vfs: do the careful dentry name access for all dentry_cmp cases

Commit 12f8ad4b0533 ("vfs: clean up __d_lookup_rcu() and dentry_cmp()
interfaces") did the careful ACCESS_ONCE() of the dentry name only for
the word-at-a-time case, even though the issue is generic.

Admittedly I don't really see gcc ever reloading the value in the middle
of the loop, so the ACCESS_ONCE() protects us from a fairly theoretical
issue. But better safe than sorry.

Also, this consolidates the common parts of the word-at-a-time and
bytewise logic, which includes checking the length.  We'll be changing
that later.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 26 insertions, 28 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index c4d2ff8b4912..5c09ad7b4a15 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -153,30 +153,9 @@ int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
  * In contrast, 'ct' and 'tcount' can be from a pathname, and do
  * need the careful unaligned handling.
  */
-static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
         unsigned long a,b,mask;
-        const unsigned char *cs;
-
-        if (unlikely(dentry->d_name.len != tcount))
-                return 1;
-        /*
-         * Be careful about RCU walk racing with rename:
-         * use ACCESS_ONCE to fetch the name pointer.
-         *
-         * NOTE! Even if a rename will mean that the length
-         * was not loaded atomically, we don't care. The
-         * RCU walk will check the sequence count eventually,
-         * and catch it. And we won't overrun the buffer,
-         * because we're reading the name pointer atomically,
-         * and a dentry name is guaranteed to be properly
-         * terminated with a NUL byte.
-         *
-         * End result: even if 'len' is wrong, we'll exit
-         * early because the data cannot match (there can
-         * be no NUL in the ct/tcount data)
-         */
-        cs = ACCESS_ONCE(dentry->d_name.name);
 
         for (;;) {
                 a = *(unsigned long *)cs;
@@ -197,13 +176,8 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 
 #else
 
-static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
-        const unsigned char *cs = dentry->d_name.name;
-
-        if (dentry->d_name.len != tcount)
-                return 1;
-
         do {
                 if (*cs != *ct)
                         return 1;
@@ -216,6 +190,30 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 
 #endif
 
+static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+{
+        if (dentry->d_name.len != tcount)
+                return 1;
+
+        /*
+         * Be careful about RCU walk racing with rename:
+         * use ACCESS_ONCE to fetch the name pointer.
+         *
+         * NOTE! Even if a rename will mean that the length
+         * was not loaded atomically, we don't care. The
+         * RCU walk will check the sequence count eventually,
+         * and catch it. And we won't overrun the buffer,
+         * because we're reading the name pointer atomically,
+         * and a dentry name is guaranteed to be properly
+         * terminated with a NUL byte.
+         *
+         * End result: even if 'len' is wrong, we'll exit
+         * early because the data cannot match (there can
+         * be no NUL in the ct/tcount data)
+         */
+        return dentry_string_cmp(ACCESS_ONCE(dentry->d_name.name), ct, tcount);
+}
+
 static void __d_free(struct rcu_head *head)
 {
         struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);