Re: [Bugme-new] [Bug 14832] New: futimens (AT_FDCWD, arg) should fail

Re: [Bugme-new] [Bug 14832] New: futimens (AT_FDCWD, arg) should fail

[Andrew Morton]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Fri, 18 Dec 2009 16:08:29 GMT
bugzilla-daemon@bugzilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=14832
> 
>            Summary: futimens (AT_FDCWD, arg) should fail
>            Product: File System
>            Version: 2.5
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: Other
>         AssignedTo: fs_other@kernel-bugs.osdl.org
>         ReportedBy: ebb9@byu.net
>         Regression: No
> 
> 
> According to POSIX 2008, futimens() shall fail with EBADF if fd is not a valid
> file descriptor.  AT_FDCWD, being negative, falls into this category.  Yet this
> program mistakenly changes the timestamps of the current directory, then
> aborts:
> 
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <stdlib.h>
> int
> main ()
> {
>   if (futimens (AT_FDCWD, NULL) != -1 || errno != EBADF)
>     abort ();
> }
> 
> Since futimens is a library call on top of utimensat, the fix is to add a
> special case to the utimensat syscall that fails with EBADF if fd is negative
> when path is NULL.
> 

Confused.

: long do_utimes(int dfd, char __user *filename, struct timespec *times, int flags)
: {
: 	int error = -EINVAL;
: 
: 	if (times && (!nsec_valid(times[0].tv_nsec) ||
: 		      !nsec_valid(times[1].tv_nsec))) {
: 		goto out;
: 	}
: 
: 	if (flags & ~AT_SYMLINK_NOFOLLOW)
: 		goto out;
: 
: 	if (filename == NULL && dfd != AT_FDCWD) {
: 		struct file *file;
: 
: 		if (flags & AT_SYMLINK_NOFOLLOW)
: 			goto out;
: 
: 		file = fget(dfd);
: 		error = -EBADF;
: 		if (!file)
: 			goto out;
: 

afacit, if filename==NULL and dfd==-1 then fget() will return NULL and
the syscall returns -EBAFD.

Your report doesn't tell us what kernel version you're testing.  We did
fix a few things ni this area, but it was a long time ago.

Re: [Bugme-new] [Bug 14832] New: futimens (AT_FDCWD, arg) should fail

[Eric Blake]

According to Andrew Morton on 12/22/2009 3:31 PM:
> Confused.
> 
> : long do_utimes(int dfd, char __user *filename, struct timespec *times, int flags)
> : {
> : 	int error = -EINVAL;
> : 
> : 	if (times && (!nsec_valid(times[0].tv_nsec) ||
> : 		      !nsec_valid(times[1].tv_nsec))) {
> : 		goto out;
> : 	}
> : 
> : 	if (flags & ~AT_SYMLINK_NOFOLLOW)
> : 		goto out;
> : 
> : 	if (filename == NULL && dfd != AT_FDCWD) {
> : 		struct file *file;
> : 
> : 		if (flags & AT_SYMLINK_NOFOLLOW)
> : 			goto out;
> : 
> : 		file = fget(dfd);
> : 		error = -EBADF;
> : 		if (!file)
> : 			goto out;
> : 
> 
> afacit, if filename==NULL and dfd==-1 then fget() will return NULL and
> the syscall returns -EBAFD.

Yes, that's true if dfd==-1.  But My complaint was not about dfd==-1, but
about dfd==AT_FDCWD, in which case, this block of code is skipped, and you
end up operating on the current directory ".".  My point is that the line:
 if (filename == NULL && dfd != AT_FDCWD) {
should probably be:
 if (filename == NULL) {
assuming that fget(AT_FDCWD) likewise fails.

> 
> Your report doesn't tell us what kernel version you're testing.  We did
> fix a few things ni this area, but it was a long time ago.

Yesterday, Ulrich patched glibc to avoid the issue from the library side
of things (I first raised the report against glibc 2 months ago, along
with a proposed patch:
http://sources.redhat.com/bugzilla/show_bug.cgi?id=10992).  But there is
still the case of newer kernels and unpatched glibc, where it would also
be nice to patch the kernel.  I reproduced the bug on
2.6.31.6-166.fc12.i686, where this test (part of coreutils' configure
script) failed with status 2:

| #include <fcntl.h>
| #include <sys/stat.h>
| #include <unistd.h>
|
| int
| main ()
| {
| struct timespec ts[2] = { { 1, UTIME_OMIT }, { 1, UTIME_NOW } };
|       int fd = creat ("conftest.file", 0600);
|       struct stat st;
|       if (fd < 0) return 1;
|       if (futimens (AT_FDCWD, NULL)) return 2;
|       if (futimens (fd, ts)) return 3;
|       sleep (1);
|       ts[0].tv_nsec = UTIME_NOW;
|       ts[1].tv_nsec = UTIME_OMIT;
|       if (futimens (fd, ts)) return 4;
|       if (fstat (fd, &st)) return 5;
|       if (st.st_ctime < st.st_atime) return 6;
|
|   ;
|   return 0;
| }


-- 
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net

Re: [Bugme-new] [Bug 14832] New: futimens (AT_FDCWD, arg) should fail

[OGAWA Hirofumi]

Eric Blake <ebb9@byu.net> writes:

>> afacit, if filename==NULL and dfd==-1 then fget() will return NULL and
>> the syscall returns -EBAFD.
>
> Yes, that's true if dfd==-1.  But My complaint was not about dfd==-1, but
> about dfd==AT_FDCWD, in which case, this block of code is skipped, and you
> end up operating on the current directory ".".  My point is that the line:
>  if (filename == NULL && dfd != AT_FDCWD) {
> should probably be:
>  if (filename == NULL) {
> assuming that fget(AT_FDCWD) likewise fails.

No, it changes utimensat() behavior. If error was EBADF, it seems fd
shouldn't be AT_FDCWD. So, this behavior would be right thing as
utimensat().

[EBADF] The path argument does not specify an absolute path and the fd
    argument is neither AT_FDCWD nor a valid file descriptor open for
    reading or searching.

>> Your report doesn't tell us what kernel version you're testing.  We did
>> fix a few things ni this area, but it was a long time ago.
>
> Yesterday, Ulrich patched glibc to avoid the issue from the library side
> of things (I first raised the report against glibc 2 months ago, along
> with a proposed patch:
> http://sources.redhat.com/bugzilla/show_bug.cgi?id=10992).  But there is
> still the case of newer kernels and unpatched glibc, where it would also
> be nice to patch the kernel.  I reproduced the bug on
> 2.6.31.6-166.fc12.i686, where this test (part of coreutils' configure
> script) failed with status 2:

-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>