On Tue, Jun 18, 2013 at 12:29:03PM -0700, Brandon Casey wrote: > > 1. Older versions of curl (and I do not recall which version off-hand, > > but it is not important) stored just the pointer. Calling code was > > required to manage the string lifetime itself. > > Daniel mentions that the change happened in libcurl 7.17. RHEL 4.X > (yes, ancient, dead, I realize) provides 7.12 and RHEL 5.X (yes, > ancient, but still widely in use) provides 7.15. Just pointing it > out. Yeah, I didn't mean to imply "we don't care about these versions", only that our analysis is different between the two sets. We have #ifdefs for curl going back to 7.7.4. That's probably excessive, but AFAIK, we would still work with such old versions. > > It could be a problem when we have multiple handles in play > > simultaneously (we invalidate the pointer that another simultaneous > > handle is using, but do not immediately reset its pointer). > > Don't we have multiple handles in play at the same time? What's going > on in get_active_slot() when USE_CURL_MULTI is defined? It appears to > be maintaining a list of "slot" 's, each with its own curl handle > initialized either by curl_easy_duphandle() or get_curl_handle(). Yes, we do; the dumb http walker will pipeline loose pack and object requests (which makes a big difference when fetching small files). The smart http code may use the curl-multi interface under the hood, but it should only have a single handle, and the use of the multi interface is just for sharing code with the dumb fetch. > So, yeah, this is what I was referring to when I mentioned > "potentially dangerous". Since the current code does not change the > size of the string, the pointer will never change, so we won't ever > invalidate a pointer that another handle is using. Agreed. I did not so much mean to dispute your "potentially dangerous" claim as clarify exactly what the potential is. :) > The other thing I thought was potentially dangerous, was just > truncating the string. Again, if there are multiple curl handles in > use (which I thought was a possibility), then merely truncating the > string that contains the username/password could potentially cause a > problem for another handle that could be in the middle of > authenticating using the string. But, I don't know if there is any > multi-processing happening within the curl library. I don't think curl does any threading; when we are not inside curl_*_perform, there is no curl code running at all (Daniel can correct me if I'm wrong on that). So I think from curl's perspective a truncation and exact rewrite is atomic, and it sees only the final content. I don't know what would happen if you truncated and put in _different_ contents. For example, if curl would have written out half of the username/password, blocked and returned from curl_multi_perform, then you update the buffer, then it resumes writing. IOW, I believe the current code is safe (though in a very subtle way), but if you were to allow password update, I'm not sure if it would be or not (and if not, you would need a per-handle buffer to make it safe). I'm fine with making the safety less subtle (e.g., your patch, with a comment added). > If we _don't_ ever use multiple curl handles, and/or if there is no > threading going on in the background within libcurl, then I don't > think there is really any danger in what the current code does. It > would just be an issue of needlessly rewriting the same string over > and over again, which is probably not a big deal depending on how > often that happens. It should be once per http request. But copying a dozen bytes is probably nothing compared to the actual request. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html