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Date: Mon, 25 Sep 2017 09:42:58 +0200
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Alexey Dobriyan <adobriyan@...il.com>, akpm@...ux-foundation.org
Cc: mtk.manpages@...il.com, linux-kernel@...r.kernel.org,
linux-api@...r.kernel.org, rdunlap@...radead.org,
tglx@...utronix.de, tixxdz@...il.com, gladkov.alexey@...il.com,
Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@...m.com>
Subject: Re: [PATCH 1/2 v2] fdmap(2)
[Not sure why original author is not in CC; added]
Hello Alexey,
On 09/24/2017 10:06 PM, Alexey Dobriyan wrote:
> From: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@...m.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).
>
> /proc, the only way to learn anything about file descriptors may not be
> available. There is unavoidable overhead associated with instantiating
> 3 dentries and 3 inodes and converting integers to strings and back.
>
> Benchmark:
>
> N=1<<22 times
> 4 opened descriptors (0, 1, 2, 3)
> opendir+readdir+closedir /proc/self/fd vs fdmap
>
> /proc 8.31 ± 0.37%
> fdmap 0.32 ± 0.72%
>From the text above, I'm still trying to understand: whose problem
does this solve? I mean, we've lived with the daemon-close-all-files
technique forever (and I'm not sure that performance is really an
important issue for the daemon case) . And you say that the effect
for lsof(1) will be buried. So, who does this new system call
really help? (Note: I'm not saying don't add the syscall, but from
explanation given here, it's not clear why we should.)
Thanks,
Michael
> FDMAP(2) Linux Programmer's Manual FDMAP(2)
>
> NAME
> fdmap - get open file descriptors of the process
>
> SYNOPSIS
> long fdmap(pid_t pid, int *fd, unsigned int nfd, int start, int flags);
>
> DESCRIPTION
> fdmap() writes open file descriptors of the process into buffer fd
> starting from the start descriptor. At most nfd elements are written.
> flags argument is reserved and must be zero.
>
> If pid is zero, syscall will work with the current process.
>
> RETURN VALUE
> On success, number of descriptors written is returned. On error, -1 is
> returned, and errno is set appropriately.
>
> ERRORS
> ESRCH No such process.
>
> EACCES Permission denied.
>
> EFAULT Invalid fd pointer.
>
> EINVAL Negative start argument.
>
> NOTES
> Glibc does not provide a wrapper for these system call; call it using
> syscall(2).
>
> EXAMPLE
> The program below demonstrates fdmap() usage.
>
> $ ./a.out $$
> 0 1 2 255
>
> $ ./a.out 42</dev/null 1023</dev/null
> 0 1 2 42
> 1023
>
> Program source
>
> #include <sys/types.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> static inline long fdmap(int pid, int *fd, unsigned int nfd, unsigned int start, int flags)
> {
> register long r10 asm ("r10") = start;
> register long r8 asm ("r8") = flags;
> long rv;
> asm volatile (
> "syscall"
> : "=a" (rv)
> : "0" (333), "D" (pid), "S" (fd), "d" (nfd), "r" (r10), "r" (r8)
> : "rcx", "r11", "cc", "memory"
> );
> return rv;
> }
>
> int main(int argc, char *argv[])
> {
> pid_t pid;
> int fd[4];
> unsigned int start;
> int n;
>
> pid = 0;
> if (argc > 1)
> pid = atoi(argv[1]);
>
> start = 0;
> while ((n = fdmap(pid, fd, sizeof(fd)/sizeof(fd[0]), start, 0)) > 0) {
> unsigned int i;
>
> for (i = 0; i < n; i++)
> printf("%u ", fd[i]);
> printf("\n");
>
> start = fd[n - 1] + 1;
> }
> return 0;
> }
>
> Linux 2017-09-21 FDMAP(2)
>
> Changelog:
>
> CONFIG_PIDMAP option
> manpage
>
>
> Signed-off-by: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@...m.com>
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
>
> ---
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> fs/Makefile | 2 +
> fs/fdmap.c | 105 ++++++++++++++++++++
> include/linux/syscalls.h | 2 +
> init/Kconfig | 7 ++
> kernel/sys_ni.c | 2 +
> tools/testing/selftests/Makefile | 1 +
> 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 +++++++++++++++++++++++++++++
> 12 files changed, 405 insertions(+)
> create mode 100644 fs/fdmap.c
> create mode 100644 tools/testing/selftests/fdmap/.gitignore
> create mode 100644 tools/testing/selftests/fdmap/Makefile
> create mode 100644 tools/testing/selftests/fdmap/fdmap.c
