Messages in this thread | | | Date | Wed, 9 Aug 2017 15:05:19 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] futex: Remove unnecessary warning from get_futex_key |
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On Wed, Aug 09, 2017 at 08:27:11AM +0100, Mel Gorman wrote: > Commit 65d8fc777f6d ("futex: Remove requirement for lock_page() in > get_futex_key()") removed an unnecessary lock_page() with the side-effect > that page->mapping needed to be treated very carefully. Two defensive > warnings were added in case any assumption was missed and the first warning > assumed a correct application would not alter a mapping backing a futex key. > Since merging, it has not triggered for any unexpected case but Mark > Rutland reported the following bug triggering due to the first warning.
[...]
> Reported-by: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: Mel Gorman <mgorman@suse.de> > Cc: stable@vger.kernel.org # 4.7+ > --- > kernel/futex.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/kernel/futex.c b/kernel/futex.c > index 16dbe4c93895..f50b434756c1 100644 > --- a/kernel/futex.c > +++ b/kernel/futex.c > @@ -670,13 +670,14 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw) > * this reference was taken by ihold under the page lock > * pinning the inode in place so i_lock was unnecessary. The > * only way for this check to fail is if the inode was > - * truncated in parallel so warn for now if this happens. > + * truncated in parallel which is almost certainly an > + * application bug. In such a case, just retry. > * > * We are not calling into get_futex_key_refs() in file-backed > * cases, therefore a successful atomic_inc return below will > * guarantee that get_futex_key() will still imply smp_mb(); (B). > */ > - if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) { > + if (!atomic_inc_not_zero(&inode->i_count)) {
I applied the same diff yesterday, and haven't seen anything go wrong with my test case and/or with Syzkaller running, so FWIW:
Tested-by: Mark Rutland <mark.rutland@arm.com>
Thanks for putting this together!
Mark.
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