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

[Marshall Clow]

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;
+inline _LIBCPP_INLINE_VISIBILITY
+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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