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Date: Fri, 20 Aug 2021 08:46:02 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Rich Felker <dalias@...c.org>
Cc: Florian Weimer <fweimer@...hat.com>,
	Olivier Galibert <galibert@...ox.com>, musl@...ts.openwall.com
Subject: Re: [PATCH] arm64/sigcontext: Synchronize the type of the
 __reserved field with the linux kernel.

* Rich Felker <dalias@...c.org> [2021-08-19 09:38:41 -0400]:
> On Thu, Aug 19, 2021 at 07:54:12AM +0200, Florian Weimer wrote:
> > * Rich Felker:
> > 
> > > On Thu, Aug 19, 2021 at 12:52:23AM +0200, Olivier Galibert wrote:
> > >> clang's compiler-rt sanitizer_linux.cpp expects the __reserved field
> > >> to be convertible to u8 *.  So let's.
> > >> ---
> > >>  arch/aarch64/bits/signal.h | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >> 
> > >> diff --git a/arch/aarch64/bits/signal.h b/arch/aarch64/bits/signal.h
> > >> index 5098c734..a46997e3 100644
> > >> --- a/arch/aarch64/bits/signal.h
> > >> +++ b/arch/aarch64/bits/signal.h
> > >> @@ -19,7 +19,7 @@ typedef struct sigcontext {
> > >>  	unsigned long fault_address;
> > >>  	unsigned long regs[31];
> > >>  	unsigned long sp, pc, pstate;
> > >> -	long double __reserved[256];
> > >> +	unsigned char __reserved[4096] __attribute__((__aligned__(16)));
> > >>  } mcontext_t;
> > >>  
> > >>  #define FPSIMD_MAGIC 0x46508001
> > >
> > > The member name __reserved is not API, much less its particular type.
> > 
> > The name is called __reserved, but it is actually part of the API.
> > We learned this when we tried to rename it:
> > 
> >   <https://sourceware.org/bugzilla/show_bug.cgi?id=22742>
> > 
> > The name and its __ prefix are rather unfortunate, but we are stuck with
> > it.
> 
> I question the reasoning there. Just because there are users of it
> doesn't mean it's API, *especially* if the users are things like
> sanitizer lib that regularly poke at internals that are not interface
> contracts. Use of a name like __reserved as API is *really* bad since
> it could even be something like a macro for an attribute in the
> implementation, rather than something available as a member name.
> 
> My interpretation was that it's something like the powerpc reserved
> space where there's a separate pointer into it, which you're supposed
> to access it by. But that doesn't seem to be the case here, so I'm not
> sure what the right way to access it is. Do you have a list of
> software that's actually poking at it so we can evaluate the situation
> better for figuring out what to do?

__reserved holds sigcontext extensions so i'd expect
code accessing the extended register state from a
signal handler would use it (includes fp/simd regs).

e.g. chromium tests seem to use it:

https://source.chromium.org/search?q=uc_mcontext.__reserved

but the ucontext access code in breakpad is written
in asm so the c struct is not used in actual code.

qemu seems to define its own types. and i don't
immediately know of other tools that poke at
sigcontext.

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