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
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
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
> 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 majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html