Re: Linux powerpc new system call instruction and ABI

Excerpts from Dmitry V. Levin's message of May 19, 2021 11:26 pm:

On Wed, May 19, 2021 at 08:59:05PM +1000, Nicholas Piggin wrote:

quoted

Excerpts from Dmitry V. Levin's message of May 19, 2021 8:24 pm:

quoted

On Wed, May 19, 2021 at 12:50:24PM +1000, Nicholas Piggin wrote:
[...]

quoted

With this patch, I think the ptrace ABI should mostly be fixed. I think 
a problem remains with applications that look at system call return 
registers directly and have powerpc specific error cases. Those probably
will just need to be updated unfortunately. Michael thought it might be
possible to return an indication via ptrace somehow that the syscall is
using a new ABI, so such apps can be updated to test for it. I don't 
know how that would be done.

Is there any sane way for these applications to handle the scv case?
How can they tell that the scv semantics is being used for the given
syscall invocation?  Can this information be obtained e.g. from struct
pt_regs?

Not that I know of. Michael suggested there might be a way to add 
something. ptrace_syscall_info has some pad bytes, could
we use one for flags bits and set a bit for "new system call ABI"?

PTRACE_GET_SYSCALL_INFO is an architecture-agnostic API, it hides all
architecture-specific details behind struct ptrace_syscall_info which has
the same meaning on all architectures.  ptrace_syscall_info.exit contains
both rval and is_error fields to support every architecture regardless of
its syscall ABI.

ptrace_syscall_info.exit is extensible, but every architecture would have
to define a method of telling whether the system call follows the "new
system call ABI" conventions to export this bit of information.

It's already architecture speicfic if you look at registers of syscall 
exit state so I don't see a problem with a flag that ppc can use for
ABI.

This essentially means implementing something like
static inline long syscall_get_error_abi(struct task_struct *task, struct pt_regs *regs)
for every architecture, and using it along with syscall_get_error
in ptrace_get_syscall_info_exit to initialize the new field in
ptrace_syscall_info.exit structure.

Yes this could work. Other architectures can just use a generic
implementation if they don't define their own so that's easy. And
in userspace they can continue to ignore the flag.

quoted

As a more hacky thing you could make a syscall with -1 and see how
the error looks, and then assume all syscalls will be the same.

This would be very unreliable because sc and scv are allowed to intermingle,
so every syscall invocation can follow any of these two error handling
conventions.

quoted

Or... is it possible at syscall entry to peek the address of
the instruction which caused the call and see if that was a
scv instruction? That would be about as reliable as possible
without having that new flag bit.

No other architecture requires peeking into tracee memory just to find out
the syscall ABI.  This would make powerpc the most ugly architecture for
ptracing.

I wonder why can't this information be just exported to the tracer via
struct pt_regs?

It might be able to, I don't see why that would be superior though.

Where could you put it... I guess it could go in the trap field in a 
high bit. But could that break things that just test for syscall 
trap number (and don't care about register ABI)? I'm not sure.

Thanks,
Nick