Re: [PATCTH 0/2] pthread_mutexattr_setrobust() and pthread_mutex_consistent()

[Yubin Ruan <ablacktshirt@gmail.com>]

On Tue, Sep 12, 2017 at 02:41:29PM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Yubin,
> 
> [...]
> > +.B PTHREAD_MUTEX_ROBUST
> > +can be set on a mutex attribute object so that when the owner of the mutex
> > +dies or when the process containing such a locked mutex performs
> > +.IR execve (2)
> > +, any future attempts to call
> > +.IR pthread_mutex_lock (3)
> > +on this mutex will suceed and return
> > +.B EOWNERDEAD
> > +to indicate that the original owner no longer exists and the mutex is left in
> > +an inconsistent state. 
> How did you verify the point regarding execve(2)? I don't see this
> detailed mentioned in the standards or in the glibc source.

Please see below the program I used to verify that. I haven't go into too much
detail in the POSIX standard, though. I think I must have read it at [1] or
somewhere else (don't remember...).

And also, it is mentioned at [1] that when the process containing such a locked
mutex unmaps the memory containing the mutex, the mutex is unlocked... I think
this is trivial so I don't add it.

Thanks,
Yubin

[1]: https://docs.oracle.com/cd/E19253-01/816-5168/pthread-mutexattr-setrobust-np-3c/index.html

/************ verify-execve.c *****************/
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define VERIFY_KEY 20170010
#define ERROR_ON(func_name) \
    fprintf(stderr, "error: " #func_name ": line[%d]: %s\n", __LINE__, strerror(errno));

int main(int argc, char *argv[]) {
    int shmid = -1;
    struct shm *shm = NULL;
    mode_t previous_umask = -1;
    int ret_code = 0;
    pthread_mutex_t *mutexp = NULL;
    pthread_mutexattr_t attr;
    pid_t pid = 0;
    char *const * execve_arg = {"cat", NULL};
    char *const * execve_env = {NULL};

    previous_umask = umask(0);
    shmid = shmget(VERIFY_KEY, sizeof(pthread_mutex_t), IPC_CREAT | 0666);
    if (shmid < 0) {
        ERROR_ON(shmget);
        return -1;
    }

    shm = (struct shm *)shmat(shmid, NULL, 0);
    if ((void *)-1 == shm) {
        ERROR_ON(shmat);
        return -1;
    }
    memset(shm, 0, sizeof(pthread_mutex_t));

    printf("Successfully attached shared memory, trying to lock\n");

    //initialize the lock
    mutexp = (pthread_mutex_t *)shm;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST);
    pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
    pthread_mutex_init(mutexp, &attr);

    ret_code = pthread_mutex_lock(mutexp);
    if (0 == ret_code) {
        printf("successfull acquired the lock. Going to fork/execve now\n");
    } else {
        ERROR_ON(pthread_mutex_lock);
        return -1;
    }

    pid = fork();
    if (0 == pid) {
        printf("child would sleep for 2 sec and then lock the mutex\n");
        sleep(2);
        ret_code = pthread_mutex_lock(mutexp);
        if (EOWNERDEAD == ret_code) {
            printf("child see EOWNERDEAD returned. Verification completed\n");
            pthread_mutex_consistent(mutexp);
            pthread_mutex_unlock(mutexp);
            exit(0);
        } else {
            printf("child see [%d] returned\n", ret_code);
            exit(1);
        }
    } else {
        printf("parent going to execve(/bin/cat)\n");
        execve("/bin/cat", execve_arg, execve_env);
    }
    return 0;
}