Linux powerpc new system call instruction and ABI

[Nicholas Piggin]

Excerpts from Dmitry V. Levin's message of May 20, 2021 9:27 am:
> On Thu, May 20, 2021 at 08:51:53AM +1000, Nicholas Piggin wrote:
>> 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.
> 
> To be honest, I don't see anything architecture-specific in
> PTRACE_GET_SYSCALL_INFO API.  Yes, it's implementation uses various
> functions defined in asm/syscall.h, but this doesn't make the interface
> architecture-specific.

No. But a field or flag it exports could be architecture dependent.
It doesn't detract independence from the rest of the ABI. That said...

> PTRACE_GET_SYSCALL_INFO saves its users from necessity to be aware of
> tracee registers.  That's why the only place where strace has to deal
> with tracee registers nowadays is syscall tampering.  The most reliable
> solution is to introduce PTRACE_SET_SYSCALL_INFO, this would make the
> whole syscall abi issue irrelevant for ptracers, maybe the time has come
> to implement it.
> 
> Unfortunately, extending ptrace API takes time, and it's not going to be
> backported to older kernels anyway, but scv-enabled kernels are already
> in the wild, so we need a quick powerpc-specific fix that would be
> backported to all maintained scv-enabled kernels.
> 
> [...]
>> > 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.
> 
> Looks like struct pt_regs.trap already contains the information that could
> be used to tell 'sc' from 'scv': if (pt_regs.trap & ~0xf) == 0x3000, then
> it's scv.  Is my reading of arch/powerpc/include/asm/ptrace.h correct?

Hmm, I think it is. Certainly in the kernel regs struct it is, I had in 
my mind that we put it to 0xc00 when populating the user struct for
compatibility, but it seems not. So I guess this would work.

Thanks,
Nick