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authorAdhemerval Zanella <adhemerval.zanella@linaro.org>2017-05-05 11:31:38 -0300
committerAdhemerval Zanella <adhemerval.zanella@linaro.org>2017-05-22 18:13:35 -0300
commitf1a67a2c78601599be51a17250ca02c7d830d79d (patch)
treeac463077dcbc47e15e3b5346c445d9ddd38fcf0d /libio/freopen64.c
parentd26db8fbb4787905590f207d56026e915b3bd5b3 (diff)
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libio: Avoid dup already opened file descriptor [BZ#21393]
As described in BZ#21398 (close as dup of 21393) report current freopen implementation fails when one tries to freopen STDIN_FILENO, STDOUT_FILENO, or STDERR_FILENO. Although on bug report the discussion leads to argue if a close followed by a freopen on the standard file is a valid operation, the underlying issue is not really the check for dup3 returned value, but rather calling it if the returned file descriptor is equal as the input one. So for a quality of implementation this patch avoid calling dup3 for the aforementioned case. It also adds a dup3 error case check for the two possible failures, with one being Linux only: EINTR and EBUSY. The EBUSY issue is better explained on this stackoverflow thread [1], but in a short it is due the internal Linux implementation which allows a race condition window for dup2 due the logic dissociation of file descriptor allocation and actual VFS 'install' operation. For both outliers failures all allocated memory is freed and a NULL FILE* is returned. With this patch the example on BZ#21398 is now actually possible (I used as the testcase for the bug report). Checked on x86_64-linux-gnu. [BZ #21393] * libio/freopen.c (freopen): Avoid dup already opened file descriptor and add a check for dup3 failure. * libio/freopen64.c (freopen64): Likewise. * libio/tst-freopen.c (do_test): Rename to do_test_basic and use libsupport. (do_test_bz21398): New test. * manual/stdio.texi (freopen): Add documentation of EBUSY failure. [1] http://stackoverflow.com/questions/23440216/race-condition-when-using-dup2
Diffstat (limited to 'libio/freopen64.c')
-rw-r--r--libio/freopen64.c22
1 files changed, 18 insertions, 4 deletions
diff --git a/libio/freopen64.c b/libio/freopen64.c
index adf749a070d..1e56de616c4 100644
--- a/libio/freopen64.c
+++ b/libio/freopen64.c
@@ -59,17 +59,31 @@ freopen64 (const char *filename, const char *mode, FILE *fp)
/* unbound stream orientation */
result->_mode = 0;
- if (fd != -1)
+ if (fd != -1 && _IO_fileno (result) != fd)
{
- __dup3 (_IO_fileno (result), fd,
- (result->_flags2 & _IO_FLAGS2_CLOEXEC) != 0
- ? O_CLOEXEC : 0);
+ /* At this point we have both file descriptors already allocated,
+ so __dup3 will not fail with EBADF, EINVAL, or EMFILE. But
+ we still need to check for EINVAL and, due Linux internal
+ implementation, EBUSY. It is because on how it internally opens
+ the file by splitting the buffer allocation operation and VFS
+ opening (a dup operation may run when a file is still pending
+ 'install' on VFS). */
+ if (__dup3 (_IO_fileno (result), fd,
+ (result->_flags2 & _IO_FLAGS2_CLOEXEC) != 0
+ ? O_CLOEXEC : 0) == -1)
+ {
+ _IO_file_close_it (result);
+ result = NULL;
+ goto end;
+ }
__close (_IO_fileno (result));
_IO_fileno (result) = fd;
}
}
else if (fd != -1)
__close (fd);
+
+end:
if (filename == NULL)
free ((char *) gfilename);
_IO_release_lock (fp);