vfs: Don't exchange "short" filenames unconditionally.

Only exchange source and destination filenames
if flags contain RENAME_EXCHANGE.
In case if executable file was running and replaced by
other file /proc/PID/exe should still show correct file name,
not the old name of the file by which it was replaced.

The scenario when this bug manifests itself was like this:
* ALT Linux uses rpm and start-stop-daemon;
* during a package upgrade rpm creates a temporary file
for an executable to rename it upon successful unpacking;
* start-stop-daemon is run subsequently and it obtains
the (nonexistant) temporary filename via /proc/PID/exe
thus failing to identify the running process.

Note that "long" filenames (> DNAiME_INLINE_LEN) are still
exchanged without RENAME_EXCHANGE and this behaviour exists
long enough (should be fixed too apparently).
So this patch is just an interim workaround that restores
behavior for "short" names as it was before changes
introduced by commit da1ce0670c14 ("vfs: add cross-rename").

See https://lkml.org/lkml/2014/9/7/6 for details.

AV: the comments about being more careful with ->d_name.hash
than with ->d_name.name are from back in 2.3.40s; they
became obsolete by 2.3.60s, when we started to unhash the
target instead of swapping hash chain positions followed
by d_delete() as we used to do when dcache was first
introduced.

Acked-by: Miklos Szeredi <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Fixes: da1ce0670c14 "vfs: add cross-rename"
Signed-off-by: Mikhail Efremov <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/dcache.c

···
 }
 EXPORT_SYMBOL(dentry_update_name_case);
 
-static void switch_names(struct dentry *dentry, struct dentry *target)
+static void switch_names(struct dentry *dentry, struct dentry *target,
+			 bool exchange)
 {
 	if (dname_external(target)) {
 		if (dname_external(dentry)) {
···
 			 */
 			unsigned int i;
 			BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
+			if (!exchange) {
+				memcpy(dentry->d_iname, target->d_name.name,
+						target->d_name.len + 1);
+				dentry->d_name.hash_len = target->d_name.hash_len;
+				return;
+			}
 			for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
 				swap(((long *) &dentry->d_iname)[i],
 				     ((long *) &target->d_iname)[i]);
···
  * When switching names, the actual string doesn't strictly have to
  * be preserved in the target - because we're dropping the target
  * anyway. As such, we can just do a simple memcpy() to copy over
- * the new name before we switch.
- *
- * Note that we have to be a lot more careful about getting the hash
- * switched - we have to switch the hash value properly even if it
- * then no longer matches the actual (corrupted) string of the target.
- * The hash value has to match the hash queue that the dentry is on..
+ * the new name before we switch, unless we are going to rehash
+ * it.  Note that if we *do* unhash the target, we are not allowed
+ * to rehash it without giving it a new name/hash key - whether
+ * we swap or overwrite the names here, resulting name won't match
+ * the reality in filesystem; it's only there for d_path() purposes.
+ * Note that all of this is happening under rename_lock, so the
+ * any hash lookup seeing it in the middle of manipulations will
+ * be discarded anyway.  So we do not care what happens to the hash
+ * key in that case.
  */
 /*
  * __d_move - move a dentry
···
 			   d_hash(dentry->d_parent, dentry->d_name.hash));
 	}
 
-
 	/* Switch the names.. */
-	switch_names(dentry, target);
+	switch_names(dentry, target, exchange);
 
 	/* ... and switch them in the tree */
 	if (IS_ROOT(dentry)) {