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Date: Thu, 20 May 2021 08:51:53 +1000
From: Nicholas Piggin <npiggin@...il.com>
To: "Dmitry V. Levin" <ldv@...linux.org>, Michael Ellerman
<mpe@...erman.id.au>
Cc: Joakim Tjernlund <Joakim.Tjernlund@...inera.com>,
libc-alpha@...rceware.org, libc-dev@...ts.llvm.org, linux-api@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, Matheus Castanho <msc@...ux.ibm.com>,
musl@...ts.openwall.com
Subject: 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:
>> Excerpts from Dmitry V. Levin's message of May 19, 2021 8:24 pm:
>> > On Wed, May 19, 2021 at 12:50:24PM +1000, Nicholas Piggin wrote:
>> > [...]
>> >> 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.
>
>> 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.
>
>> 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
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