> create mode 100644 tools/testing/selftests/fdmap/fdmap.h
> create mode 100644 tools/testing/selftests/fdmap/fdmap_test.c
>
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index 5aef183e2f85..9bfe5f79674f 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -339,6 +339,7 @@
> 330 common pkey_alloc sys_pkey_alloc
> 331 common pkey_free sys_pkey_free
> 332 common statx sys_statx
> +333 common fdmap sys_fdmap
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/Makefile b/fs/Makefile
> index 7bbaca9c67b1..27476a66c18e 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -13,6 +13,8 @@ obj-y := open.o read_write.o file_table.o super.o \
> pnode.o splice.o sync.o utimes.o \
> stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
>
> +obj-$(CONFIG_FDMAP) += fdmap.o
> +
> ifeq ($(CONFIG_BLOCK),y)
> obj-y += buffer.o block_dev.o direct-io.o mpage.o
> else
> diff --git a/fs/fdmap.c b/fs/fdmap.c
> new file mode 100644
> index 000000000000..274e6c5b7c9c
> --- /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 (512/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;
> +}
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 95606a2d556f..d393d844facb 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -936,5 +936,7 @@ 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);
>
> #endif
> diff --git a/init/Kconfig b/init/Kconfig
> index 78cb2461012e..952d13b7326d 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1400,6 +1400,13 @@ config MEMBARRIER
>
> If unsure, say Y.
>
> +config FDMAP
> + bool "fdmap() system call" if EXPERT
> + default y
> + help
> + Enable fdmap() system call that allows to query file descriptors
> + in binary form avoiding /proc overhead.
> +
> config EMBEDDED
> bool "Embedded system"
> option allnoconfig_y
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 8acef8576ce9..d61fa27d021e 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -258,3 +258,5 @@ cond_syscall(sys_membarrier);
> cond_syscall(sys_pkey_mprotect);
> cond_syscall(sys_pkey_alloc);
> cond_syscall(sys_pkey_free);
> +
> +cond_syscall(sys_fdmap);
> diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
> index 26ce4f7168be..e8d63c27c865 100644
> --- a/tools/testing/selftests/Makefile
> +++ b/tools/testing/selftests/Makefile
> @@ -5,6 +5,7 @@ TARGETS += cpufreq
> TARGETS += cpu-hotplug
> TARGETS += efivarfs
> TARGETS += exec
> +TARGETS += fdmap
> TARGETS += firmware
> TARGETS += ftrace
> TARGETS += futex
> diff --git a/tools/testing/selftests/fdmap/.gitignore b/tools/testing/selftests/fdmap/.gitignore
> new file mode 100644
> index 000000000000..9a9bfdac1cc0
> --- /dev/null
> +++ b/tools/testing/selftests/fdmap/.gitignore
> @@ -0,0 +1 @@
> +fdmap_test
> diff --git a/tools/testing/selftests/fdmap/Makefile b/tools/testing/selftests/fdmap/Makefile
> new file mode 100644
> index 000000000000..bf9f051f4b63
> --- /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 $@
> diff --git a/tools/testing/selftests/fdmap/fdmap.c b/tools/testing/selftests/fdmap/fdmap.c
> new file mode 100644
> index 000000000000..66725b0201e0
> --- /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" (333),
> + "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;
> +}
> diff --git a/tools/testing/selftests/fdmap/fdmap.h b/tools/testing/selftests/fdmap/fdmap.h
> new file mode 100644
> index 000000000000..c501111b2bbd
> --- /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
> diff --git a/tools/testing/selftests/fdmap/fdmap_test.c b/tools/testing/selftests/fdmap/fdmap_test.c
> new file mode 100644
> index 000000000000..6f448406d96a
> --- /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(333, 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(333, 0, fds, sizeof(int), INT_MAX, 0);
> + ASSERT_EQ(0, ret);
> +}
> +
> +TEST(einval) {
> + int ret;
> +
> + ret = syscall(333, 0, NULL, 0, -1, 0);
> + ASSERT_EQ(-1, ret);
> + ASSERT_EQ(EINVAL, errno);
> +
> + ret = syscall(333, 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(333, 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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--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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