Date: Thu, 20 May 2021 12:40:36 +1000 From: Nicholas Piggin <npiggin@...il.com> To: "Dmitry V. Levin" <ldv@...linux.org> 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, Michael Ellerman <mpe@...erman.id.au>, 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 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
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