[PATCH -mm] utrace: fix double free re __rcu_process_callbacks()

From: Alexey Dobriyan
Date: Tue Apr 24 2007 - 05:02:49 EST

The following patch fixes double free manifesting itself as crash in

The problem is with check_dead_utrace() conditionally scheduling
"struct utrace" for freeing but not cleaning struct task_struct::utrace
pointer leaving it reachable:

tsk->utrace_flags = flags;
if (flags)

OTOH, utrace_release_task() first clears ->utrace pointer, then frees
struct utrace itself:

Roland inserted some debugging into 2.6.21-rc6-mm1 so that aforementined
double free couldn't be reproduced without seeing
BUG at kernel/utrace.c:176 first. It triggers if one struct utrace were
passed to rcu_utrace_free() second time.

With patch applied I no longer see¹ BUG message and double frees on
2-way P3, 8-way ia64, Core 2 Duo boxes. Testcase is at the first link.

I _think_ it adds leak if utrace_reap() takes branch without freeing
but, well, I hope Roland will give me some clue on how to fix it too.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxx>

kernel/utrace.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

¹ But I see whole can of other bugs! I think they were already lurking
but weren't easily reproducable without hitting double-free first.
FWIW, it's
oops at the beginning of remove_engine()
NULL ->report_quiesce call which is absent in ptrace utrace ops

--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -205,7 +205,6 @@ utrace_clear_tsk(struct task_struct *tsk
if (utrace->u.live.signal == NULL) {
if (likely(tsk->utrace != NULL)) {
- rcu_assign_pointer(tsk->utrace, NULL);
tsk->utrace_flags &= UTRACE_ACTION_NOREAP;
@@ -305,10 +304,7 @@ check_dead_utrace(struct task_struct *ts

tsk->utrace_flags = flags;
- if (flags)
- spin_unlock(&utrace->lock);
- else
- rcu_utrace_free(utrace);
+ spin_unlock(&utrace->lock);

* Now we're finished updating the utrace state.

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