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Levin" , Michael Ellerman Cc: Joakim Tjernlund , libc-alpha@sourceware.org, libc-dev@lists.llvm.org, linux-api@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Matheus Castanho , musl@lists.openwall.com References: <20210519132656.GA17204@altlinux.org> In-Reply-To: <20210519132656.GA17204@altlinux.org> MIME-Version: 1.0 Message-Id: <1621464056.o9t21cquw8.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: [musl] 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 thi= nk=20 >> >> a problem remains with applications that look at system call return=20 >> >> registers directly and have powerpc specific error cases. Those proba= bly >> >> 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=20 >> >> know how that would be done. >> >=20 >> > 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? >>=20 >> Not that I know of. Michael suggested there might be a way to add=20 >> something. ptrace_syscall_info has some pad bytes, could >> we use one for flags bits and set a bit for "new system call ABI"? >=20 > 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. >=20 > 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=20 exit state so I don't see a problem with a flag that ppc can use for ABI. >=20 > 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. >=20 >> 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. >=20 > This would be very unreliable because sc and scv are allowed to interming= le, > so every syscall invocation can follow any of these two error handling > conventions. >=20 >> 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. >=20 > No other architecture requires peeking into tracee memory just to find ou= t > the syscall ABI. This would make powerpc the most ugly architecture for > ptracing. >=20 > 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=20 high bit. But could that break things that just test for syscall=20 trap number (and don't care about register ABI)? I'm not sure. Thanks, Nick