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journal: guarantee async-signal-safety in sd_journald_sendv

signal(7) provides a list of functions which may be called from a
signal handler. Other functions, which only call those functions and
don't access global memory and are reentrant are also safe.
sd_j_sendv was mostly OK, but would call mkostemp and writev in a
fallback path, which are unsafe.

Being able to call sd_j_sendv in a async-signal-safe way is important
because it allows it be used in signal handlers.

Safety is achieved by replacing mkostemp with open(O_TMPFILE) and an
open-coded writev replacement which uses write. Unfortunately,
O_TMPFILE is only available on kernels >= 3.11. When O_TMPFILE is
unavailable, an open-coded mkostemp is used.

https://bugzilla.gnome.org/show_bug.cgi?id=722889
keep-around/ba91431154ad7bac82ddf0a540ec1b40db62d782
Zbigniew Jędrzejewski-Szmek 9 years ago
parent
commit
65b3903ff5
  1. 3
      .gitignore
  2. 7
      Makefile.am
  3. 31
      man/sd_journal_print.xml
  4. 2
      src/journal/journal-send.c
  5. 1
      src/shared/def.h
  6. 16
      src/shared/missing.h
  7. 54
      src/shared/util.c
  8. 3
      src/shared/util.h
  9. 51
      src/test/test-tmpfiles.c
  10. 26
      src/test/test-util.c

3
.gitignore

@ -33,7 +33,7 @@
/hostnamectl
/install-tree
/journalctl
/libsystemd-login.c
/libsystemd-*.c
/libtool
/localectl
/loginctl
@ -184,6 +184,7 @@
/test-strxcpyx
/test-tables
/test-time
/test-tmpfiles
/test-udev
/test-unit-file
/test-unit-name

7
Makefile.am

@ -1126,6 +1126,7 @@ tests += \
test-utf8 \
test-ellipsize \
test-util \
test-tmpfiles \
test-namespace \
test-date \
test-sleep \
@ -1232,6 +1233,12 @@ test_util_SOURCES = \
test_util_LDADD = \
libsystemd-core.la
test_tmpfiles_SOURCES = \
src/test/test-tmpfiles.c
test_tmpfiles_LDADD = \
libsystemd-shared.la
test_namespace_SOURCES = \
src/test/test-namespace.c

31
man/sd_journal_print.xml

@ -153,8 +153,8 @@
for details) instead of the format string. Each
structure should reference one field of the entry to
submit. The second argument specifies the number of
structures in the
array. <function>sd_journal_sendv()</function> is
structures in the array.
<function>sd_journal_sendv()</function> is
particularly useful to submit binary objects to the
journal where that is necessary.</para>
@ -220,6 +220,19 @@ sd_journal_send("MESSAGE=Hello World, this is PID %lu!", (unsigned long) getpid(
variable itself is not altered.</para>
</refsect1>
<refsect1>
<title>Async signal safety</title>
<para><function>sd_journal_sendv()</function> is "async signal
safe" in the meaning of <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para>
<para><function>sd_journal_print</function>,
<function>sd_journal_printv</function>,
<function>sd_journal_send</function>, and
<function>sd_journal_perror</function> are
not async signal safe.</para>
</refsect1>
<refsect1>
<title>Notes</title>
@ -233,6 +246,16 @@ sd_journal_send("MESSAGE=Hello World, this is PID %lu!", (unsigned long) getpid(
file.</para>
</refsect1>
<refsect1>
<title>History</title>
<para><function>sd_journal_sendv()</function> was
modified to guarantee async-signal-safety in
systemd-209. Before that, it would behave safely only
when entry size was small enough to fit in one (large)
datagram.</para>
</refsect1>
<refsect1>
<title>See Also</title>
@ -243,7 +266,9 @@ sd_journal_send("MESSAGE=Hello World, this is PID %lu!", (unsigned long) getpid(
<citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>perror</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>

2
src/journal/journal-send.c

@ -314,7 +314,7 @@ _public_ int sd_journal_sendv(const struct iovec *iov, int n) {
if (buffer_fd < 0)
return buffer_fd;
n = writev(buffer_fd, w, j);
n = writev_safe(buffer_fd, w, j);
if (n < 0) {
close_nointr_nofail(buffer_fd);
return -errno;

1
src/shared/def.h

@ -44,6 +44,7 @@
#define LOWERCASE_LETTERS "abcdefghijklmnopqrstuvwxyz"
#define UPPERCASE_LETTERS "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define LETTERS LOWERCASE_LETTERS UPPERCASE_LETTERS
#define ALPHANUMERICAL LETTERS DIGITS
#define REBOOT_PARAM_FILE "/run/systemd/reboot-param"

