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SubjectRe: Fix for sparc64 cpu hangs.
From: David Miller <>
Date: Tue, 06 Nov 2007 20:34:33 -0800 (PST)

> [FUTEX]: Fix address computation in compat code.

Sorry, I just noticed there is a second handle_futex_death()
call in compat_exit_robust_list() which has the same
address computation bug.

Here is an updated patch:

[FUTEX]: Fix address computation in compat code.

compat_exit_robust_list() computes a pointer to the
futex entry in userspace as follows:

(void __user *)entry + futex_offset

'entry' is a 'struct robust_list __user *', and
'futex_offset' is a 'compat_long_t' (typically a 's32').

Things explode if the 32-bit sign bit is set in futex_offset.

Type promotion sign extends futex_offset to a 64-bit value before
adding it to 'entry'.

This triggered a problem on sparc64 running 32-bit applications which
would lock up a cpu looping forever in the fault handling for the
userspace load in handle_futex_death().

Compat userspace runs with address masking (wherein the cpu zeros out
the top 32-bits of every effective address given to a memory operation
instruction) so the sparc64 fault handler accounts for this by
zero'ing out the top 32-bits of the fault address too.

Since the kernel properly uses the compat_uptr interfaces, kernel side
accesses to compat userspace work too since they will only use
addresses with the top 32-bit clear.

Because of this compat futex layer bug we get into the following loop
when executing the get_user() load near the top of handle_futex_death():

1) load from address '0xfffffffff7f16bd8', FAULT
2) fault handler clears upper 32-bits, processes fault
for address '0xf7f16bd8' which succeeds
3) goto #1

I want to thank Bernd Zeimetz, Josip Rodin, and Fabio Massimo Di Nitto
for their tireless efforts helping me track down this bug.

Signed-off-by: David S. Miller <>

diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 00b5726..1931457 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -30,6 +30,15 @@ fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
return 0;

+static void __user *futex_uaddr(struct robust_list *entry,
+ compat_long_t futex_offset)
+ compat_uptr_t base = ptr_to_compat(entry);
+ void __user *uaddr = compat_ptr(base + futex_offset);
+ return uaddr;
* Walk curr->robust_list (very carefully, it's a userspace list!)
* and mark any locks found there dead, and notify any waiters.
@@ -76,11 +85,13 @@ void compat_exit_robust_list(struct task_struct *curr)
* A pending lock might already be on the list, so
* dont process it twice:
- if (entry != pending)
- if (handle_futex_death((void __user *)entry + futex_offset,
- curr, pi))
- return;
+ if (entry != pending) {
+ void __user *uaddr = futex_uaddr(entry,
+ futex_offset);

+ if (handle_futex_death(uaddr, curr, pi))
+ return;
+ }
if (rc)
uentry = next_uentry;
@@ -94,9 +105,11 @@ void compat_exit_robust_list(struct task_struct *curr)

- if (pending)
- handle_futex_death((void __user *)pending + futex_offset,
- curr, pip);
+ if (pending) {
+ void __user *uaddr = futex_uaddr(pending, futex_offset);
+ handle_futex_death(uaddr, curr, pip);
+ }

asmlinkage long
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