Change in functionality of futex() system call.

[David Oliver]

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.
  }
}


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