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bpo-32777: Fix _Py_set_inheritable async-safety in subprocess (GH-5560) 

Fix a rare but potential pre-exec child process deadlock in subprocess on POSIX systems when marking file descriptors inheritable on exec in the child process.  This bug appears to have been introduced in 3.4 with the inheritable file descriptors support.

This also changes Python/fileutils.c `set_inheritable` to use the "slow" two `fcntl` syscall path instead of the "fast" single `ioctl` syscall path when asked to be async signal safe (by way of being asked not to raise exceptions).  `ioctl` is not a POSIX async-signal-safe approved function.

ref: http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html
This commit is contained in:
Alexey Izbyshev 2018-02-06 09:09:34 +03:00 committed by Gregory P. Smith
parent 22864bc8e4
commit c1e46e94de
4 changed files with 26 additions and 7 deletions
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3
Include/fileutils.h
View File

@ -152,6 +152,9 @@ PyAPI_FUNC(int) _Py_get_inheritable(int fd);
PyAPI_FUNC(int) _Py_set_inheritable(int fd, int inheritable,
int *atomic_flag_works);
PyAPI_FUNC(int) _Py_set_inheritable_async_safe(int fd, int inheritable,
int *atomic_flag_works);
PyAPI_FUNC(int) _Py_dup(int fd);
#ifndef MS_WINDOWS

3
Misc/NEWS.d/next/Library/2018-02-05-21-28-28.bpo-32777.C-wIXF.rst Normal file
View File

@ -0,0 +1,3 @@
Fix a rare but potential pre-exec child process deadlock in subprocess on
POSIX systems when marking file descriptors inheritable on exec in the child
process. This bug appears to have been introduced in 3.4.

8
Modules/_posixsubprocess.c
View File

@ -169,7 +169,7 @@ make_inheritable(PyObject *py_fds_to_keep, int errpipe_write)
called. */
continue;
}
if (_Py_set_inheritable((int)fd, 1, NULL) < 0)
if (_Py_set_inheritable_async_safe((int)fd, 1, NULL) < 0)
return -1;
}
return 0;
@ -431,21 +431,21 @@ child_exec(char *const exec_array[],
dup2() removes the CLOEXEC flag but we must do it ourselves if dup2()
would be a no-op (issue #10806). */
if (p2cread == 0) {
if (_Py_set_inheritable(p2cread, 1, NULL) < 0)
if (_Py_set_inheritable_async_safe(p2cread, 1, NULL) < 0)
goto error;
}
else if (p2cread != -1)
POSIX_CALL(dup2(p2cread, 0)); /* stdin */
if (c2pwrite == 1) {
if (_Py_set_inheritable(c2pwrite, 1, NULL) < 0)
if (_Py_set_inheritable_async_safe(c2pwrite, 1, NULL) < 0)
goto error;
}
else if (c2pwrite != -1)
POSIX_CALL(dup2(c2pwrite, 1)); /* stdout */
if (errwrite == 2) {
if (_Py_set_inheritable(errwrite, 1, NULL) < 0)
if (_Py_set_inheritable_async_safe(errwrite, 1, NULL) < 0)
goto error;
}
else if (errwrite != -1)

19
Python/fileutils.c
View File

@ -913,6 +913,7 @@ _Py_stat(PyObject *path, struct stat *statbuf)
}
/* This function MUST be kept async-signal-safe on POSIX when raise=0. */
static int
get_inheritable(int fd, int raise)
{
@ -958,6 +959,8 @@ _Py_get_inheritable(int fd)
return get_inheritable(fd, 1);
}
/* This function MUST be kept async-signal-safe on POSIX when raise=0. */
static int
set_inheritable(int fd, int inheritable, int raise, int *atomic_flag_works)
{
@ -1014,8 +1017,10 @@ set_inheritable(int fd, int inheritable, int raise, int *atomic_flag_works)
#else
#if defined(HAVE_SYS_IOCTL_H) && defined(FIOCLEX) && defined(FIONCLEX)
if (ioctl_works != 0) {
if (ioctl_works != 0 && raise != 0) {
/* fast-path: ioctl() only requires one syscall */
/* caveat: raise=0 is an indicator that we must be async-signal-safe
* thus avoid using ioctl() so we skip the fast-path. */
if (inheritable)
request = FIONCLEX;
else
@ -1086,8 +1091,7 @@ make_non_inheritable(int fd)
}
/* Set the inheritable flag of the specified file descriptor.
On success: return 0, on error: raise an exception if raise is nonzero
and return -1.
On success: return 0, on error: raise an exception and return -1.
If atomic_flag_works is not NULL:
@ -1108,6 +1112,15 @@ _Py_set_inheritable(int fd, int inheritable, int *atomic_flag_works)
return set_inheritable(fd, inheritable, 1, atomic_flag_works);
}
/* Same as _Py_set_inheritable() but on error, set errno and
don't raise an exception.
This function is async-signal-safe. */
int
_Py_set_inheritable_async_safe(int fd, int inheritable, int *atomic_flag_works)
{
return set_inheritable(fd, inheritable, 0, atomic_flag_works);
}
static int
_Py_open_impl(const char *pathname, int flags, int gil_held)
{