libmount: mnt_resolve_target: tiptoe around active mount points

Current code in mnt_fs_match_target() and mnt_table_find_target()
already does not canonicalize active mount points (when read from
mountinfo), because they are already canonicalized by the kernel.
Calling realpath(fs->target) on a mount point can hang -- e.g. if the
NFS server is unreachable.

This patch optionally extends this strategy to the general case, that is
when @fs does not directly come from the kernel through mountinfo (for
instance, it may have been parsed from /etc/fstab).

Given @mtab parsed from mountinfo, and if mnt_cache_set_targets(cache,
mtab) is used, then mnt_fs_match_target() and mnt_table_find_target()
check whether @fs->target is a known mount point in the cached
mountinfo, before attempting to canonicalize @fs->target, no matter
where @fs itself comes from. If found in the cached mountinfo,
@fs->target is not canonicalized.

[kzak@redhat.com: - don't allocate libmnt_iter,
                  - add docs for mnt_cache_set_targets(),
                  - fallback to mnt_resolve_path() if no cache->mtab specified,
                  - use streq_except_trailing_slash() to compare paths]

Signed-off-by: Eric Rannaud <e@nanocritical.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/libmount/src/fs.c b/libmount/src/fs.c
index 21ef0f747..82541083a 100644
--- a/libmount/src/fs.c
+++ b/libmount/src/fs.c
@@ -1413,7 +1413,9 @@ int mnt_fs_append_comment(struct libmnt_fs *fs, const char *comm)
  *	1) compare @target with @fs->target
  *	2) realpath(@target) with @fs->target
  *	3) realpath(@target) with realpath(@fs->target) if @fs is not from
- *	   /proc/self/mountinfo.
+ *	   /proc/self/mountinfo. However, if mnt_cache_set_targets(cache,
+ *	   mtab) was called, and the path @fs->target is found in @mtab,
+ *	   this comparison is not performed (see mnt_resolve_target()).
  *
  * The 2nd and 3rd attempts are not performed when @cache is NULL.
  *
@@ -1438,7 +1440,7 @@ int mnt_fs_match_target(struct libmnt_fs *fs, const char *target,
 
 		/* 3) - canonicalized and canonicalized */
 		if (!rc && cn && !mnt_fs_is_kernel(fs) && !mnt_fs_is_swaparea(fs)) {
-			char *tcn = mnt_resolve_path(fs->target, cache);
+			char *tcn = mnt_resolve_target(fs->target, cache);
 			rc = (tcn && strcmp(cn, tcn) == 0);
 		}
 	}