Re: [PATCH] Avoid concurrency problem in ldconfig (bug 23973)

[Florian Weimer <fweimer at redhat dot com>]

* Andreas Schwab:

> Use a unique name for the temporary file when updating the ld.so cache, so
> that two concurrent runs of ldconfig don't write to the same file.
>
> 	* elf/cache.c (save_cache): Use unique temporary name.
> 	(save_aux_cache): Likewise.

The downside of this change is that if ldconfig is interrupted, the
temporary file never goes away.

Ideally, we would use O_TMPFILE if supported by the (file) system, but
that can get quite involved.

> diff --git a/elf/cache.c b/elf/cache.c
> index b8e9e6ccc3..ec7d94b0bc 100644
> --- a/elf/cache.c
> +++ b/elf/cache.c
> @@ -427,12 +427,12 @@ save_cache (const char *cache_name)
>    /* Write out the cache.  */
>  
>    /* Write cache first to a temporary file and rename it later.  */
> -  char *temp_name = xmalloc (strlen (cache_name) + 2);
> -  sprintf (temp_name, "%s~", cache_name);
> +  char *temp_name;
> +  if (asprintf (&temp_name, "%s.XXXXXX", cache_name) < 0)
> +    error (EXIT_FAILURE, errno, _("Can't allocate temporary name for cache file"));
>  
>    /* Create file.  */
> -  int fd = open (temp_name, O_CREAT|O_WRONLY|O_TRUNC|O_NOFOLLOW,
> -		 S_IRUSR|S_IWUSR);
> +  int fd = mkostemp (temp_name, O_NOFOLLOW);

I think you can use mkstemp because O_NOFOLLOW is implied by its use of
O_EXCL.

> +  int fd = mkostemp (temp_name, O_NOFOLLOW);

Likewise.

Thanks,
Florian