On Fri, Jul 05, 2019 at 05:01:33PM +0200, Tomas Härdin wrote: > ons 2019-07-03 klockan 10:46 +0200 skrev Michael Niedermayer: > > On Wed, Jul 03, 2019 at 09:41:41AM +0200, Reimar Döffinger wrote: > > > > > > > > > On 03.07.2019, at 08:29, Michael Niedermayer <mich...@niedermayer.cc> > > > wrote: > > > > > > > On Tue, Jul 02, 2019 at 08:42:43PM -0300, James Almer wrote: > > > > > > > > > > How many cases are there in the codebase where cnt can be 0, and dst > > > > > or > > > > > src NULL, without it having been checked before calling memcpy? And > > > > > from > > > > > those, which would not be from situations where the code should have > > > > > instead aborted and returned ENOMEM, or EINVAL if either of them are > > > > > function arguments? > > > > > > > > There are around 2500 occurances of memcpy in the codebase > > > > To awnser your question it would be needed to review all of them and in > > > > many > > > > cases their surrounding code. > > > > So that is unlikely to be awnsered by anyone accuratly > > > > > > > > Also iam not sure i understand why you ask or why this would matter > > > > the suggested function allows to simplify cases where the NULL can > > > > occur, not where it cannot or should not. That is this is intended for > > > > the cases where we already have or are adding explicit checks to > > > > avoid the NULL case. > > > > > > > > i could rename this to av_memcpy_nullsafe which would make it clearer > > > > but > > > > also its more to write and read > > > > > > I admit I thought that a worthwhile idea originally, > > > but I have to think back to a long time ago that every function > > > added to our "API" has a cost of people having to know about it and > > > how to use it. > > > And if it's currently only 2 places that would benefit I think > > > James is right to ask if it makes sense. > > > Of course another question might be if it might make sense to > > > replace all memcpy uses with it. > > > I mean, isn't it naturally expected behaviour that the pointers > > > would be ignored if the copy amount is 0? There might be a lot of > > > code assuming that we do not know about... > > > > in addition to the 2 there are these related commits found by very dumb git > > log greps > > In further addition there would be cases that deal with src == dst, > > something we > > could add a check for in av_memcpy() too > > All of these proposed "solutions" are equally horrible
>
> I'm going to raise the issue of formal verification again (Frama-C and
> friends), because crap like this should be checked at compile time, not
> with flakey runtime checks.
>
> I gave adding ACSL markup to parts of lavu a stab a while back, and it
> looked somewhat promising. Bit hacks present a bit of a problem
> however..
As we are already off topic, heres an example to test static
analysis, does this trigger undefined behavior by executing the memcpy
for some user input ?
void f(unsigned bigint a) {
bigint i;
for (i = 2; (((bigint)1 << a) + 1) % i; i++)
;
if (a > 20 && i > ((bigint)1 << a))
memcpy(NULL, NULL, 0);
}
i know its a lame example but just to remind that static analysis has
limitations. (your mail sounds a bit like static analysis could replace
everything ...)
Thanks
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
In fact, the RIAA has been known to suggest that students drop out
of college or go to community college in order to be able to afford
settlements. -- The RIAA
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