Re: [PATCH v3] futex: Fix regression with read only mappings
From: Shawn Bohrer
Date: Wed Jun 29 2011 - 10:57:05 EST
On Tue, Jun 28, 2011 at 04:55:59PM -0700, Darren Hart wrote:
>
>
> On 06/28/2011 10:38 AM, Shawn Bohrer wrote:
> > On Tue, Jun 28, 2011 at 07:52:06AM -0700, Darren Hart wrote:
> >>
> >>
> >> On 06/28/2011 03:54 AM, Peter Zijlstra wrote:
> >>> On Mon, 2011-06-27 at 17:22 -0500, Shawn Bohrer wrote:
> >>>> commit 7485d0d3758e8e6491a5c9468114e74dc050785d (futexes: Remove rw
> >>>> parameter from get_futex_key()) in 2.6.33 introduced a user-mode
> >>>> regression in that it additionally prevented futex operations on a
> >>>> region within a read only memory mapped file. For example this breaks
> >>>> workloads that have one or more reader processes doing a FUTEX_WAIT on a
> >>>> futex within a read only shared mapping, and a writer processes that has
> >>>> a writable mapping issuing the FUTEX_WAKE.
> >>>>
> >>>> This fixes the regression for futex operations that should be valid on
> >>>> RO mappings by trying a RO get_user_pages_fast() when the RW
> >>>> get_user_pages_fast() fails so as not to slow down the common path of
> >>>> writable anonymous maps and bailing when we used the RO path on
> >>>> anonymous memory.
> >>>>
> >>>> While fixing the regression this patch opens up two possible bad
> >>>> scenarios as identified by KOSAKI Motohiro:
> >>>>
> >>>> 1) This patch also allows FUTEX_WAIT on RO private mappings which have
> >>>> the following corner case.
> >>>>
> >>>> Thread-A: call futex(FUTEX_WAIT, memory-region-A).
> >>>> get_futex_key() return inode based key.
> >>>> sleep on the key
> >>>> Thread-B: call mprotect(PROT_READ|PROT_WRITE, memory-region-A)
> >>>> Thread-B: write memory-region-A.
> >>>> COW happen. This process's memory-region-A become related
> >>>> to new COWed private (ie PageAnon=1) page.
> >>>> Thread-B: call futex(FUETX_WAKE, memory-region-A).
> >>>> get_futex_key() return mm based key.
> >>>> IOW, we fail to wake up Thread-A.
> >>>>
> >>>> 2) Current futex code doesn't handle zero page properly.
> >>>>
> >>>> Read mode get_user_pages() can return zero page, but current futex
> >>>> code doesn't handle it at all. Then, zero page makes infinite loop
> >>>> internally.
> >>>>
> >>>> This Patch is based on Peter Zijlstra's initial patch with modifications to
> >>>> only allow RO mappings for futex operations that need VERIFY_READ access.
> >>>>
> >>>> Reported-by: David Oliver <david@xxxxxxxxxxxxxxx>
> >>>> Signed-off-by: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxxx>
> >>>
> >>> Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> >>
> >> I think we need to address #2 above first.
> >
> > I believe the following contrived case triggers the zero page problem:
> >
> > #include <errno.h>
> > #include <fcntl.h>
> > #include <stdint.h>
> > #include <linux/futex.h>
> > #include <sys/mman.h>
> > #include <sys/syscall.h>
> > #include <unistd.h>
> > #include <stdio.h>
> >
> >
> > int main(int argc, char *argv[])
> > {
> > int fd, *futex, rc, val = 42;
> > struct timespec ts = {.tv_sec = 2, .tv_nsec = 0 };
> >
> > fd = open("/tmp/futex_test", O_RDWR|O_CREAT, 0644);
> > write(fd, &val, 4);
> > futex = (int *)mmap(0, sizeof(int), PROT_READ, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
> > rc = syscall(SYS_futex, futex, FUTEX_WAIT, val, &ts, 0, 0);
> > printf("rc=%d errno=%d\n", rc, errno);
> > }
> >
> > Running the program above with my patch applied will spin in the
> > kernel at 100% CPU usage.
> >
> >> 1) Is the loop killable?
> >
> > Yes, SIGINT causes the program to return.
>
> OK, so while it is killable... I'd really rather not introduce a busy loop in the
> kernel. Especially not after we removed it with:
>
> http://lkml.org/lkml/2010/1/13/136
>
> How about adding something like this. I _think_ the only way to get a ZERO_PAGE
> here now is with the contrived testcase below.
>
> $ git diff
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 1737a66..a5417c2 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -316,6 +316,13 @@ again:
> if (!page_head->mapping) {
> unlock_page(page_head);
> put_page(page_head);
> + /*
> + * ZERO_PAGE pages don't have a mapping. Avoid a busy loop
> + * trying to find one. RW mapping would have COW'd (and thus
> + * have a mapping) so this page is RO and won't ever change.
> + */
> + if ((page_head == ZERO_PAGE(address)))
> + return -EFAULT;
> goto again;
> }
>
>
> This returns EFAULT for the following testcase which spun in get_futex_key with
> your V3 patch.
I've tested and confirmed this as well. I can send a v4 patch with
this included.
>
> #include <errno.h>
> #include <fcntl.h>
> #include <stdint.h>
> #include <linux/futex.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <unistd.h>
> #include <stdio.h>
>
>
> int main(int argc, char *argv[])
> {
> int fd, *futex, rc;
> struct timespec ts = {.tv_sec = 2, .tv_nsec = 0 };
>
> futex = (int *)mmap(0, sizeof(int), PROT_READ, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
> printf("futex @ %p\n", futex);
> rc = syscall(SYS_futex, futex, FUTEX_WAIT, 0, &ts, 0, 0);
> printf("rc=%d errno=%d\n", rc, errno);
> }
>
> Thanks,
>
> Darren
>
> >
> >> 2) If not, and we try to catch zero_page scenario, we need to ensure RO
> >> sparse file mapping work. Peter notes that these are probably fine,
> >> filemap.c doesn't seem to use the zero page - but we need to test.
> >>
> >> It's agreed, I believe, between myself, Peter, and Shawn that we will
> >> document Private RW mappings as unsupported.
--
Shawn
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