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Subject[PATCH] proc: test /proc/*/fd a bit (+ PF_KTHREAD is ABI!)
* Test lookup in /proc/self/fd.
"map_files" lookup story showed that lookup is not that simple.

* Test that all those symlinks open the same file.
Check with (st_dev, st_info).

* Test that kernel threads do not have anything in their /proc/*/fd/
directory.

Now this is where things get interesting.

First, kernel threads aren't pinned by /proc/self or equivalent,
thus some "atomicity" is required.

Second, ->comm can contain whitespace and ')'.
No, they are not escaped.

Third, the only reliable way to check if process is kernel thread
appears to be field #9 in /proc/*/stat.

This field is struct task_struct::flags in decimal!
Check is done by testing PF_KTHREAD flags like we do in kernel.

PF_KTREAD value is a part of userspace ABI !!!

Other methods for determining kernel threadness are not reliable:
* RSS can be 0 if everything is swapped, even while reading
from /proc/self.

* ->total_vm CAN BE ZERO if process is finishing

munmap(NULL, whole address space);

* /proc/*/maps and similar files can be empty because unmapping
everything works. Read returning 0 can't distinguish between
kernel thread and such suicide process.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

tools/testing/selftests/proc/.gitignore | 3
tools/testing/selftests/proc/Makefile | 5
tools/testing/selftests/proc/fd-001-lookup.c | 168 +++++++++++++++++++++++
tools/testing/selftests/proc/fd-002-posix-eq.c | 57 ++++++++
tools/testing/selftests/proc/fd-003-kthread.c | 178 +++++++++++++++++++++++++
tools/testing/selftests/proc/proc-uptime.h | 16 --
tools/testing/selftests/proc/proc.h | 39 +++++
tools/testing/selftests/proc/read.c | 17 --
8 files changed, 451 insertions(+), 32 deletions(-)

--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -1,3 +1,6 @@
+/fd-001-lookup
+/fd-002-posix-eq
+/fd-003-kthread
/proc-loadavg-001
/proc-self-map-files-001
/proc-self-map-files-002
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -1,6 +1,9 @@
-CFLAGS += -Wall -O2
+CFLAGS += -Wall -O2 -Wno-unused-function

