fix proc_sys_compare() handling of in-lookup dentries

There's one case where ->d_compare() can be called for an in-lookup
dentry; usually that's nothing special from ->d_compare() point of
view, but... proc_sys_compare() is weird.

The thing is, /proc/sys subdirectories can look differently for
different processes.  Up to and including having the same name
resolve to different dentries - all of them hashed.

The way it's done is ->d_compare() refusing to admit a match unless
this dentry is supposed to be visible to this caller.  The information
needed to discriminate between them is stored in inode; it is set
during proc_sys_lookup() and until it's done d_splice_alias() we really
can't tell who should that dentry be visible for.

Normally there's no negative dentries in /proc/sys; we can run into
a dying dentry in RCU dcache lookup, but those can be safely rejected.

However, ->d_compare() is also called for in-lookup dentries, before
they get positive - or hashed, for that matter.  In case of match
we will wait until dentry leaves in-lookup state and repeat ->d_compare()
afterwards.  In other words, the right behaviour is to treat the
name match as sufficient for in-lookup dentries; if dentry is not
for us, we'll see that when we recheck once proc_sys_lookup() is
done with it.

While we are at it, fix the misspelled READ_ONCE and WRITE_ONCE there.

Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)")
Reported-by: NeilBrown <neilb@brown.name>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index a3eb3b740f76647c9918b5821daef83ff1df9f6f..3604b616311c27a0ed1d91d05f1569cc2523cdc0 100644 (file)
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -42,7 +42,7 @@ static void proc_evict_inode(struct inode *inode)
 
        head = ei->sysctl;
        if (head) {
-               RCU_INIT_POINTER(ei->sysctl, NULL);
+               WRITE_ONCE(ei->sysctl, NULL);
                proc_sys_evict_inode(inode, head);
        }
 }

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index cc9d74a06ff03c14413091579f62f8f4cdec0f62..08b78150cdde1f01791c2026be025f149ddbe189 100644 (file)
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -918,17 +918,21 @@ static int proc_sys_compare(const struct dentry *dentry,
        struct ctl_table_header *head;
        struct inode *inode;
 
-       /* Although proc doesn't have negative dentries, rcu-walk means
-        * that inode here can be NULL */
-       /* AV: can it, indeed? */
-       inode = d_inode_rcu(dentry);
-       if (!inode)
-               return 1;
        if (name->len != len)
                return 1;
        if (memcmp(name->name, str, len))
                return 1;
-       head = rcu_dereference(PROC_I(inode)->sysctl);
+
+       // false positive is fine here - we'll recheck anyway
+       if (d_in_lookup(dentry))
+               return 0;
+
+       inode = d_inode_rcu(dentry);
+       // we just might have run into dentry in the middle of __dentry_kill()
+       if (!inode)
+               return 1;
+
+       head = READ_ONCE(PROC_I(inode)->sysctl);
        return !head || !sysctl_is_seen(head);
 }