16
src/shared/missing.h

@ -301,25 +301,29 @@ static inline int name_to_handle_at(int fd, const char *name, struct file_handle
#endif
#ifndef CIFS_MAGIC_NUMBER
#define CIFS_MAGIC_NUMBER 0xFF534D42
# define CIFS_MAGIC_NUMBER 0xFF534D42
#endif
#ifndef TFD_TIMER_CANCEL_ON_SET
#define TFD_TIMER_CANCEL_ON_SET (1 << 1)
# define TFD_TIMER_CANCEL_ON_SET (1 << 1)
#endif
#ifndef SO_REUSEPORT
#define SO_REUSEPORT 15
# define SO_REUSEPORT 15
#endif
#ifndef EVIOCREVOKE
#define EVIOCREVOKE _IOW('E', 0x91, int)
# define EVIOCREVOKE _IOW('E', 0x91, int)
#endif
#ifndef DRM_IOCTL_SET_MASTER
#define DRM_IOCTL_SET_MASTER _IO('d', 0x1e)
# define DRM_IOCTL_SET_MASTER _IO('d', 0x1e)
#endif
#ifndef DRM_IOCTL_DROP_MASTER
#define DRM_IOCTL_DROP_MASTER _IO('d', 0x1f)
# define DRM_IOCTL_DROP_MASTER _IO('d', 0x1f)
#endif
#ifndef TMP_MAX
# define TMP_MAX 238328
#endif

54
src/shared/util.c

@ -6109,6 +6109,53 @@ int getpeersec(int fd, char **ret) {
return 0;
}
int writev_safe(int fd, const struct iovec *w, int j) {
for (int i = 0; i < j; i++) {
size_t written = 0;
while (written < w[i].iov_len) {
ssize_t r;
r = write(fd, (char*) w[i].iov_base + written, w[i].iov_len - written);
if (r < 0 && errno != -EINTR)
return -errno;
written += r;
}
}
return 0;
}
int mkostemp_safe(char *pattern, int flags) {
char *s = pattern + strlen(pattern) - 6;
uint64_t tries = TMP_MAX;
int randfd, fd, i;
assert(streq(s, "XXXXXX"));
randfd = open("/dev/urandom", O_RDONLY);
if (randfd < 0)
return -ENOSYS;
while (tries--) {
fd = read(randfd, s, 6);
if (fd == 0)
return -ENOSYS;
for (i = 0; i < 6; i++)
s[i] = ALPHANUMERICAL[(unsigned) s[i] % strlen(ALPHANUMERICAL)];
fd = open(pattern, flags|O_EXCL|O_CREAT, S_IRUSR|S_IWUSR);
if (fd >= 0)
return fd;
if (!IN_SET(errno, EEXIST, EINTR))
return -errno;
}
return -EEXIST;
}
int open_tmpfile(const char *path, int flags) {
int fd;
char *p;
@ -6120,12 +6167,9 @@ int open_tmpfile(const char *path, int flags) {
#endif
p = strappenda(path, "/systemd-tmp-XXXXXX");
RUN_WITH_UMASK(0077) {
fd = mkostemp(p, O_RDWR|O_CLOEXEC);
}
fd = mkostemp_safe(p, O_RDWR|O_CLOEXEC);
if (fd < 0)
return -errno;
return fd;
unlink(p);
return fd;

3
src/shared/util.h

@ -850,4 +850,7 @@ bool pid_valid(pid_t pid);
int getpeercred(int fd, struct ucred *ucred);
int getpeersec(int fd, char **ret);
int writev_safe(int fd, const struct iovec *w, int j);
int mkostemp_safe(char *pattern, int flags);
int open_tmpfile(const char *path, int flags);

51
src/test/test-tmpfiles.c

@ -0,0 +1,51 @@
/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
/***
This file is part of systemd.
Copyright 2014 Zbigniew Jędrzejewski-Szmek
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include "util.h"
int main(int argc, char** argv) {
const char *p = argv[1] ?: "/tmp";
char *pattern = strappenda(p, "/systemd-test-XXXXXX");
_cleanup_close_ int fd, fd2;
_cleanup_free_ char *cmd, *cmd2;
fd = open_tmpfile(p, O_RDWR);
assert(fd >= 0);
assert_se(asprintf(&cmd, "ls -l /proc/"PID_FMT"/fd/%d", getpid(), fd) > 0);
system(cmd);
fd2 = mkostemp_safe(pattern, O_RDWR);
assert(fd >= 0);
assert_se(unlink(pattern) == 0);
assert_se(asprintf(&cmd2, "ls -l /proc/"PID_FMT"/fd/%d", getpid(), fd2) > 0);
system(cmd2);
return 0;
}

26
src/test/test-util.c

@ -28,6 +28,8 @@
#include "util.h"
#include "strv.h"
#include "def.h"
#include "fileio.h"
static void test_streq_ptr(void) {
assert_se(streq_ptr(NULL, NULL));
@ -578,6 +580,29 @@ static void test_in_set(void) {
assert_se(!IN_SET(0, 1, 2, 3, 4));
}
static void test_writev_safe(void) {
char name[] = "/tmp/test-writev_safe.XXXXXX";
_cleanup_free_ char *contents;
size_t size;
int fd, r;
struct iovec iov[3];
IOVEC_SET_STRING(iov[0], "abc\n");
IOVEC_SET_STRING(iov[1], ALPHANUMERICAL "\n");
IOVEC_SET_STRING(iov[2], "");
fd = mkstemp(name);
printf("test_writev_safe: %s", name);
r = writev_safe(fd, iov, 3);
assert(r == 0);
r = read_full_file(name, &contents, &size);
assert(r == 0);
printf("contents: %s", contents);
assert(streq(contents, "abc\n" ALPHANUMERICAL "\n"));
}
int main(int argc, char *argv[]) {
test_streq_ptr();
test_first_word();
@ -615,6 +640,7 @@ int main(int argc, char *argv[]) {
test_fstab_node_to_udev_node();
test_get_files_in_directory();
test_in_set();
test_writev_safe();
return 0;
}

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