From: john stultz Date: Mon, 14 Aug 2006 06:24:24 +0000 (-0700) Subject: [PATCH] futex_handle_fault always fails X-Git-Tag: v2.6.18-rc5~87 X-Git-Url: http://ftp.safe.ca/?a=commitdiff_plain;h=e579dcbf23604cb33c08b5c3c3ac06ca36e7c683;hp=e579dcbf23604cb33c08b5c3c3ac06ca36e7c683;p=safe%2Fjmp%2Flinux-2.6 [PATCH] futex_handle_fault always fails We found this issue last week w/ the -RT kernel, but it seems the same issue is in mainline as well. Basically it is possible for futex_unlock_pi to return without actually freeing the lock. This is due to buggy logic in the use of futex_handle_fault() and its attempt argument in a failure case. Looking at futex.c the logic is as follows: 1) In futex_unlock_pi() we start w/ ret=0 and we go down to the first futex_atomic_cmpxchg_inatomic(), where we find uval==-EFAULT. We then jump to the pi_faulted label. 2) From pi_faulted: We increment attempt, unlock the sem and hit the retry label. 3) From the retry label, with ret still zero, we again hit EFAULT on the first futex_atomic_cmpxchg_inatomic(), and again goto the pi_faulted label. 4) Again from pi_faulted: we increment attempt and enter the conditional, where we call futex_handle_fault. 5) futex_handle_fault fails, and we goto the out_unlock_release_sem label. 6) From out_unlock_release_sem we return, and since ret is still zero, we return without error, while never actually unlocking the lock. Issue #1: at the first futex_atomic_cmpxchg_inatomic() we should probably be setting ret=-EFAULT before jumping to pi_faulted: However in our case this doesn't really affect anything, as the glibc we're using ignores the error value from futex_unlock_pi(). Issue #2: Look at futex_handle_fault(), its first conditional will return -EFAULT if attempt is >= 2. However, from the "if(attempt++) futex_handle_fault(attempt)" logic above, we'll *never* call futex_handle_fault when attempt is less then two. So we never get a chance to even try to fault the page in. The following patch addresses these two issues by 1) Always setting ret to -EFAULT if futex_handle_fault fails, and 2) Removing the = in futex_handle_fault's (attempt >= 2) check. I'm really not sure this is the right fix, but wanted to bring it up so folks knew the issue is alive and well in the current -git tree. From looking at the git logs the logic was first introduced (then later copied to other places) in the following commit almost a year ago: http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=4732efbeb997189d9f9b04708dc26bf8613ed721;hp=5b039e681b8c5f30aac9cc04385cc94be45d0823 Cc: Rusty Russell Cc: Ingo Molnar Acked-by: Thomas Gleixner Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman ---