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Subject[PATCH] robust futex thread exit race
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Date
Hi Ingo,
I finally found the bug that causes tst-robust8 from the glibc to fail
on s390x. Turned out to be a common code problem with the processing of
the robust futex list. The patch below fixes the bug for me.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.

--
Subject: [PATCH] robust futex thread exit race

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Calling handle_futex_death in exit_robust_list for the different
robust mutexes of a thread basically frees the mutex. Another
thread might grab the lock immediately which updates the next
pointer of the mutex. fetch_robust_entry over the next pointer
might therefore branch into the robust mutex list of a different
thread. This can cause two problems: 1) some mutexes held by
the dead thread are not getting freed and 2) some mutexs held by
a different thread are freed.
The next point need to be read before calling handle_futex_death.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---

diff -urpN linux-2.6/kernel/futex.c linux-2.6-patched/kernel/futex.c
--- linux-2.6/kernel/futex.c 2007-08-23 11:14:33.000000000 +0200
+++ linux-2.6-patched/kernel/futex.c 2007-09-30 16:31:57.000000000 +0200
@@ -1943,9 +1943,10 @@ static inline int fetch_robust_entry(str
void exit_robust_list(struct task_struct *curr)
{
struct robust_list_head __user *head = curr->robust_list;
- struct robust_list __user *entry, *pending;
- unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
+ struct robust_list __user *entry, *next_entry, *pending;
+ unsigned int limit = ROBUST_LIST_LIMIT, pi, next_pi, pip;
unsigned long futex_offset;
+ int rc;

/*
* Fetch the list head (which was registered earlier, via
@@ -1965,12 +1966,13 @@ void exit_robust_list(struct task_struct
if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
return;

- if (pending)
- handle_futex_death((void __user *)pending + futex_offset,
- curr, pip);
-
while (entry != &head->list) {
/*
+ * Fetch the next entry in the list before calling
+ * handle_futex_death:
+ */
+ rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
+ /*
* A pending lock might already be on the list, so
* don't process it twice:
*/
@@ -1978,11 +1980,10 @@ void exit_robust_list(struct task_struct
if (handle_futex_death((void __user *)entry + futex_offset,
curr, pi))
return;
- /*
- * Fetch the next entry in the list:
- */
- if (fetch_robust_entry(&entry, &entry->next, &pi))
+ if (rc)
return;
+ entry = next_entry;
+ pi = next_pi;
/*
* Avoid excessively long or circular lists:
*/
@@ -1991,6 +1992,10 @@ void exit_robust_list(struct task_struct

cond_resched();
}
+
+ if (pending)
+ handle_futex_death((void __user *)pending + futex_offset,
+ curr, pip);
}

long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,

-
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