> The POSIX.1 draft spec for futimens()/utimensat() says:
>
> Only a process with the effective user ID equal to the
> user ID of the file, *or with write access to the file*,
> or with appropriate privileges may use futimens() or
> utimensat() with a null pointer as the times argument
> or with both tv_nsec fields set to the special value
> UTIME_NOW.
>
> The important piece here is "with write access to the file", and
> this matters for futimens(), which deals with an argument that
> is a file descriptor referring to the file whose timestamps are
> being updated, The standard is saying that the "writability"
> check is based on the file permissions, not the access mode with
> which the file is opened. (This behavior is consistent with the
> semantics of FreeBSD's futimes().) However, Linux is currently
> doing the latter -- futimens(fd, times) is a library
> function implemented as
>
> utimensat(fd, NULL, times, 0)
>
> and within the utimensat() implementation we have the code:
>
> f = fget(dfd); // dfd is 'fd'
> ...
> if (f) {
> if (!(f->f_mode & FMODE_WRITE))
> goto mnt_drop_write_and_out;
>
> The check should instead be based on the file permissions.
>
> Thanks to Miklos for pointing out how to do this check.
>
> CC: Miklos Szeredi <miklos@xxxxxxxxxx>
> CC: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> CC: Ulrich Drepper <drepper@xxxxxxxxxx>
> Signed-off-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
>
> --- linux-2.6.26-rc4/fs/utimes.c 2008-06-03 23:13:31.000000000 +0200
> +++ linux-2.6.26-rc4-utimensat-fix-v4/fs/utimes.c 2008-06-03 23:15:12.000000000 +0200
> @@ -137,7 +137,8 @@
>
> if (!is_owner_or_cap(inode)) {
> if (f) {
> - if (!(f->f_mode & FMODE_WRITE))
> + error = permission(inode, MAY_WRITE, NULL);
> + if (error)
> goto mnt_drop_write_and_out;
> } else {
> error = vfs_permission(&nd, MAY_WRITE);
At which point the "if (f)" and the "else" branches become equivalent
(the nameidata isn't interesting in the other case either). So that
could be written as:
if (!is_owner_or_cap(inode)) {
error = permission(inode, MAY_WRITE, NULL);
if (error)
goto mnt_drop_write_and_out;
}
Miklos
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
