Re: [Qemu-devel] [PATCH] qdict: fix unbounded stack for qdict_array_entries

[Peter Xu]

On Mon, Mar 21, 2016 at 02:58:25PM -0600, Eric Blake wrote:
> On 03/09/2016 06:36 PM, Peter Xu wrote:
> > Sorry to forgot CCing Eric/Markus/Kevin.
> > 
> > This patch title is not correct, which should be:
> > 
> > "Fix unbounded stack warning for qdict_array_entries"
> 
> Keep the 'qdict:' prefix, but yes, adding "warning" helps the commit
> message.
> 
> > 
> > Do I need to re-send with the same content?
> 
> For just the title adjustment, it's up to the maintainer.  Often, a
> maintainer will make small changes like that before sending a pull request.
> 
> > 
> > I'm using g_strdup_printf() here, considering it's most convenient,
> > safe, and as long as it's called rarely only when quorum device
> > opens.
> 
> On the other hand, this information might have been useful...
> 
> > 
> > Thanks.
> > Peter
> > 
> > On Wed, Mar 09, 2016 at 02:03:38PM +0800, Peter Xu wrote:
> >> Signed-off-by: Peter Xu <address@hidden>
> 
> ...in the commit body proper (explaining why you are always allocating,
> because it is not a hot path).  So a v2 might indeed be easier.
> 
> >> +++ b/qobject/qdict.c
> >> @@ -704,19 +704,16 @@ int qdict_array_entries(QDict *src, const char 
> >> *subqdict)
> >>      for (i = 0; i < INT_MAX; i++) {
> >>          QObject *subqobj;
> >>          int subqdict_entries;
> >> -        size_t slen = 32 + subqdict_len;
> >> -        char indexstr[slen], prefix[slen];
> >> -        size_t snprintf_ret;
> >> +        char *prefix = g_strdup_printf("%s%u.", subqdict, i);
> 
> If we were worried that this could be a hot path, you could add a %n and
> &len here...
> 
> >>  
> >> -        snprintf_ret = snprintf(indexstr, slen, "%s%u", subqdict, i);
> >> -        assert(snprintf_ret < slen);
> >> +        subqdict_entries = qdict_count_prefixed_entries(src, prefix);
> >>  
> >> -        subqobj = qdict_get(src, indexstr);
> >> +        /* Remove ending "." */
> >> +        prefix[strlen(prefix) - 1] = 0x00;
> 
> ...to avoid the strlen() call here.  But this is not a hot path, and %n
> always makes me worry about security, so I'm fine with your approach.
> 
> However, 0x00 is a rather verbose way of writing 0 (and even if you want
> verbosity, '\0' is more idiomatic 0x00).
> 
> At this point, if you send a v2 with s/0x00/0/ and the improved commit
> message, you can also include:
> Reviewed-by: Eric Blake <address@hidden>

Will respin just like above, and with you r-b. Thanks!

-- peterx