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

[Christian Brauner <christian at brauner dot io>]

On Thu, Apr 18, 2019 at 02:01:55PM +0200, Florian Weimer wrote:
> * 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.

Just saw this fly by so sorry if I miss the the necessary context: If
there doesn't need to be a file on disk what about using memfd_create()
on kernels that support it?

> 
> > 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