Re: Q: sys_futex() && timespec_valid()

From: Oleg Nesterov
Date: Mon Jun 28 2010 - 11:04:33 EST

On 06/28, Jakub Jelinek wrote:
> On Mon, Jun 28, 2010 at 03:58:25PM +0200, Oleg Nesterov wrote:
> > Honestly, it looks a bit strange to me that you blame the correct code,
> Whether that is correct or not is what is being disputed.

OK. I only argued with the "buggy" term. Once again, the code works
as expected.

> > and at the same time you ignore the test-case which hangs because the
> > kernel returns -EFAULT saying that this is the caller's problem.
> The userspace code reads the abstime->tv_nsec value, so if it wouldn't
> be valid address, the code would already segfault. And that's fine, POSIX
> certainly allows that, reporting EFAULT isn't required. Well, it doesn't
> read abstime->tv_sec in the assembly version, so if you try hard, you can
> avoid the segfault, yet get EFAULT from futex syscall by putting abstime
> 8 bytes before start of some page with previous page not mmapped.

And this is exactly what I did to prove that (in my opinion) libc needs
fixes anyway, even if we change the kernel to treat tv_sec < 0 specially.

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/mman.h>

pthread_rwlock_t rwlock;

static struct timespec *make_efault_ts(void)
int page_size = sysconf(_SC_PAGESIZE);
void *ptr = mmap(0, 2 * page_size, PROT_READ|PROT_WRITE,
munmap(ptr, page_size);
return ptr + page_size - sizeof(long);

static void *thread_func(void *arg)
int ret = pthread_rwlock_timedwrlock(&rwlock, make_efault_ts());
printf("lock: ret=%d %m\n", ret);
return NULL;

int main(int argv, char *argc[])
pthread_t tid;

pthread_rwlock_init(&rwlock, NULL);

pthread_create(&tid, NULL, thread_func, NULL);
pthread_join(tid, NULL);

return 0;

It may hang or segfault on your machine, this depends on libc version.
It hangs on the testing machine which also suffers from the reported
timespec_valid() issue. I did this test-case looking at
"objdump -d /lib64/".

To me, this looks like a bug in libc, but I won't insist.


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