[cfe-dev] RFC: Adding attribute(nonnull) to things in libc++

Reid Kleckner rnk at google.com
Mon Jun 1 09:57:17 PDT 2015

Why should memset / memcpy be attribute nonnull? Is there standardese that
supports that?

On Mon, Jun 1, 2015 at 7:20 AM, Marshall Clow <mclow.lists at gmail.com> wrote:

> This weekend, I got an email from Nuno Lopes informing me that UBSAN now
> paid attention to attribute(nonnull), and he was having some problems with
> it going off when using libc++.
> I did some investigation, and found that he was exactly right - there were
> places (deep inside the vector code, for example) which called
> std::memcpy(null, null, 0) - which is definitely UB.
> In an ideal world, our C library would define ::memcpy with the correct
> annotations, and libc++ would import that into namespace std, and we'd be
> golden.
> But we don't have a C library - we use whatever is provided by the system
> we're running on, so that's not really an option.
> For my testing, I changed libc++'s <cstring> header:
> -using ::memcpy;
> +void* memcpy(void* __s1, const void* __s2, size_t __n)
> __attribute__((nonnull(1, 2)))
> +{ return ::memcpy(__s1, __s2, __n); }
> (similarly for memmove and memcmp), and I found several cases of simple
> code that now UBSAN fires off on:
> such as:    std::vector<int> v;  v.push_back(1);
> and :          int *p = NULL;  std::copy(p,p,p);
> This seems fairly useful to me.
> I would like to hear other people's opinions about:
> * Is adding this kind of UB detection something that people want in libc++?
> * What do people think about wrapping the C library functions to enable
> UBSAN to catch them (this is a separate Q from the first Q, because I can
> see putting these kind of parameter checks into functions that have no
> counterpart in the C library). Sadly, this would NOT affect calls to
> ::memcpy (for example), just std::memcpy.
> * Is that the best way to annotate the declarations? Is there a more
> portable, standard way to do this (things that start with double
> underscores worry me). In any case, I would probably wrap this in a macro
> to support compilers that don't understand whatever mechanism we end up
> using.
> Thanks
> -- Marshall
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