From: Nicholas Piggin <npiggin@gmail.com>
To: "Dmitry V. Levin" <ldv@altlinux.org>
Cc: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>,
libc-alpha@sourceware.org, libc-dev@lists.llvm.org,
linux-api@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Michael Ellerman <mpe@ellerman.id.au>,
Matheus Castanho <msc@linux.ibm.com>,
musl@lists.openwall.com
Subject: [musl] Re: Linux powerpc new system call instruction and ABI
Date: Thu, 20 May 2021 12:40:36 +1000 [thread overview]
Message-ID: <1621478238.xha1ow4ujh.astroid@bobo.none> (raw)
In-Reply-To: <20210519232726.GA24134@altlinux.org>
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
next prev parent reply other threads:[~2021-05-20 2:40 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-11 8:12 [musl] " Nicholas Piggin
2020-06-11 8:12 ` [musl] [PATCH 1/2] powerpc/64s/exception: treat NIA below __end_interrupts as soft-masked Nicholas Piggin
2020-07-24 13:25 ` [musl] " Michael Ellerman
2020-06-11 8:12 ` [musl] [PATCH 2/2] powerpc/64s: system call support for scv/rfscv instructions Nicholas Piggin
2020-07-23 6:47 ` [musl] " Michael Ellerman
2020-07-23 16:48 ` Christophe Leroy
2020-07-24 10:45 ` Michael Ellerman
2020-06-11 21:02 ` [musl] Re: Linux powerpc new system call instruction and ABI Segher Boessenkool
2020-06-14 9:26 ` Nicholas Piggin
2021-05-18 23:13 ` Dmitry V. Levin
2021-05-19 2:50 ` Nicholas Piggin
2021-05-19 5:01 ` Nicholas Piggin
2021-05-21 19:40 ` Matheus Castanho
2021-05-21 19:52 ` Florian Weimer
2021-05-21 20:00 ` Matheus Castanho
2021-05-21 20:52 ` Dmitry V. Levin
2021-05-24 12:11 ` Matheus Castanho
2021-05-24 20:33 ` Matheus Castanho
2021-05-19 10:24 ` Dmitry V. Levin
2021-05-19 10:59 ` Nicholas Piggin
2021-05-19 12:39 ` Tulio Magno Quites Machado Filho
2021-05-19 13:26 ` Dmitry V. Levin
2021-05-19 22:51 ` Nicholas Piggin
2021-05-19 23:27 ` Dmitry V. Levin
2021-05-20 2:40 ` Nicholas Piggin [this message]
2021-05-20 3:06 ` Dmitry V. Levin
2021-05-20 5:12 ` Nicholas Piggin
2021-05-19 7:33 ` Joakim Tjernlund
2021-05-19 7:55 ` Nicholas Piggin
2021-05-19 8:08 ` Joakim Tjernlund
2021-05-19 8:42 ` Nicholas Piggin
2021-05-19 11:12 ` Nicholas Piggin
2021-05-19 14:38 ` Segher Boessenkool
2021-05-19 15:06 ` Joakim Tjernlund
2021-05-19 15:22 ` Segher Boessenkool
2021-05-19 15:36 ` Rich Felker
2021-05-19 18:09 ` Joakim Tjernlund
2021-05-19 23:48 ` Rich Felker
2021-05-20 1:06 ` Dmitry V. Levin
2021-05-20 2:45 ` Nicholas Piggin
2021-05-20 2:59 ` Dmitry V. Levin
2021-05-20 7:20 ` Nicholas Piggin
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