Hi Al,
Certainly, I never checked grab_super. Lesson learned.
Thanks a lot for taking the time to write this master class. I really
appreciate it. :)
--
Gustavo
a reproducer), we have some other crap going on and need to investigate
that, but even in that case, the patch is wrong]
As for how to investigate that kind of thing... Look:
The code in question is
if (fc->user_ns != old->s_user_ns) {
spin_unlock(&sb_lock);
if (s) {
up_write(&s->s_umount);
destroy_unused_super(s);
}
return ERR_PTR(-EBUSY);
}
if (!grab_super(old))
goto retry;
if (s) {
up_write(&s->s_umount);
destroy_unused_super(s);
s = NULL;
}
return old;
Your hypothesis is that we can get to that return old; with sb_lock
held. That would almost certainly be a bad thing, since elsewhere
in the same function we have
spin_unlock(&sb_lock);
get_filesystem(s->s_type);
register_shrinker(&s->s_shrink);
return s;
which appears to return an object with sb_lock dropped, with no obvious
way for a caller to tell one from another. Even if such a way existed
(it does, actually), that kind of calling conventions would be highly
bug-prone.
The next question is, when would we get to that return old; with
sb_lock held? We do, apparently, hold it before the if (fc->...)
above (again, strictly speaking not proven yet, but that's the
most likely assumption). So if grab_super(old) returns true and
we are holding sb_lock, either we do have a problem, or something
subtle is going on.
The obvious next target of examination is grab_super(). Which comes
with
/**
* grab_super - acquire an active reference
* @s: reference we are trying to make active
*
* Tries to acquire an active reference. grab_super() is used when we
* had just found a superblock in super_blocks or fs_type->fs_supers
* and want to turn it into a full-blown active reference. grab_super()
^^^^^^^^^^^^
* is called with sb_lock held and drops it. Returns 1 in case of
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* success, 0 if we had failed (superblock contents was already dead or
* dying when grab_super() had been called). Note that this is only
* called for superblocks not in rundown mode (== ones still on ->fs_supers
* of their type), so increment of ->s_count is OK here.
*/
and looking for references to sb_lock yields the underscored sentence. Now,
if that is true (which is not guaranteed - comments can become stale), we do
not need to drop sb_lock after the call of grab_super() - it's already been
dropped by grab_super() itself.
And looking at the actual code we have
static int grab_super(struct super_block *s) __releases(sb_lock)
{
s->s_count++;
spin_unlock(&sb_lock);
^^^^^^^^^^^^^^^^^^^^^---------- dropped, indeed
down_write(&s->s_umount);
if ((s->s_flags & SB_BORN) && atomic_inc_not_zero(&s->s_active)) {
put_super(s);
return 1;
}
up_write(&s->s_umount);
put_super(s);
return 0;
}
... and not regained, unless put_super() does something fishy.
static void put_super(struct super_block *sb)
{
spin_lock(&sb_lock);
__put_super(sb);
spin_unlock(&sb_lock);
}
OK, put_super() definitely returns with sb_lock not held, and therefore so does
grab_super(). In other words, the comment does match the reality and trying
to drop sb_lock right after the call of grab_super() would be 100% wrong.
That disproves your hypothesis. For the sake of completeness, let's finish the
analysis of sget_fc() wrt sb_lock.
struct super_block *sget_fc(struct fs_context *fc,
int (*test)(struct super_block *, struct fs_context *),
int (*set)(struct super_block *, struct fs_context *))
{
if (!(fc->sb_flags & SB_KERNMOUNT) &&
fc->purpose != FS_CONTEXT_FOR_SUBMOUNT) {
/* Don't allow mounting unless the caller has CAP_SYS_ADMIN
* over the namespace.
