ret1, ret2;
+ size_t size1, size2, i;
+
+ struct rlimit rlim = {
+ .rlim_cur = 600000,
+ .rlim_max = 600000
+ };
+ ASSERT_EQ(0, setrlimit(RLIMIT_NOFILE, &rlim));
+ for (int i = 0; i < 500000; i++)
+ dup(0);
+
+ ret1 = fdmap_full(0, &fds1, &size1);
+ ret2 = fdmap_proc(0, &fds2, &size2);
+ ASSERT_EQ(ret2, ret1);
+ ASSERT_EQ(size2, size1);
+ for (i = 0; i < size1; i++)
+ ASSERT_EQ(fds2[i], fds1[i]);
+ free(fds1);
+ free(fds2);
+}
+
+TEST(child) {
+ int pipefd[2];
+ int *fds1, *fds2, ret1, ret2, i;
+ size_t size1, size2;
+ char byte = 0;
+ pid_t pid;
+
+ ASSERT_NE(-1, pipe(pipefd));
+ pid = fork();
+ ASSERT_NE(-1, pid);
+ if (!pid) {
+ read(pipefd[0], &byte, 1);
+ close(pipefd[0]);
+ close(pipefd[1]);
+ exit(0);
+ }
+
+ ret1 = fdmap_full(0, &fds1, &size1);
+ ret2 = fdmap_proc(0, &fds2, &size2);
+ ASSERT_EQ(ret2, ret1);
+ ASSERT_EQ(size2, size1);
+ for (i = 0; i < size1; i++)
+ ASSERT_EQ(fds2[i], fds1[i]);
+ free(fds1);
+ free(fds2);
+
+ write(pipefd[1], &byte, 1);
+ close(pipefd[0]);
+ close(pipefd[1]);
+ waitpid(pid, NULL, 0);
+}
+
+TEST_HARNESS_MAIN
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