TEST_GEN_PROGS :=
+TEST_GEN_PROGS += fd-001-lookup
+TEST_GEN_PROGS += fd-002-posix-eq
+TEST_GEN_PROGS += fd-003-kthread
TEST_GEN_PROGS += proc-loadavg-001
TEST_GEN_PROGS += proc-self-map-files-001
TEST_GEN_PROGS += proc-self-map-files-002
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/proc/fd-001-lookup.c
@@ -0,0 +1,168 @@
+/*
+ * Copyright © 2018 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+// Test /proc/*/fd lookup.
+#define _GNU_SOURCE
+#undef NDEBUG
+#include <assert.h>
+#include <dirent.h>
+#include <errno.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include "proc.h"
+
+/* lstat(2) has more "coverage" in case non-symlink pops up somehow. */
+static void test_lookup_pass(const char *pathname)
+{
+ struct stat st;
+ ssize_t rv;
+
+ memset(&st, 0, sizeof(struct stat));
+ rv = lstat(pathname, &st);
+ assert(rv == 0);
+ assert(S_ISLNK(st.st_mode));
+}
+
+static void test_lookup_fail(const char *pathname)
+{
+ struct stat st;
+ ssize_t rv;
+
+ rv = lstat(pathname, &st);
+ assert(rv == -1 && errno == ENOENT);
+}
+
+static void test_lookup(unsigned int fd)
+{
+ char buf[64];
+ unsigned int c;
+ unsigned int u;
+ int i;
+
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%u", fd);
+ test_lookup_pass(buf);
+
+ /* leading junk */
+ for (c = 1; c <= 255; c++) {
+ if (c == '/')
+ continue;
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%c%u", c, fd);
+ test_lookup_fail(buf);
+ }
+
+ /* trailing junk */
+ for (c = 1; c <= 255; c++) {
+ if (c == '/')
+ continue;
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%u%c", fd, c);
+ test_lookup_fail(buf);
+ }
+
+ for (i = INT_MIN; i < INT_MIN + 1024; i++) {
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%d", i);
+ test_lookup_fail(buf);
+ }
+ for (i = -1024; i < 0; i++) {
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%d", i);
+ test_lookup_fail(buf);
+ }
+ for (u = INT_MAX - 1024; u <= (unsigned int)INT_MAX + 1024; u++) {
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%u", u);
+ test_lookup_fail(buf);
+ }
+ for (u = UINT_MAX - 1024; u != 0; u++) {
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%u", u);
+ test_lookup_fail(buf);
+ }
+
+
+}
+
+int main(void)
+{
+ struct dirent *de;
+ unsigned int fd, target_fd;
+
+ if (unshare(CLONE_FILES) == -1)
+ return 1;
+
+ /* Wipe fdtable. */
+ do {
+ DIR *d;
+
+ d = opendir("/proc/self/fd");
+ if (!d)
+ return 1;
+
+ de = xreaddir(d);
+ assert(de->d_type == DT_DIR);
+ assert(streq(de->d_name, "."));
+
+ de = xreaddir(d);
+ assert(de->d_type == DT_DIR);
+ assert(streq(de->d_name, ".."));
+next:
+ de = xreaddir(d);
+ if (de) {
+ unsigned long long fd_ull;
+ unsigned int fd;
+ char *end;
+
+ assert(de->d_type == DT_LNK);
+
+ fd_ull = xstrtoull(de->d_name, &end);
+ assert(*end == '\0');
+ assert(fd_ull == (unsigned int)fd_ull);
+
+ fd = fd_ull;
+ if (fd == dirfd(d))
+ goto next;
+ close(fd);
+ }
+
+ closedir(d);
+ } while (de);
+
+ /* Now fdtable is clean. */
+
+ fd = open("/", O_PATH|O_DIRECTORY);
+ assert(fd == 0);
+ test_lookup(fd);
+ close(fd);
+
+ /* Clean again! */
+
+ fd = open("/", O_PATH|O_DIRECTORY);
+ assert(fd == 0);
+ /* Default RLIMIT_NOFILE-1 */
+ target_fd = 1023;
+ while (target_fd > 0) {
+ if (dup2(fd, target_fd) == target_fd)
+ break;
+ target_fd /= 2;
+ }
+ assert(target_fd > 0);
+ close(fd);
+ test_lookup(target_fd);
+ close(target_fd);
+
+ return 0;
+}
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/proc/fd-002-posix-eq.c
@@ -0,0 +1,57 @@
+/*
+ * Copyright © 2018 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+// Test that open(/proc/*/fd/*) opens the same file.
+#undef NDEBUG
+#include <assert.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main(void)
+{
+ int fd0, fd1, fd2;
+ struct stat st0, st1, st2;
+ char buf[64];
+ int rv;
+
+ fd0 = open("/", O_DIRECTORY|O_RDONLY);
+ assert(fd0 >= 0);
+
+ snprintf(buf, sizeof(buf), "/proc/self/fd/%u", fd0);
+ fd1 = open(buf, O_RDONLY);
+ assert(fd1 >= 0);
+
+ snprintf(buf, sizeof(buf), "/proc/thread-self/fd/%u", fd0);
+ fd2 = open(buf, O_RDONLY);
+ assert(fd2 >= 0);
+
+ rv = fstat(fd0, &st0);
+ assert(rv == 0);
+ rv = fstat(fd1, &st1);
+ assert(rv == 0);
+ rv = fstat(fd2, &st2);
+ assert(rv == 0);
+
+ assert(st0.st_dev == st1.st_dev);
+ assert(st0.st_ino == st1.st_ino);
+
+ assert(st0.st_dev == st2.st_dev);
+ assert(st0.st_ino == st2.st_ino);
+
+ return 0;
+}
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/proc/fd-003-kthread.c
@@ -0,0 +1,178 @@
+/*
+ * Copyright © 2018 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+// Test that /proc/$KERNEL_THREAD/fd/ is empty.
+#define _GNU_SOURCE
+#undef NDEBUG
+#include <sys/syscall.h>
+#include <assert.h>
+#include <dirent.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "proc.h"
+
+#define PF_KHTREAD 0x00200000
+
+/*
+ * Test for kernel threadness atomically with openat().
+ *
+ * Return /proc/$PID/fd descriptor if process is kernel thread.
+ * Return -1 if a process is userspace process.
+ */
+static int kernel_thread_fd(unsigned int pid)
+{
+ unsigned int flags = 0;
+ char buf[4096];
+ int dir_fd, fd;
+ ssize_t rv;
+
+ snprintf(buf, sizeof(buf), "/proc/%u", pid);
+ dir_fd = open(buf, O_RDONLY|O_DIRECTORY);
+ if (dir_fd == -1)
+ return -1;
+
+ /*
+ * Believe it or not, struct task_struct::flags is directly exposed
+ * to userspace!
+ */
+ fd = openat(dir_fd, "stat", O_RDONLY);
+ if (fd == -1) {
+ close(dir_fd);
+ return -1;
+ }
+ rv = read(fd, buf, sizeof(buf));
+ close(fd);
+ if (0 < rv && rv <= sizeof(buf)) {
+ unsigned long long flags_ull;
+ char *p, *end;
+ int i;
+
+ assert(buf[rv - 1] == '\n');
+ buf[rv - 1] = '\0';
+
+ /* Search backwards: ->comm can contain whitespace and ')'. */
+ for (i = 0; i < 43; i++) {
+ p = strrchr(buf, ' ');
+ assert(p);
+ *p = '\0';
+ }
+
+ p = strrchr(buf, ' ');
+ assert(p);
+
+ flags_ull = xstrtoull(p + 1, &end);
+ assert(*end == '\0');
+ assert(flags_ull == (unsigned int)flags_ull);
+
+ flags = flags_ull;
+ }
+
+ fd = -1;
+ if (flags & PF_KHTREAD) {
+ fd = openat(dir_fd, "fd", O_RDONLY|O_DIRECTORY);
+ }
+ close(dir_fd);
+ return fd;
+}
+
+static void test_readdir(int fd)
+{
+ DIR *d;
+ struct dirent *de;
+
+ d = fdopendir(fd);
+ assert(d);
+
+ de = xreaddir(d);
+ assert(streq(de->d_name, "."));
+ assert(de->d_type == DT_DIR);
+
+ de = xreaddir(d);
+ assert(streq(de->d_name, ".."));
+ assert(de->d_type == DT_DIR);
+
+ de = xreaddir(d);
+ assert(!de);
+}
+
+static inline int sys_statx(int dirfd, const char *pathname, int flags,
+ unsigned int mask, void *stx)
+{
+ return syscall(SYS_statx, dirfd, pathname, flags, mask, stx);
+}
+
+static void test_lookup_fail(int fd, const char *pathname)
+{
+ char stx[256] __attribute__((aligned(8)));
+ int rv;
+
+ rv = sys_statx(fd, pathname, AT_SYMLINK_NOFOLLOW, 0, (void *)stx);
+ assert(rv == -1 && errno == ENOENT);
+}
+
+static void test_lookup(int fd)
+{
+ char buf[64];
+ unsigned int u;
+ int i;
+
+ for (i = INT_MIN; i < INT_MIN + 1024; i++) {
+ snprintf(buf, sizeof(buf), "%d", i);
+ test_lookup_fail(fd, buf);
+ }
+ for (i = -1024; i < 1024; i++) {
+ snprintf(buf, sizeof(buf), "%d", i);
+ test_lookup_fail(fd, buf);
+ }
+ for (u = INT_MAX - 1024; u < (unsigned int)INT_MAX + 1024; u++) {
+ snprintf(buf, sizeof(buf), "%u", u);
+ test_lookup_fail(fd, buf);
+ }
+ for (u = UINT_MAX - 1024; u != 0; u++) {
+ snprintf(buf, sizeof(buf), "%u", u);
+ test_lookup_fail(fd, buf);
+ }
+}
+
+int main(void)
+{
+ unsigned int pid;
+ int fd;
+
+ /*
+ * In theory this will loop indefinitely if kernel threads are exiled
+ * from /proc.
+ *
+ * Start with kthreadd.
+ */
+ pid = 2;
+ while ((fd = kernel_thread_fd(pid)) == -1 && pid < 1024) {
+ pid++;
+ }
+ /* EACCES if run as non-root. */
+ if (pid >= 1024)
+ return 1;
+
+ test_readdir(fd);
+ test_lookup(fd);
+
+ return 0;
+}
--- a/tools/testing/selftests/proc/proc-uptime.h
+++ b/tools/testing/selftests/proc/proc-uptime.h
@@ -20,21 +20,7 @@
#include <stdlib.h>
#include <unistd.h>

