Re: [PATCH] http.c: don't rewrite the user:passwd string multiple times

[Brandon Casey <drafnel@gmail.com>]

On Tue, Jun 18, 2013 at 3:13 PM, Jeff King <peff@peff.net> wrote:
> On Tue, Jun 18, 2013 at 12:29:03PM -0700, Brandon Casey wrote:

>> > 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. :)

Ah, yes, I did read your sentence "It could be a problem when we have
multiple handles in play simultaneously" as "It could be a problem [at
some point in the future] when we [modify the code to] have multiple
handles in play simultaneously, [but since we are not doing that now,
it is not a problem]".  Now that I read that sentence again, I see you
are alluding to the dumb http walker code path that I was also
thinking about.

>> 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).

Ok, will do.

-Brandon