On Tue, Oct 31, 2017 at 02:51:56PM -0700, Luis R. Rodriguez wrote:
> Linux filesystems cannot set extra file attributes (stx_attributes as per
> statx(2)) on a symbolic link as ioctl(2) with FS_IOC_SETFLAGS is used for
> this purpose, and *all* ioctl(2) calls on a symbolic link yield EBADF.
> This is because ioctl(2) tries to obtain struct fd from the symbolic link
> file descriptor passed using fdget(), fdget() in turn always returns no
> file set when a file descriptor is open with O_PATH. As per symlink(2)
> O_PATH and O_NOFOLLOW must *always* be used when you want to get the file
> descriptor of a symbolic link, and this holds true for Linux, as such extra
> file attributes cannot possibly be set on symbolic links on Linux.
>
> Given this Linux filesystems should treat extra file attributes set on
> symbolic links as corruption and fix them.
>
> The TL;DR:
>
> How I discovered this was finding a corrupted filesystem with symbolic
> links with the extra file attribute append (STATX_ATTR_APPEND) set. Symbolic
> links with the attribute append set cannot be removed as they are treated as
> if a file was set with the immutable flag set. Standard tools however cannot
> remove *any* attribute flag:
>
> # chattr -a symlink
> chattr: Operation not supported while reading flags on b
>
> If you end up with these symbolic links userspace cannot remove them.
>
> Likewise one cannot use the same tool to *set* this extra file attribute on
> a symbolic link using chattr:
> # rm -f y z
> # ln -s y z
> # chattr +a z
> chattr: Operation not supported while reading flags on z
>
> What makes this puzzling was one cannot even list attributes on symlinks
> using lsattr either:
>
> # rm -f a b
> # ln -s a b
> # lsattr b
> lsattr: Operation not supported While reading flags on b
>
> The above was due to commit 023d111e92195 ("chattr.1.in: Document the
> compression attribute flags E, X, and ...") merged on e2fsprogs v1.28 since
> August 2002. That commit just refers to the fact that attributes were only
> allowed after that commit for directories and regular files due to Debian
> bug 152029 [0]. This bug was filed since lsattr lsattr would segfault when
> used against a special file of an old DRM buggy driver, the ioctl seem to
> have worked but crashed lsattr with its information. The bug report doesn't
> list any specific reasoning for not allowing attributes on symlinks though.
>
> Crafting your own tool to query the extra file attributes with the new
> shiny statx(2) works, and if a symbolic link has the extra attribute
> flag set statx(2) would inform userspace of this. statx(2) is only used
> for getting file information, not for setting them.
>
> This all meant that if you end up with the append extra file attribute
> set on a symlink you need special tools to try to remove it and currently
> that's only possible on XFS with xfs_db [1] [2].
>
> Fix XFS filesystems which have these extra file attributes set as the only
> way they could have been set was through corruption.
>
> [0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=152029
> [1] xfs_db -x -c 'inode inode-number' -c 'write core.append 1' /dev/device
> [2] xfs_db -x -c 'inode inode-number' -c 'write core.append 0' /dev/device
>
> Cc: Tso Ted <tytso@xxxxxxx>
> Cc: Nikolay Borisov <nborisov@xxxxxxxx>
> Cc: Flex Liu <fliu@xxxxxxxx>
> Cc: Jake Norris <jake.norris@xxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> ---
>
> On this v2 I've provided a much better explanation as to why these
> extra file attributes don't make sense on Linux, and trimmed the flags
> we venture out to clear to *only* match what statx defines. It may be
> possible to clear more dino->di_flags and maybe even dino->di_flags2
> for symbolic links however that those be determined separately as the
> other flags' semantics are clarified for setting.
>
> repair/dinode.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/repair/dinode.c b/repair/dinode.c
> index 15ba8cc22b39..6288e42de15e 100644
> --- a/repair/dinode.c
> +++ b/repair/dinode.c
> @@ -2482,6 +2482,27 @@ _("bad (negative) size %" PRId64 " on inode %" PRIu64 "\n"),
> FS_XFLAG_EXTSIZE);
> }
> }
> + if (flags & (XFS_DIFLAG_IMMUTABLE | XFS_DIFLAG_APPEND |
> + XFS_DIFLAG_NODUMP)) {
> + /*
> + * ioctl(fd, *) and so ioctl(fd, FS_IOC_SETFLAGS)
> + * yields EBADF on symlinks as they have O_PATH set.
> + * "Extra file attributes", stx_attributes, as per
> + * statx(2) cannot be set on symlinks on Linux.
> + */
> + if (di_mode && S_ISLNK(di_mode) &&
> + !S_ISREG(di_mode) && !S_ISDIR(di_mode)) {
I don't think we can be a link and a file at the same time, right?
Does this DIFLAG clearing applies to bdev/cdev/fifo/socket files too?
--D
> + if (!uncertain) {
> + do_warn(
> + _("file or directory attributes set on symlink inode %" PRIu64 "\n"),
> + lino);
> + }
> + flags &= ~(XFS_DIFLAG_IMMUTABLE |
> + XFS_DIFLAG_APPEND |
> + XFS_DIFLAG_NODUMP);
> + }
> + }
> +
> if (!verify_mode && flags != be16_to_cpu(dino->di_flags)) {
> if (!no_modify) {
> do_warn(_("fixing bad flags.\n"));
> --
> 2.14.2
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
