Messages in this thread Patch in this message |  | | Date | Tue, 28 Jun 2011 16:55:59 -0700 | | From | Darren Hart <> | | Subject | Re: [PATCH v3] futex: Fix regression with read only mappings |
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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@rgmadvisors.com> >>>> Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com> >>> >>> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> >> >> 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.
#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. >> >> -- >> Darren Hart >> Intel Open Source Technology Center >> Yocto Project - Linux Kernel > > > --------------------------------------------------------------- > This email, along with any attachments, is confidential. If you > believe you received this message in error, please contact the > sender immediately and delete all copies of the message. > Thank you. >
-- Darren Hart Intel Open Source Technology Center Yocto Project - Linux Kernel
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