Re: [musl] REG_SP Definition for RISC-V

[Mark Corbin <mark@dibsco.co.uk>]

On Monday, 3 February 2020 15:24:27 GMT Rich Felker wrote:
> On Mon, Feb 03, 2020 at 03:17:15PM +0000, Mark Corbin wrote:
> > 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.
> 
> I think using a symbolic name is both more informative and more
> portable (since the layout of the saved registers is an OS choice,
> nothing universal to the architecture). The question is just where the
> macro should be obtained from. As long as glibc (and any other
> platforms that might be relevant?) has a sys/reg.h, it wouldn't hurt
> to just add the include and continue using the macro, regardless of
> whether musl moves it later.
> 

Glibc and uClibc don't have a sys/reg.h - is there a way that it could be 
included conditionally for musl only?

Thanks

Mark