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Date: Mon, 06 Jun 2011 18:16:29 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: David Oliver <david@...advisors.com>, linux-kernel@...r.kernel.org,
Shawn Bohrer <sbohrer@...advisors.com>,
Zachary Vonler <zvonler@...advisors.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Hugh Dickins <hughd@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Darren Hart <dvhart@...ux.intel.com>,
Ingo Molnar <mingo@...e.hu>
Subject: Re: Change in functionality of futex() system call.
On Mon, 2011-06-06 at 18:11 +0200, Peter Zijlstra wrote:
> On Mon, 2011-06-06 at 17:23 +0200, Eric Dumazet wrote:
> > Le lundi 06 juin 2011 à 09:28 -0500, David Oliver a écrit :
> > > Hello,
> > >
> > > The functionality of the futex() system call appears to have changed
> > > between versions 2.6.18 and 2.6.32.28.
> > >
> > > Specifically, performing a FUTEX_WAIT on a read-only mapped location
> > > results in an EFAULT. Although other operations, such as FUTEX_WAKE,
> > > are only meaningful for writable locations, FUTEX_WAIT is useful for
> > > processes with read-only access to a memory-mapped file.
> > >
> > > The code below illustrates the changed behavior (each of the EXPECT
> > > operations succeed on the older kernel, the ASSERTs pass in each
> > > case), assuming the file /tmp/futex_test exists and contains int(42).
> > >
> > > With the older kernel, the syscall() suspends until another process
> > > changes the file and issues a FUTEX_WAKE, whereas the new behavior is
> > > for an EFAULT error, independent of the file contents.
> > >
> > > Let me know if you need further clarification.
> > >
> > > Cheers!
> > >
> > > David Oliver.
> > >
> > >
> > > #include <errno.h>
> > > #include <fcntl.h>
> > > #include <stdint.h>
> > > typedef uint32_t u32; // for futex.h
> > > #include <linux/futex.h>
> > > #include <sys/mman.h>
> > > #include <sys/syscall.h>
> > > #include <unistd.h>
> > > #include "gtest/gtest.h" // test framework to illustrate issue.
> > >
> > >
> > > TEST(Futex, futex_in_read_only_file_is_ok) {
> > > int fd = open("/tmp/futex_test", O_RDONLY);
> > > ASSERT_GE(fd, 0);
> > > int* futex = static_cast<int *>(mmap(0, sizeof(int), PROT_READ,
> > > MAP_SHARED, fd, 0));
> > > ASSERT_NE((int *)(0), futex);
> > >
> > > int rc = syscall(SYS_futex, futex, FUTEX_WAIT, 42, 0, 0, 0);
> > >
> > > EXPECT_NE(-1, rc); // fails.
> > > if (rc == -1) {
> > > EXPECT_NE(errno, EFAULT); // fails.
> > > }
> > > }
> > >
> >
> > Right you are, this came from commit 7485d0d3758e8e6491a5 (futexes:
> > Remove rw parameter from get_futex_key()) in 2.6.33
> >
> > commit 7485d0d3758e8e6491a5c9468114e74dc050785d
> > Author: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Date: Tue Jan 5 16:32:43 2010 +0900
> >
> > futexes: Remove rw parameter from get_futex_key()
> >
> > Currently, futexes have two problem:
> >
> > A) The current futex code doesn't handle private file mappings properly.
> >
> > get_futex_key() uses PageAnon() to distinguish file and
> > anon, which can cause the following bad scenario:
> >
> > 1) thread-A call futex(private-mapping, FUTEX_WAIT), it
> > sleeps on file mapping object.
> > 2) thread-B writes a variable and it makes it cow.
> > 3) thread-B calls futex(private-mapping, FUTEX_WAKE), it
> > wakes up blocked thread on the anonymous page. (but it's nothing)
> >
> > B) 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.
> >
> > The solution is to use write mode get_user_page() always for
> > page lookup. It prevents the lookup of both file page of private
> > mappings and zero page.
> >
> > Performance concerns:
> >
> > Probaly very little, because glibc always initialize variables
> > for futex before to call futex(). It means glibc users never see
> > the overhead of this patch.
> >
> > Compatibility concerns:
> >
> > This patch has few compatibility issues. After this patch,
> > FUTEX_WAIT require writable access to futex variables (read-only
> > mappings makes EFAULT). But practically it's not a problem,
> > glibc always initalizes variables for futexes explicitly - nobody
> > uses read-only mappings.
>
> Urgh,. maybe something like the below but with more conditionals that
> enable the extra logic only for FUTEX_WAIT..
>
> The idea is to try a RO gup() when the RW gup() fails so as not to slow
> down the common path of writable anonymous maps and bail when we used
> the RO path on anonymous memory.
>
> ---
> diff --git a/kernel/futex.c b/kernel/futex.c
> index fe28dc2..11f2ad1 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -234,7 +234,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
> unsigned long address = (unsigned long)uaddr;
> struct mm_struct *mm = current->mm;
> struct page *page, *page_head;
> - int err;
> + int err, ro = 0;
>
> /*
> * The futex address must be "naturally" aligned.
> @@ -262,6 +262,10 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
>
> again:
> err = get_user_pages_fast(address, 1, 1, &page);
> + if (err == -EFAULT) {
> + err = get_user_pages_fast(address, 1, 0, &page);
> + ro = 1;
> + }
> if (err < 0)
> return err;
>
> @@ -316,6 +320,11 @@ again:
> * the object not the particular process.
> */
> if (PageAnon(page_head)) {
> + if (ro) {
> + err = -EFAULT;
> + goto out;
> + }
> +
> key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
> key->private.mm = mm;
> key->private.address = address;
> @@ -327,9 +336,10 @@ again:
>
> get_futex_key_refs(key);
>
> +out:
> unlock_page(page_head);
> put_page(page_head);
> - return 0;
> + return err;
> }
>
> static inline void put_futex_key(union futex_key *key)
>
Hmm, wouldn't that still be susceptible to the zero-page thing if: we
create a writable private file map of a sparse file, touch a page and
then remap the thing RO?
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