[Libguestfs] [PATCH libnbd] generator: Fix race condition when validating h->state.

[Eric Blake]

On 6/4/19 6:23 AM, Richard W.M. Jones wrote:
> The code for (eg) nbd_aio_pread starts by checking this outside
> the lock:
> 
>   if (!(nbd_aio_is_ready (h) || nbd_aio_is_processing (h))) {
> 
> However there can still be a race condition even though h->state is
> atomic:
> 
>   Thread A                         Thread B
>   (in a call that holds h->lock)   (calling nbd_aio_pread)
>   --------------------------------------------------------------------
>   h->state is processing
>                                    checks nbd_aio_is_ready
>                                      (it's false)
>   h->state is moved to READY
>                                    checks nbd_aio_is_processing
>                                      (it's false)
>                                    validation check fails
> 
> However the state was valid so the validation check should have
> succeeded.
> 
> Fixes commit e63a11736930c381a79a8cc2d03844cfff5db3ef.
> 
> Thanks: Eric Blake for identifying the race condition above.
> ---
>  generator/generator | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/generator/generator b/generator/generator
> index ea6f739..8068762 100755
> --- a/generator/generator
> +++ b/generator/generator
> @@ -2820,10 +2820,9 @@ let generate_lib_api_c () =
>        pr "  nbd_internal_reset_error (\"nbd_%s\");\n" name;
>        pr "\n"
>      );
> +    if is_locked then
> +      pr "  pthread_mutex_lock (&h->lock);\n";
>      if permitted_states <> [] then (
> -      pr "  /* We can check the state outside the handle lock because the\n";
> -      pr "   * the state is atomic.\n";
> -      pr "   */\n";
>        let tests =
>          List.map (
>            function

Doesn't this code emit 'return -1' on error,

> @@ -2842,8 +2841,6 @@ let generate_lib_api_c () =
>        pr "  }\n";
>        pr "\n"
>      );
> -    if is_locked then
> -      pr "  pthread_mutex_lock (&h->lock);\n";

but returning -1 without releasing the lock is wrong. I would expect a
bit more change to keep the locking sane, but once that is in place, the
patch does solve one race.

>      pr "  ret = nbd_unlocked_%s (h" name;
>      let argnames = List.flatten (List.map name_of_arg args) in
>      List.iter (pr ", %s") argnames;
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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