Re: [PATCH 6/6] mqueue: fix mq_open() file descriptor leak on user-space processes

[Xiaotian Feng <xtfeng@gmail.com>]

On Tue, Feb 23, 2010 at 3:04 PM, André Goddard Rosa
<andre.goddard@gmail.com> wrote:
> It can be triggered by the following test program:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <mqueue.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <sys/stat.h>
> #include <sys/resource.h>
>
> int main(int argc, char *argv[])
> {
>        struct rlimit limit;
>        char queue_name[] = "/mq_open/bug";
>        char tmp_name[]   = "/tmp/tmp";
>        int fd, i = 1;
>
>        if (getrlimit(RLIMIT_NOFILE, &limit) != 0) {
>                printf("%s\n", "Failed to get RLIMIT_NOFILE");
>                return EXIT_FAILURE;
>        }
>        printf("Max number of open files is: %d\n", limit.rlim_cur);
>
>        while (i <= limit.rlim_cur) {
>                mqd_t queue;
>
>                errno = 0;
>                queue = mq_open(queue_name, O_CREAT |O_RDWR, S_IRUSR | S_IWUSR
>                    , NULL);
>                if (queue != (mqd_t)-1) {
>                        /* Success opening mqueue, no leak will happen */
>                        printf("Successfully opened an mqueue[%d]\n", queue);
>                        printf("mq_close(%d) = %d\n", queue, mq_close(queue));
>                        return EXIT_SUCCESS;
>                }
>                /* Failed to open mqueue, maybe a leak is happening... */
>                if (errno == EMFILE)
>                {
>                        printf("\nRun out of file descriptors");
>                        break;
>                }
>                printf("\rLeaking [%d] files?!?!", i++);
>                fflush(stdout);
>                usleep(500);
>        }
>        /* Double check that no file descriptor is available anymore indeed */
>        putchar('\n');
>        errno = 0;
>        fd = open(tmp_name, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
>        if (fd == -1) {
>                printf("open() failed: %s\n", strerror(errno));
>                if (errno == EMFILE) {
>                        printf("%s\n", "Cannot open new files, fds exhausted");
>                        return EXIT_FAILURE;
>                }
>        } else
>                printf("close(%d) = %d\n", fd, close(fd));
>        printf("%s\n", "Expected output: kernel is not leaking any fds!");
>
>        return EXIT_SUCCESS;
> }
>
> ## Preparing for testing
>
> $ touch /tmp/tmp
> $ gcc -g main_mq_open_fd_leak.c -lrt
>
> ## Linux kernel with the fix applied:
>
> $ ./a.out
> Max number of open files is: 1024
> Leaking [1024] files?!?!
> close(3) = 0
> Expected output: kernel is not leaking any fds!
>
> ## Linux kernel without the fix:
>
> ## Shell execution:
>
> $ ./a.out
> Max number of open files is: 1024
> Leaking [1019] files?!?!
> Run out of file descriptors
> Segmentation fault
>
> ## Valgrind execution:
>
> $ valgrind --track-fds=yes ./a.out
> ==2895== Memcheck, a memory error detector
> ==2895== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
> ==2895== Using Valgrind-3.6.0.SVN and LibVEX; rerun with -h for copyright info
> ==2895== Command: ./a.out
> ==2895==
> Max number of open files is: 1024
> Leaking [1024] files?!?!
> open() failed: Too many open files
> Cannot open new files, fds exhausted
> ==2895==
> ==2895== FILE DESCRIPTORS: 5 open at exit.
> ==2895== Open file descriptor 13:
> ==2895==    <inherited from parent>
> ==2895==
> ==2895== Open file descriptor 12:
> ==2895==    <inherited from parent>
> ==2895==
> ==2895== Open file descriptor 2: /dev/pts/1
> ==2895==    <inherited from parent>
> ==2895==
> ==2895== Open file descriptor 1: /dev/pts/1
> ==2895==    <inherited from parent>
> ==2895==
> ==2895== Open file descriptor 0: /dev/pts/1
> ==2895==    <inherited from parent>
> ==2895==
> ==2895==
> ==2895== HEAP SUMMARY:
> ==2895==     in use at exit: 0 bytes in 0 blocks
> ==2895==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
> ==2895==
> ==2895== All heap blocks were freed -- no leaks are possible
> ==2895==
> ==2895== For counts of detected and suppressed errors, rerun with: -v
> ==2895== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
>
> When not running valgrind, user-space program segfaults trying to execute
> strerror(errno). With valgrind, it executes successfully and prints the
> 5 open files: stdin, stdout, stderr, pipe[0] and pipe[1].
>
> Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>

If kernel failed to lookup the dentry in mqueue,  the fd allocated by
get_unused_fd_flags will be leaked then.
I think this is a good catch ;-)

Acked-by: Xiaotian Feng <xtfeng@gmail.com>

> ---
>  ipc/mqueue.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/mqueue.c b/ipc/mqueue.c
> index e47c9eb..b6cb064 100644
> --- a/ipc/mqueue.c
> +++ b/ipc/mqueue.c
> @@ -710,7 +710,7 @@ SYSCALL_DEFINE4(mq_open, const char __user *, u_name, int, oflag, mode_t, mode,
>        dentry = lookup_one_len(name, ipc_ns->mq_mnt->mnt_root, strlen(name));
>        if (IS_ERR(dentry)) {
>                error = PTR_ERR(dentry);
> -               goto out_err;
> +               goto out_putfd;
>        }
>        mntget(ipc_ns->mq_mnt);
>
> @@ -749,7 +749,6 @@ out:
>        mntput(ipc_ns->mq_mnt);
>  out_putfd:
>        put_unused_fd(fd);
> -out_err:
>        fd = error;
>  out_upsem:
>        mutex_unlock(&ipc_ns->mq_mnt->mnt_root->d_inode->i_mutex);
> --
> 1.7.0.87.g0901d
>
> --
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