-static unsigned long long xstrtoull(const char *p, char **end)
-{
- if (*p == '0') {
- *end = (char *)p + 1;
- return 0;
- } else if ('1' <= *p && *p <= '9') {
- unsigned long long val;
-
- errno = 0;
- val = strtoull(p, end, 10);
- assert(errno == 0);
- return val;
- } else
- assert(0);
-}
+#include "proc.h"

static void proc_uptime(int fd, uint64_t *uptime, uint64_t *idle)
{
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/proc/proc.h
@@ -0,0 +1,39 @@
+#pragma once
+#undef NDEBUG
+#include <assert.h>
+#include <dirent.h>
+#include <errno.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+static inline bool streq(const char *s1, const char *s2)
+{
+ return strcmp(s1, s2) == 0;
+}
+
+static unsigned long long xstrtoull(const char *p, char **end)
+{
+ if (*p == '0') {
+ *end = (char *)p + 1;
+ return 0;
+ } else if ('1' <= *p && *p <= '9') {
+ unsigned long long val;
+
+ errno = 0;
+ val = strtoull(p, end, 10);
+ assert(errno == 0);
+ return val;
+ } else
+ assert(0);
+}
+
+static struct dirent *xreaddir(DIR *d)
+{
+ struct dirent *de;
+
+ errno = 0;
+ de = readdir(d);
+ assert(de || errno == 0);
+ return de;
+}
--- a/tools/testing/selftests/proc/read.c
+++ b/tools/testing/selftests/proc/read.c
@@ -31,22 +31,7 @@
#include <fcntl.h>
#include <unistd.h>

-static inline bool streq(const char *s1, const char *s2)
-{
- return strcmp(s1, s2) == 0;
-}
-
-static struct dirent *xreaddir(DIR *d)
-{
- struct dirent *de;
-
- errno = 0;
- de = readdir(d);
- if (!de && errno != 0) {
- exit(1);
- }
- return de;
-}
+#include "proc.h"

static void f_reg(DIR *d, const char *filename)
{
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