PRIV_CALL, VRT_re_init, and mutexes

[Stephen J. Butler]

In the varnish source code for the 6.0 release, I found one use of
PRIV_CALL in vmod_std, vmod_fileread(). This function does have locks, but
they're all to protect modifications of the global frlist variable. It does
seem like the priv parameter is used as a call site value but no care is
taken to make sure access is exclusive.

In that case it also bothers me that there's an assumption that -- and ++
are atomic operations on all platforms and compilers... some quick Googling
suggests that's not the case.


On Fri, Apr 27, 2018 at 1:49 AM, Stephen J. Butler <stephen.butler at gmail.com
> wrote:

> Sorry, didn't reply to the list:
>
> On Fri, Apr 27, 2018 at 1:48 AM, Stephen J. Butler <
> stephen.butler at gmail.com> wrote:
>
>> Hmm. If PRIV_CALL is private to the call site, then I think
>> in vmod_bodyaccess.c, vmod_rematch_req_body()there's a problem. In that
>> case it calls VRE_compile()to initiate a regex without a lock on a
>> PRIV_CALL structure.
>>
>> On Fri, Apr 27, 2018 at 1:37 AM, Guillaume Quintard <
>> guillaume at varnish-software.com> wrote:
>>
>>> Hi, and welcome!
>>>
>>> In this case, priv comes from PRIV_CALL, meaning it's going to be the
>>> same for all the request/vcl that call the function from the same spot.
>>> This is done to cache the regex, so you only compile for the first call.
>>>
>>> However, I'm thinking that you may have the same priv with a different
>>> string, and then everything goes to hell. Could anyone familiar with
>>> PRIV_CALL confirm?
>>>
>>> Cheers,
>>>
>>> --
>>> Guillaume Quintard
>>>
>>> On Fri, Apr 27, 2018 at 8:05 AM, Stephen J. Butler <
>>> stephen.butler at gmail.com> wrote:
>>>
>>>> I'm a new developer/user for Varnish, and was looking to extend a
>>>> module with some regex support (vmod_cookie if you must know). I don't know
>>>> anything about vmod development or Varnish threading, so to get an idea of
>>>> how to do this I looked at the vmod_header module.
>>>>
>>>> But I'm confused about why they thought they needed a mutex to
>>>> protected a call to VRT_re_init. Is this just a useless, historic thing
>>>> that got left in for some reason?
>>>>
>>>> https://github.com/varnish/varnish-modules/blob/master/src/v
>>>> mod_header.c
>>>>
>>>> If you look at vmod_remove it has a 2nd parameter that's in the vcc as
>>>> PRIV_CALL. This parameter is initialized with the string value from the
>>>> last parameter (a regex pattern). It does this by calling
>>>> header_init_re(). And if you look at that function you see:
>>>>
>>>> /*
>>>> * Initialize the regex *s on priv, if it hasn't already been done.
>>>> * XXX: We have to recheck the condition after grabbing the lock to
>>>> avoid a
>>>> * XXX: race condition.
>>>> */
>>>> static void
>>>> header_init_re(struct vmod_priv *priv, const char *s)
>>>> {
>>>> if (priv->priv == NULL) {
>>>> assert(pthread_mutex_lock(&header_mutex) == 0);
>>>> if (priv->priv == NULL) {
>>>> VRT_re_init(&priv->priv, s);
>>>> priv->free = VRT_re_fini;
>>>> }
>>>> pthread_mutex_unlock(&header_mutex);
>>>> }
>>>> }
>>>> My reading of the docs makes me think that this "priv" only lives for
>>>> each single call of the function. There should be no reason to protect it
>>>> with a global mutex. Do the internals of VRT_re_init() require this? I
>>>> can't imagine how, because if it did then this method of protecting it is
>>>> broken anyway.
>>>>
>>>> I suspect this is leftover from some rewrite or updating of the module
>>>> but wanted to check.
>>>>
>>>>
>>>> _______________________________________________
>>>> varnish-dev mailing list
>>>> varnish-dev at varnish-cache.org
>>>> https://www.varnish-cache.org/lists/mailman/listinfo/varnish-dev
>>>>
>>>
>>>
>>
>
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