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Date: Mon, 03 Feb 2020 15:17:15 +0000
From: Mark Corbin <mark@...sco.co.uk>
To: musl@...ts.openwall.com
Cc: Rich Felker <dalias@...c.org>
Subject: Re: REG_SP Definition for RISC-V

On Monday, 3 February 2020 13:32:25 GMT Rich Felker wrote:
> On Mon, Feb 03, 2020 at 11:42:30AM +0000, Mark Corbin wrote:
> > Hello
> > 
> > I'm trying to fix a build issue with libsigsegv [1] for RISC-V when
> > compiling against musl 1.1.24 (under Buildroot).
> > 
> > The build fails because the array index 'REG_SP' (for indexing into
> > uc_mcontext.__gregs[]) is not defined in arch/riscv64/bits/signal.h. This
> > constant is defined by glibc in
> > sysdeps/unix/sysv/linux/riscv/sys/ucontext.h
> > 
> > I was wondering whether the appropriate fix is just to add '#define REG_SP
> > 2' to the top of arch/riscv64/bits/signal.h ? (Note that there is a
> > REG_SP definition in arch/riscv64/bits/reg.h which isn't being included).
> > 
> > Alternatively I could submit a patch to libsigsegv to modify the index
> > into
> > the '__gregs' array to be '2' rather than 'REG_SP', however there could be
> > other glibc compatible RISC-V packages that make use of the 'REG_SP'
> > definition.
> > 
> > I'm happy to generate and submit any patches as appropriate.
> 
> Generally, we like to avoid this kind of REG_* (or even bare names)
> register macro in signal.h since it's highly namespace-polluting (can
> break software using them for its own purposes that has no knowledge
> that some arch has a reg by that name in its signal.h bits) and only
> expose them under _GNU_SOURCE when we do. Right now musl has them
> exposed via <sys/reg.h>. I'm not sure if there's any precedent for
> that or if glibc only has them in <signal.h>

I spent some time looking for a good method of handling this, but couldn't 
really find any consistency between architectures. I think that most of them 
access the appropriate register array using a numeric value rather than a 
register name in this scenario.

> 
> So my leaning would be to leave it as it is and ask applications to
> include <sys/reg.h> if they want these macros. But if it looks like
> this is contrary to what maintainers of other software want to do, we
> could consider putting them under _GNU_SOURCE with <signal.h> like
> many other archs do.

I guess that it would probably be best to change the libsigsegv code to use a 
value of '2' instead of the REG_SP definition. I'll look at submitting a patch 
to the project.

Thanks

Mark Corbin



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