Re: [musl] REG_SP Definition for RISC-V

[Palmer Dabbelt <palmer@dabbelt.com>]

On Tue, 04 Feb 2020 06:26:31 PST (-0800), dalias@libc.org wrote:
> On Tue, Feb 04, 2020 at 10:03:59AM +0000, Mark Corbin wrote:
>> 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?
>
> If you want a configure test to detect it the yes; otherwise no. But
> this suggests the way we did it is wrong. We should not be making this
> kind of mess. I should probably just move the definitions...

The glibc definitions are in sys/ucontext.h as that's also where the relevant
structures are defined.  They're all within a _GNU_SOURCE to avoid polluting
the namespace too much.  Maybe the best bet is to have a riscv64-specific
sys/ucontext.h?  I don't see any other ports with their own sys/ucontext.h,
though.