Re: [git pull] d_revalidate pile

Al Viro
2025-01-28T00:27:01Z

On Mon, Jan 27, 2025 at 06:26:58PM -0500, Sasha Levin wrote:
> On Mon, Jan 27, 2025 at 10:40:59PM +0000, Al Viro wrote:
> > On Mon, Jan 27, 2025 at 09:34:56PM +0000, Al Viro wrote:
> >
> > > If so, then
> > > a) it's a false positive (and IIRC, it's not the first time
> > > kfence gets confused by that)
> > > b) your bisection will probably converge to bdd9951f60f9
> > > "dissolve external_name.u into separate members" which is where we'd
> > > ended up with offsetof(struct external_name, name) being 4 modulo 8.
> > >
> > > As a quick test, try to flip the order of head and count in
> > > struct external_name and see if that makes the warning go away.
> > > If it does, I'm pretty certain that theory above is correct.
> >
> > Not quite... dentry_string_cmp() assumes that ->d_name.name is
> > word-aligned, so load_unaligned_zeropad() is done only to the
> > second string (the one we compare against).
>
> Sorry for the silence on my end: this issue doesn't reproduce
> consistently, so I need to do more runs for these tests.

Updated version force-pushed; delta is

diff --git a/fs/dcache.c b/fs/dcache.c
index 695406e48937..903142b324e9 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -295,10 +295,16 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
return dentry_string_cmp(cs, ct, tcount);
}

+/*
+ * long names are allocated separately from dentry and never modified.
+ * Refcounted, freeing is RCU-delayed. See take_dentry_name_snapshot()
+ * for the reason why ->count and ->head can't be combined into a union.
+ * dentry_string_cmp() relies upon ->name[] being word-aligned.
+ */
struct external_name {
- struct rcu_head head; // ->head and ->count can't be combined
- atomic_t count; // see take_dentry_name_snapshot()
- unsigned char name[];
+ atomic_t count;
+ struct rcu_head head;
+ unsigned char name[] __aligned(sizeof(unsigned long));
};

static inline struct external_name *external_name(struct dentry *dentry)

Could you recheck that one (23e8b451dea4)? I'll send an update pull request
if nothing wrong shows up.