[Bug middle-end/104854] -Wstringop-overread should not warn for strnlen, strndup and strncmp

["siddhesh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104854

--- Comment #8 from Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #7)
> Moving warnings into the analyzer and scaling it up to be able to run by
> default, during development, sounds like a good long-term plan.  Until that

That's not quite what I'm suggesting here.  I'm not a 100% convinced that those
are the right heuristics at all; the size argument for strnlen, strndup and
strncmp does not intend to describe the size of the passed strings.  It is only
recommended security practice that the *n* variant functions be used instead of
their unconstrained relatives to mitigate overflows.  In fact in more common
cases the size argument (especially in case of strnlen and strncmp) may
describe a completely different buffer or some other application-specific
property.

This is different from the -Wformat-overflow, where there is a clear
relationship between buffer, the format string and the string representation of
input numbers and we're only tweaking is the optimism level of the warnings. 
So it is not just a question of levels of verosity/paranoia.

In that context, using size to describe the underlying buffer of the source
only makes sense only for a subset of uses, making this heuristic quite noisy. 
So what I'm actually saying is: the heuristic is too noisy but if we insist on
keeping it, it makes sense as an analyzer warning where the user *chooses* to
look for pessimistic scenarios and is more tolerant of noisy heuristics.

> happens, rather than gratuitously removing warnings that we've added over
> the years, just because they fall short of the ideal 100% efficacy (as has
> been known and documented), making them easier to control seems like a
> better approach.

It's not just a matter of efficacy here IMO.  The heuristic for strnlen,
strncmp and strndup overreads is too loose for it to be taken seriously.