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

[Andreas Schwab <schwab@suse.de>]

On Apr 18 2019, Florian Weimer <fweimer@redhat.com> 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.

That shortens the window, but won't close it, since we need a name to
pass to rename in any case.

>> 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.

Yes, O_NOFOLLOW is useless anyway, since mk[o]stemp is about creating a
new file.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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