[PATCH v2 1/2] string: _FORTIFY_SOURCE=3 using __builtin_dynamic_object_size

[Florian Weimer]

* Siddhesh Poyarekar via Libc-alpha:

> diff --git a/NEWS b/NEWS
> index 0820984547..4167f34c13 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -28,6 +28,11 @@ Major new features:
>    The 32-bit RISC-V port requires at least Linux 5.4, GCC 7.1 and binutils
>    2.28.
>  
> +* A new fortification level _FORTIFY_SOURCE=3 is available.  At this level,
> +  glibc may use additional checks that may be computationally expensive.  At
> +  present these checks are available only on LLVM 9 and later.  The latest GCC
> +  available at this time (10.2) does not support this level of fortification.

I think it's still a fixed overhead per call (i.e., not proportional to
buffer size or stack depth).  So the performance warning is perhaps a
bit too strong (likewise in the manual).

Rest of the patch looks okay to me.  Debian Code Search doesn't show any
users of __objsize0 reference, so the macro name should be safe,
although it does not have a __glibc_ prefix.

Thanks,
Florian
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