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, 5 insertions, 3 deletions

diff --git a/libmount/src/tab.c b/libmount/src/tab.c
index 77260ab96..fd6a7d8e2 100644
--- a/libmount/src/tab.c
+++ b/libmount/src/tab.c
@@ -880,8 +880,10 @@ struct libmnt_fs *mnt_table_find_mountpoint(struct libmnt_table *tb,
  *
  * Try to lookup an entry in the given tab, three iterations are possible, the first
  * with @path, the second with realpath(@path) and the third with realpath(@path)
- * against realpath(fs->target). The 2nd and 3rd iterations are not performed
- * when the @tb cache is not set (see mnt_table_set_cache()).
+ * against realpath(fs->target). The 2nd and 3rd iterations are not performed when
+ * the @tb cache is not set (see mnt_table_set_cache()). If
+ * mnt_cache_set_targets(cache, mtab) was called, the 3rd iteration skips any
+ * @fs->target found in @mtab (see mnt_resolve_target()).
  *
  * Returns: a tab entry or NULL.
  */
@@ -933,7 +935,7 @@ struct libmnt_fs *mnt_table_find_target(struct libmnt_table *tb, const char *pat
 		    || (*fs->target == '/' && *(fs->target + 1) == '\0'))
 		       continue;
 
-		p = mnt_resolve_path(fs->target, tb->cache);
+		p = mnt_resolve_target(fs->target, tb->cache);
 		/* both canonicalized, strcmp() is fine here */
 		if (p && strcmp(cn, p) == 0)
 			return fs;