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

Arnd Bergmann [off-list ref] writes:

On Thu, Apr 29, 2021 at 7:23 PM Eric W. Biederman [off-list ref] wrote:

quoted

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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.

quoted

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?

On sparc si_trapno is only set when SIGILL ILL_TRP is set.  So we can
limit si_trapno to that combination, and it should not be a problem for
a new signal/si_code pair to use that storage.  Precisely because it is
new.

Similarly on alpha si_trapno is only set for:

SIGFPE {FPE_INTOVF, FPE_INTDIV, FPE_FLTOVF, FPE_FLTDIV, FPE_FLTUND,
FPE_FLTINV, FPE_FLTRES, FPE_FLTUNK} and SIGTRAP {TRAP_UNK}.

Placing si_trapno into the union would also make the problem that the
union is pointer aligned a non-problem as then the union immediate
follows a pointer.

I hadn't had a chance to look before but we must deal with this.  The
definition of perf_sigtrap in 42dec9a936e7696bea1f27d3c5a0068cd9aa95fd
is broken on sparc, alpha, and ia64 as it bypasses the code in
kernel/signal.c that ensures the si_trapno or the ia64 special fields
are set.

Not to mention that perf_sigtrap appears to abuse si_errno.

The code is only safe if the analysis that says we can move si_trapno
and perhaps the ia64 fields into the union is correct.  It looks like
ia64 much more actively uses it's signal extension fields including for
SIGTRAP, so I am not at all certain the generic definition of
perf_sigtrap is safe on ia64.

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.

If we localize the use of si_trapno to just a few special cases on alpha
and sparc I think we don't even need to worry about breaking userspace
on any architecture.  It will complicate siginfo_layout, but it is a
complication that reflects reality.

I don't have a clue how any of this affects ia64.  Does perf work on
ia64?  Does perf work on sparc, and alpha?

If perf works on ia64 we need to take a hard look at what is going on
there as well.

Eric