Re: Change in functionality of futex() system call.

From: Eric Dumazet
Date: Mon Jun 06 2011 - 11:23:48 EST


Le lundi 06 juin 2011 Ã 09:28 -0500, David Oliver a Ãcrit :
> 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@xxxxxxxxxxxxxx>
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.

Reported-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Darren Hart <dvhltc@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxx>
Cc: Ulrich Drepper <drepper@xxxxxxxxx>
LKML-Reference: <20100105162633.45A2.A69D9226@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>



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