Re: [Xen-devel] [PATCH] libxl: libxl__spawn_qdisk_backend closes fds

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On 17/03/14 17:32, Ian Jackson wrote:
> This function needs to close both null and logfile_w on both error and
> normal exits.  (The child gets its own copy during the fork, and the
> parent doesn't need them any more.)
> 
> Use the standard initialise-to-unallocated, always-free style; the
> label "out" only makes the callback if rc is nonzero.
> 
> Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> Coverity-ID: 1130517 and 1130518

There's a similar patch in
<21134.22006.356685.408733@xxxxxxxxxxxxxxxxxxxxxxxx>.

> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: coverity@xxxxxxxxxxxxxx
> ---
>  tools/libxl/libxl_dm.c |   15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
> index 8abed7b..d28ea4d 100644
> --- a/tools/libxl/libxl_dm.c
> +++ b/tools/libxl/libxl_dm.c
> @@ -1392,7 +1392,7 @@ void libxl__spawn_qdisk_backend(libxl__egc *egc, 
> libxl__dm_spawn_state *dmss)
>      flexarray_t *dm_args;
>      char **args;
>      const char *dm;
> -    int logfile_w, null, rc;
> +    int logfile_w = -1, null = -1, rc;
>      uint32_t domid = dmss->guest_domid;
>  
>      /* Always use qemu-xen as device model */
> @@ -1448,12 +1448,21 @@ void libxl__spawn_qdisk_backend(libxl__egc *egc, 
> libxl__dm_spawn_state *dmss)
>          libxl__exec(gc, null, logfile_w, logfile_w, dm, args, NULL);
>      }
>  
> +    rc = 0;
> +
> + out:
> +    if (logfile_w >= 0) close(logfile_w);
> +    if (null >= 0) close(null);
> +
> +    /* rc is nonzero iff we had an error; if we had no error then
> +     * spawn succeeded and we will continue in a further callback */
> +    if (rc)
> +        dmss->callback(egc, dmss, rc);

Using the device_model_spawn_outcome helper instead of directly calling
the callback will print a nice error message in case spawn has failed.

>      return;
>  
>  error:
>      assert(rc);
> -    dmss->callback(egc, dmss, rc);
> -    return;
> +    goto out;

This jump backwards is kind of strange IMHO, why not just rename the
error label to out and use it instead for both the error and non-error
exit paths?

Roger.


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