Re: [PATCH v3] futex: Fix regression with read only mappings

[Shawn Bohrer]

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.

> 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.
> 
> -- 
> Darren Hart
> Intel Open Source Technology Center
> Yocto Project - Linux Kernel


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