Re: siginfo_t ABI break on sparc64 from si_addr_lsb move 3y ago

[Arnd Bergmann <arnd@arndb.de>]

On Thu, Apr 29, 2021 at 7:23 PM Eric W. Biederman <ebiederm@xmission.com> wrote:

> > Which option do you prefer? Are there better options?
>
> Personally the most important thing to have is a single definition
> shared by all architectures so that we consolidate testing.
>
> A little piece of me cries a little whenever I see how badly we
> implemented the POSIX design.  As specified by POSIX the fields can be
> place in siginfo such that 32bit and 64bit share a common definition.
> Unfortunately we did not addpadding after si_addr on 32bit to
> accommodate a 64bit si_addr.
>
> I find it unfortunate that we are adding yet another definition that
> requires translation between 32bit and 64bit, but I am glad
> that at least the translation is not architecture specific.  That common
> definition is what has allowed this potential issue to be caught
> and that makes me very happy to see.
>
> Let's go with Option 3.
>
> Confirm BUS_MCEERR_AR, BUS_MCEERR_AO, SEGV_BNDERR, SEGV_PKUERR are not
> in use on any architecture that defines __ARCH_SI_TRAPNO, and then fixup
> the userspace definitions of these fields.
>
> To the kernel I would add some BUILD_BUG_ON's to whatever the best
> maintained architecture (sparc64?) that implements __ARCH_SI_TRAPNO just
> to confirm we don't create future regressions by accident.
>
> I did a quick search and the architectures that define __ARCH_SI_TRAPNO
> are sparc, mips, and alpha.  All have 64bit implementations.

I think you (slightly) misread: mips has "#undef __ARCH_SI_TRAPNO", not
"#define __ARCH_SI_TRAPNO". This means it's only sparc and
alpha.

I can see that the alpha instance was added to the kernel during linux-2.5,
but never made it into the glibc or uclibc copy of the struct definition, and
musl doesn't support alpha or sparc. Debian codesearch only turns up
sparc (and BSD) references to si_trapno.

> I did a quick search and the architectures that define __ARCH_SI_TRAPNO
> are sparc, mips, and alpha.  All have 64bit implementations.  A further
> quick search shows that none of those architectures have faults that
> use BUS_MCEERR_AR, BUS_MCEERR_AO, SEGV_BNDERR, SEGV_PKUERR, nor do
> they appear to use mm/memory-failure.c
>
> So it doesn't look like we have an ABI regression to fix.

Even better!

So if sparc is the only user of _trapno and it uses none of the later
fields in _sigfault, I wonder if we could take even more liberty at
trying to have a slightly saner definition. Can you think of anything that
might break if we put _trapno inside of the union along with _perf
and _addr_lsb?

I suppose in theory sparc64 or alpha might start using the other
fields in the future, and an application might be compiled against
mismatched headers, but that is unlikely and is already broken
with the current headers.

       Arnd