Re: NULL

[John Spencer <maillist-musl@barfooze.de>]

On 01/09/2013 01:18 PM, Szabolcs Nagy wrote:
> * John Spencer<maillist-musl@barfooze.de>  [2013-01-09 12:02:29 +0100]:
>> at this point the issue in evince is not fixed, i still need to
>> create a patch based on their current git master, subscribe to their
>> ML, send them the patch and hope that they haven't changed their
>> mind.
> i think you are overcomplicating this issue
>
> this is clearly a violation of the standard and that's what
> you should tell to the maintainers:
>
> using NULL in the argument of variadic functions is ub both
> in c and c++

many developers don't care about the standard. they take the stance:
"works for me, if you want it patched then do it yourself and we'll 
eventually merge"
> the problem is worse in c++ because it actually breaks code
> in practice with the usual c++ definition of NULL, in c the
> usual definition happens to work
>
>> so for me, there are 3 options how to deal with issue in the future:
>>
>> 1) go into the trouble of debugging all future C++ apps i will port,
> i think this issue should be catched with automated tools, not debugging:
> static code analyzer or runtime instrumentation of vararg functions

yes, as i proposed in 3)

>> (as a side note: when i googled for g_signal_emit, i found multiple
>> bug reports to multiple projects, of which the backtraces showed
>> exactly the kind of varargs UB that i encountered.
>> all of them were not fixed, but merely worked around. apparently the
>> developers of these projects didnt find the real cause of their
>> bugs.
>> this is not unlikely, as it is very hard to find out whats going on.
>> https://bugs.launchpad.net/compiz/+bug/932382
>> https://trac.transmissionbt.com/ticket/1135 ... )
> may be this should get a bit more publicity,
> it's an easy to fix bug

yes, once you know the details, it's easy to fix.
but when you don't, you'll have a hard time figuring out where the 
segfault comes from.
i agree that this should get more publicity.

>> 2) change musl so it is compatible with those apps. this would mean:
>> #if defined(__GNUC__)&&  defined(__cplusplus__)
>> #define NULL __null
> i think this is not needed, you can have a definition
> in c++ that "happens to work" just like the (void*)0
> in c:
>
> #define NULL 0L

yes, that'll work as well.

> but this is just a workaround, the bugs still need to be fixed

again, some ppl don't agree, even if you cite the standard.
i even heard things like "using a cast is ugly (or C'ish), we prefer NULL"

> (in c++11 we could use nullptr which has std::nullptr_t type
> which converts to (void*)0 in vararg context, but c++11 is not
> widely used yet)
>
>> 3) create some kind of code analysis tool that will scan C++ code
> i don't know any good open source static code analyzer for
> c and c++, maybe the clang based one can do the job

did you look at cppcheck ? i think this could be the right tool for the job.
if we raise awareness of the issue, i'm sure they'll add a check for this.

> i tried splint but that does not catch it
>
> catching null pointers in general can be tricky as 0 may be a
> valid argument to a variadic function and the code analyzer has
> no way to tell if it's meant to be a pointer or not
>
> but if NULL is used without a cast then that's an error both in
> c and c++ as both languages allow various different definitions
> of NULL which may have different size and representation in
> vararg context
>
>> i'm welcoming any comments on the issue. i'm especially interested
>> what the gentoo developers think.
> i wonder what makes them special :)
>
well, from what i heard on IRC they started to work  on a musl port 2 
weeks ago (but it got silent since...).
since they have likely more packages than sabotage (350) this issue 
could cause them major pain.