Re: [PATCH v2] x86: signal: move save_altstack_ex out of generic headers

[Al Viro]

On Fri, Apr 03, 2020 at 04:16:06PM -0700, Nick Desaulniers wrote:
> In some configurations (clang+KASAN), sas_ss_reset() may emit calls to
> memset().  This is a problem for SMAP protections on x86, which should
> try to minimize calls to any function not already on short whitelist, in
> order to prevent leaking AC flags or being used as a gadget.
> 
> Linus noted that unsafe_save_altstack() only has callsites in the
> arch-specific arch/x86/kernel/signal.c, and shouldn't be defined in arch
> independent headers.
> 
> Split the logic of unsafe_save_altstack() into two, and move the definitions
> to arch/x86/include/asm/sigframe.h.  This does less work with the SMAP
> guards down.

Just move that into signal_delivered() and that's it.  SMAP or no SMAP -
doing that until the sigframe is set and we are committed to entering
the handler is wrong.