[libc] Expand usage of LIBC_ADD_NULL_CHECKS.

[nickdesaulniers]

1896ee3 added a config option to check for nullptr args to math.h functions. I think we could expand the usage of this macro to additional functions that accept pointer args that are defined by the standard to be non-null (or UB occurs in the case of NULL) (example: memcpy). This would allow us to provide a hardened config.

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Nick Desaulniers (nickdesaulniers)

1896ee3 added a config option to check for nullptr args to math.h functions. I think we could expand the usage of this macro to additional functions that accept pointer args that are defined by the standard to be non-null (or UB occurs in the case of NULL) (example: `memcpy`). This would allow us to provide a hardened config.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

1896ee3 added a config option to check for nullptr args to math.h functions. I think we could expand the usage of this macro to additional functions that accept pointer args that are defined by the standard to be non-null (or UB occurs in the case of NULL) (example: `memcpy`). This would allow us to provide a hardened config.

[AlyElashram]

I can take a look at this if possible 👍. Can you please assign this to me @nickdesaulniers

[nickdesaulniers]

done!

[AlyElashram]

@nickdesaulniers I have a quick question in regards to memcpy and memcmp. The Standard does not define that either of the pointers shouldn't be null and does not specify if there is undefined behavior if either is null.

Should I still add nullchecks to both ?
Here's the Standard I'm currently referencing

[AlyElashram]

@michaelrj-google @SchrodingerZhu any ideas on the question above here. Nick seems OOO for the week.

[nickdesaulniers]

Here's the Standard I'm currently referencing

I recommend using the latest draft from https://www.open-std.org/jtc1/sc22/wg14/www/projects (worth bookmarking that page). Today that draft would be N3301.

The Standard does not define that either of the pointers shouldn't be null and does not specify if there is undefined behavior if either is null.

Here's the relevant citations:

7.26.1.2:

Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4.

7.1.4:

If an argument to a function has an invalid value (such as ... a null pointer...) ... the behavior is undefined.

So I would add null checks to both.