• Mel Gorman's avatar
    futex: Remove unnecessary warning from get_futex_key · 48fb6f4d
    Mel Gorman authored
    Commit 65d8fc77 ("futex: Remove requirement for lock_page() in
    get_futex_key()") removed an unnecessary lock_page() with the
    side-effect that page->mapping needed to be treated very carefully.
    
    Two defensive warnings were added in case any assumption was missed and
    the first warning assumed a correct application would not alter a
    mapping backing a futex key.  Since merging, it has not triggered for
    any unexpected case but Mark Rutland reported the following bug
    triggering due to the first warning.
    
      kernel BUG at kernel/futex.c:679!
      Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
      Modules linked in:
      CPU: 0 PID: 3695 Comm: syz-executor1 Not tainted 4.13.0-rc3-00020-g307fec773ba3 #3
      Hardware name: linux,dummy-virt (DT)
      task: ffff80001e271780 task.stack: ffff000010908000
      PC is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679
      LR is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679
      pc : [<ffff00000821ac14>] lr : [<ffff00000821ac14>] pstate: 80000145
    
    The fact that it's a bug instead of a warning was due to an unrelated
    arm64 problem, but the warning itself triggered because the underlying
    mapping changed.
    
    This is an application issue but from a kernel perspective it's a
    recoverable situation and the warning is unnecessary so this patch
    removes the warning.  The warning may potentially be triggered with the
    following test program from Mark although it may be necessary to adjust
    NR_FUTEX_THREADS to be a value smaller than the number of CPUs in the
    system.
    
        #include <linux/futex.h>
        #include <pthread.h>
        #include <stdio.h>
        #include <stdlib.h>
        #include <sys/mman.h>
        #include <sys/syscall.h>
        #include <sys/time.h>
        #include <unistd.h>
    
        #define NR_FUTEX_THREADS 16
        pthread_t threads[NR_FUTEX_THREADS];
    
        void *mem;
    
        #define MEM_PROT  (PROT_READ | PROT_WRITE)
        #define MEM_SIZE  65536
    
        static int futex_wrapper(int *uaddr, int op, int val,
                                 const struct timespec *timeout,
                                 int *uaddr2, int val3)
        {
            syscall(SYS_futex, uaddr, op, val, timeout, uaddr2, val3);
        }
    
        void *poll_futex(void *unused)
        {
            for (;;) {
                futex_wrapper(mem, FUTEX_CMP_REQUEUE_PI, 1, NULL, mem + 4, 1);
            }
        }
    
        int main(int argc, char *argv[])
        {
            int i;
    
            mem = mmap(NULL, MEM_SIZE, MEM_PROT,
                   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    
            printf("Mapping @ %p\n", mem);
    
            printf("Creating futex threads...\n");
    
            for (i = 0; i < NR_FUTEX_THREADS; i++)
                pthread_create(&threads[i], NULL, poll_futex, NULL);
    
            printf("Flipping mapping...\n");
            for (;;) {
                mmap(mem, MEM_SIZE, MEM_PROT,
                     MAP_FIXED | MAP_SHARED | MAP_ANONYMOUS, -1, 0);
            }
    
            return 0;
        }
    Reported-and-tested-by: default avatarMark Rutland <mark.rutland@arm.com>
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Cc: stable@vger.kernel.org # 4.7+
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    48fb6f4d
futex.c 93.8 KB