Re: [Xen-devel] [PATCH 2/3] libxl: Hold the atfork lock while closing carefd

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-02-24 at 14:19 +0000, Ian Jackson wrote:
> This avoids the process being forked while a carefd is recorded in the
> list but the actual fd has been closed.  If that happened, a
> subsequent libxl_postfork_child_noexec would attempt to close the fd
> again.  If we are lucky that results in a harmless warning; but if we
> are unlucky the fd number has been reused and we close an unrelated
> fd.
> 
> This race has not been observed anywhere as far as we are aware.
> 
> Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>

Acked-by: Ian Campbell <Ian.Campbell@xxxxxxxxxx>

> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> ---
>  tools/libxl/libxl_fork.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/libxl/libxl_fork.c b/tools/libxl/libxl_fork.c
> index 8421296..fa15095 100644
> --- a/tools/libxl/libxl_fork.c
> +++ b/tools/libxl/libxl_fork.c
> @@ -184,9 +184,9 @@ void libxl_postfork_child_noexec(libxl_ctx *ctx)
>  int libxl__carefd_close(libxl__carefd *cf)
>  {
>      if (!cf) return 0;
> +    atfork_lock();
>      int r = cf->fd < 0 ? 0 : close(cf->fd);
>      int esave = errno;
> -    atfork_lock();
>      LIBXL_LIST_REMOVE(cf, entry);
>      atfork_unlock();
>      free(cf);



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