*/
if (!(fc->fs_type->fs_flags & FS_USERNS_MOUNT) &&
!capable(CAP_SYS_ADMIN))
return ERR_PTR(-EPERM);
else if (!ns_capable(fc->user_ns, CAP_SYS_ADMIN))
return ERR_PTR(-EPERM);
}
retry:
spin_lock(&sb_lock);
OK, we definitely do not want to call that with sb_lock held - doing so would
either return ERR_PTR(-EPERM) or deadlock. So the calling conventions include
"caller is not holding sb_lock". If so, everything up to retry: should be
executed without sb_lock held, and subsequent code is with sb_lock held.
if (test) {
hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
if (!test(old, fc))
continue;
'test' callback should be callable with sb_lock held. Note that
at least in case when it returns false it must not have dropped
sb_lock - the list we are walking is protected by sb_lock.
if (fc->user_ns != old->s_user_ns) {
spin_unlock(&sb_lock);
... in which case we also want sb_lock not dropped by test() since we
drop it ourselves.
if (s) {
up_write(&s->s_umount);
destroy_unused_super(s);
destroy_unused_super() is called without sb_lock here and examination
shows that it doesn't touch sb_lock itself.
}
return ERR_PTR(-EBUSY);
}
... and in this case we also want sb_lock not dropped by test() either,
since grab_super() will drop it.
if (!grab_super(old))
goto retry;
we either go back to 'retry:' with sb_lock not held (same as in the
case of reaching retry: without goto) or coninue to
if (s) {
up_write(&s->s_umount);
destroy_unused_super(s);
same as above, called without sb_lock, doesn't touch it.
s = NULL;
}
return old;
... and we return without sb_lock held.
}
}
Here (after the if (test) body) we do hold sb_lock
if (!s) {
spin_unlock(&sb_lock);
drop and call alloc_super(), which doesn't touch sb_lock
s = alloc_super(fc);
if (!s)
return ERR_PTR(-ENOMEM);
goto retry;
... and either return without sb_lock or go back to retry:, with
the same conditions as on other paths leading there. Incidentally,
since alloc_super() very clearly blocks (GFP_USER kzalloc the very
first thing in there), the calling conventions for sget_fc() include
not just "must not be holding sb_lock" but "must not be holding any
spinlock".
}
s->s_fs_info = fc->s_fs_info;
err = set(s, fc);
OK, so 'set()' is also called under sb_lock.
if (err) {
s->s_fs_info = NULL;
spin_unlock(&sb_lock);
... and at least in case of error must not drop it, since we'd do that
ourselves.
up_write(&s->s_umount);
destroy_unused_super(s);
return ERR_PTR(err);
same as in earlier cases
}
fc->s_fs_info = NULL;
s->s_type = fc->fs_type;
strlcpy(s->s_id, s->s_type->name, sizeof(s->s_id));
list_add_tail(&s->s_list, &super_blocks);
hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
spin_unlock(&sb_lock);
OK, so 'set()' must not drop sb_lock in any cases. And from that point
on sb_lock is not held (neither get_filesystem() nor register_shrinker()
touch it)
get_filesystem(s->s_type);
register_shrinker(&s->s_shrink);
return s;
}
So we arrive to the following:
* sget_fc() must not be called with any spinlocks (sb_lock included) held.
* in all cases it returns with sb_lock not held.
* test() and set() callbacks are always called under sb_lock and should not
drop it.
Looking at the shape of that code strengthens the last one to "even
drop-and-retake is not allowed". With that kind of loop over hlist, dropping
and retaking sb_lock in test() might blow up. And as for set() callback,
we clearly don't want to create a new instance when an existing one would
satisfy the test() predicate. And dropping/retaking sb_lock would've
allowed another caller to come and insert a new instance while our
set() has not been holding sb_lock, ending up with just that once we
return and get to hlist_add_head() there.
In other words,
* called without any spinlocks held
* returns with no spinlocks held
* callbacks are always called under sb_lock and must not touch it.
Verifying that callers (and all possible callbacks) satisfy those rules
is left as an exercise for reader...
