umount: never 'stat' the path when "-c" is given.

It is currently not possible to reliably and automatically
unmount an NFS filesystem. If the server is not available, the
umount command will hang.

The hang can be avoided by using "-l" or "-f", but neither
of these are appropriate for automatic use such as by an
automounter (e.g automountd or systemd).

"-l" will unmount even if the filesystem is in use, which
an automounter generally doesn't want.  If the filesystem
is in use, then the umount should fail.

"-f" can cause the filesystem to abort pending transactions
which might break filesystem semantics.  This can be useful
in the hands of a sysadmin, but not when used by an
automatic tool.

umount has another option, "-c" aka "--no-canonicalize"
which avoids some "stat" calls.
Currently this doesn't avoid all calls to
canonicalize_path()
as
  mnt_context_prepare_umount() ->
    lookup_umount_fs() ->
      mnt_context_find_umount_fs() ->
        mnt_context_get_mtab_for_target() ->
	  mnt_resolve_path() ->
	    canonicalize_path_and_cache() ->
	      canonicalize_path()

leads to that function being called.

The "-c" option could be taken to mean "I know what I'm
doing, this really is the path to a mount point, I just want
you to unmount it".  Given that, it seems suitable to
extend this to avoid all 'stat' calls on the mountpoint.

It is already appropriate for any automount program to pass
"-c" to "umount", so they can be changed to do so at any
time.
With the patch below, "-c" will result in the mountpoint
never being "stat"ed, so umount won't hang on an
inaccessible server.

This isn't quite sufficient, for NFS at least, as the usage
of libmount in umount.nfs still calls 'stat' on the mount
point.
"-c" isn't passed to the umount helper, but it is reasonable
for such helpers to assume "-c" because "umount" will have
canonicalized the path when that is appropriate.

So, this patch treats "-c" much like "-l" and "-f" when
deciding whether it is safe to 'stat' the path.

Signed-off-by: NeilBrown <neilb@suse.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/libmount/src/context_umount.c b/libmount/src/context_umount.c
index e663a703c..693891def 100644
--- a/libmount/src/context_umount.c
+++ b/libmount/src/context_umount.c
@@ -77,6 +77,7 @@ int mnt_context_find_umount_fs(struct libmnt_context *cxt,
 	 * it's usable only for canonicalized stuff (e.g. kernel mountinfo).
 	 */
 	if (!mnt_context_mtab_writable(cxt) && *tgt == '/' &&
+	    !mnt_context_is_nocanonicalize(cxt) &&
 	    !mnt_context_is_force(cxt) && !mnt_context_is_lazy(cxt))
 		rc = mnt_context_get_mtab_for_target(cxt, &mtab, tgt);
 	else
@@ -245,6 +246,7 @@ static int lookup_umount_fs(struct libmnt_context *cxt)
 	    && !mnt_context_mtab_writable(cxt)
 	    && !mnt_context_is_force(cxt)
 	    && !mnt_context_is_lazy(cxt)
+	    && !mnt_context_is_nocanonicalize(cxt)
 	    && !mnt_context_is_loopdel(cxt)
 	    && mnt_stat_mountpoint(tgt, &st) == 0 && S_ISDIR(st.st_mode)
 	    && !has_utab_entry(cxt, tgt)) {