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When reopening a stream backed by an open file descriptor, do not close

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the existing file descriptor.  Instead, let dup2() atomically close the
old file descriptor when assigning the newly opened file to the same
descriptor.  This closes a race in a multithreaded application where a
concurrent open() could allocate the existing file descriptor in between
the calls to close() and dup2().

PR:		threads/79887
Submitted by:	Dmitrij Tejblum  tejblum of yandex-team.ru
Reviewed by:	davidxu
MFC after:	1 week
This commit is contained in:
John Baldwin 2010-12-09 20:28:30 +00:00
parent d1cf854b5d
commit cc3d85727d
1 changed files with 9 additions and 12 deletions
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21
lib/libc/stdio/freopen.c
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@ -150,14 +150,6 @@ freopen(file, mode, fp)
/* Get a new descriptor to refer to the new file. */
f = _open(file, oflags, DEFFILEMODE);
if (f < 0 && isopen) {
/* If out of fd's close the old one and try again. */
if (errno == ENFILE || errno == EMFILE) {
(void) (*fp->_close)(fp->_cookie);
isopen = 0;
f = _open(file, oflags, DEFFILEMODE);
}
}
sverrno = errno;
finish:
@ -165,9 +157,11 @@ finish:
* Finish closing fp. Even if the open succeeded above, we cannot
* keep fp->_base: it may be the wrong size. This loses the effect
* of any setbuffer calls, but stdio has always done this before.
*
* Leave the existing file descriptor open until dup2() is called
* below to avoid races where a concurrent open() in another thread
* could claim the existing descriptor.
*/
if (isopen)
(void) (*fp->_close)(fp->_cookie);
if (fp->_flags & __SMBF)
free((char *)fp->_bf._base);
fp->_w = 0;
@ -186,6 +180,8 @@ finish:
memset(&fp->_mbstate, 0, sizeof(mbstate_t));
if (f < 0) { /* did not get it after all */
if (isopen)
(void) (*fp->_close)(fp->_cookie);
fp->_flags = 0; /* set it free */
FUNLOCKFILE(fp);
errno = sverrno; /* restore in case _close clobbered */
@ -197,11 +193,12 @@ finish:
* to maintain the descriptor. Various C library routines (perror)
* assume stderr is always fd STDERR_FILENO, even if being freopen'd.
*/
if (wantfd >= 0 && f != wantfd) {
if (wantfd >= 0) {
if (_dup2(f, wantfd) >= 0) {
(void)_close(f);
f = wantfd;
}
} else
(void)_close(fp->_file);
}
/*