Returning a pidfd allows a process to keep a race-free handle for a
child process, otherwise, the caller will need to either use pidfd_open
(which still might be subject to TOCTOU) or keep the old racy interface
base on pid_t.
To correct use pifd_spawn, the kernel must support not only returning
the pidfd with clone/clone3 but also waitid (P_PIDFD) (added on Linux
5.4). If kernel does not support the waitid, pidfd return ENOSYS.
It avoids the need to racy workarounds, such as reading the procfs
fdinfo to get the pid to use along with other wait interfaces.
These interfaces are similar to the posix_spawn and posix_spawnp, with
the only difference being it returns a process file descriptor (int)
instead of a process ID (pid_t). Their prototypes are:
int pidfd_spawn (int *restrict pidfd,
const char *restrict file,
const posix_spawn_file_actions_t *restrict facts,
const posix_spawnattr_t *restrict attrp,
char *const argv[restrict],
char *const envp[restrict])
int pidfd_spawnp (int *restrict pidfd,
const char *restrict path,
const posix_spawn_file_actions_t *restrict facts,
const posix_spawnattr_t *restrict attrp,
char *const argv[restrict_arr],
char *const envp[restrict_arr]);
A new symbol is used instead of a posix_spawn extension to avoid
possible issues with language bindings that might track the return
argument lifetime. Although on Linux pid_t and int are interchangeable,
POSIX only states that pid_t should be a signed integer.
Both symbols reuse the posix_spawn posix_spawn_file_actions_t and
posix_spawnattr_t, to void rehash posix_spawn API or add a new one. It
also means that both interfaces support the same attribute and file
actions, and a new flag or file action on posix_spawn is also added
automatically for pidfd_spawn.
Also, using posix_spawn plumbing allows the reusing of most of the
current testing with some changes:
- waitid is used instead of waitpid since it is a more generic
interface.
- tst-posix_spawn-setsid.c is adapted to take into consideration that
the caller can check for session id directly. The test now spawns
itself and writes the session id as a file instead.
- tst-spawn3.c need to know where pidfd_spawn is used so it keeps an
extra file description unused.
Checked on x86_64-linux-gnu on Linux 4.15 (no CLONE_PIDFD or waitid
support), Linux 5.4 (full support), and Linux 6.2.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
I used these shell commands:
../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")
and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 7061 files FOO.
I then removed trailing white space from math/tgmath.h,
support/tst-support-open-dev-null-range.c, and
sysdeps/x86_64/multiarch/strlen-vec.S, to work around the following
obscure pre-commit check failure diagnostics from Savannah. I don't
know why I run into these diagnostics whereas others evidently do not.
remote: *** 912-#endif
remote: *** 913:
remote: *** 914-
remote: *** error: lines with trailing whitespace found
...
remote: *** error: sysdeps/unix/sysv/linux/statx_cp.c: trailing lines
I used these shell commands:
../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")
and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 6694 files FOO.
I then removed trailing white space from benchtests/bench-pthread-locks.c
and iconvdata/tst-iconv-big5-hkscs-to-2ucs4.c, to work around this
diagnostic from Savannah:
remote: *** pre-commit check failed ...
remote: *** error: lines with trailing whitespace found
remote: error: hook declined to update refs/heads/master
As noted in bug 13888, and as I noted previously in
<https://sourceware.org/ml/libc-alpha/2000-10/msg00111.html>, various
tests used hardcoded paths in /tmp, so posing issues for simultaneous
test runs from different build directories.
This patch fixes such uses of hardcoded file names to put them in the
build directory instead (in the case of stdio-common/bug5 the file
names are changed as well, to avoid a conflict with the name bug5.out
also used for the automatic test output redirection). It also fixes
test-installation.pl likewise (that was using filenames with $$ in
them rather than strictly hardcoded names, but that's still not good
practice for temporary file naming).
Note that my list of files changed is not identical to that in bug
13888. I added tst-spawn3.c and test-installation.pl, and removed
some tests that seem to me (now) to create temporary files securely
(simply using /tmp is not itself a problem if the temporary files are
handled properly with mkstemp; I haven't checked whether those tests
used to do things insecurely). conformtest is not changed because the
makefiles always pass a --tmpdir option so the /tmp default is
irrelevant, and for the same reason there is no actual problem with
nptl/tst-umask1.c because again the makefiles always override the
default.
nptl/sockperf.c is ignored because there is no code to run it;
probably that file should actually be removed.
Some tests use the mktemp function, but I think they all use it in a
way that *is* secure (for generating names for directories / sockets /
fifos / symlinks, where the operation using the name will not follow
symlinks and so there is no potential for a symlink attack on the
account running the testsuite).
Some tests use the tmpnam function to generate temporary file names.
This is in principle insecure, but not addressed by this patch (I
consider it a separate issue from the fully hardcoded paths).
Tested for x86_64.
[BZ #13888]
* posix/Makefile (CFLAGS-tst-spawn3.c): New variable.
* posix/tst-spawn3.c (do_test): Put tst-spwan3.pid in OBJPFX, not
/tmp.
* scripts/test-installation.pl: Put temporary files in build
directory, not /tmp.
* stdio-common/Makefile (CFLAGS-bug3.c): New variable.
(CFLAGS-bug4.c): Likewise.
(CFLAGS-bug5.c): Likewise.
(CFLAGS-test-fseek.c): Likewise.
(CFLAGS-test-popen.c): Likewise.
(CFLAGS-test_rdwr.c): Likewise.
* stdio-common/bug3.c (main): Put temporary file in OBJPFX, not
/tmp.
* stdio-common/bug4.c (main): Likewise.
* stdio-common/bug5.c (main): Likewise.
* stdio-common/test-fseek.c (TESTFILE): Likewise.
* stdio-common/test-popen.c (do_test): Likewise.
* stdio-common/test_rdwr.c (main): Likewise.
On posix_spawn open file action (issued by posix_spawn_file_actions_addopen)
POSIX states that if fildes was already an open file descriptor, it shall be
closed before the new file is openedi [1]. This avoid pontential issues when
posix_spawn plus addopen action is called with the process already at maximum
number of file descriptor opened and also for multiple actions on single-open
special paths (like /dev/watchdog).
This fixes its behavior on Linux posix_spawn implementation and also adds
a tests to check for its behavior.
Checked on x86_64.
* posix/Makefile (tests): Add tst-spawn3.
* posix/tst-spawn3.c: New file.
* sysdeps/unix/sysv/linux/spawni.c (__spawni_child): Close file descriptor
if it is already opened for open action.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn_file_actions_addclose.html
