On Tue, Jan 28, 2025 at 3:59 PM Christophe Leroy <christophe.le...@csgroup.eu> wrote: > > > Le 27/01/2025 à 19:13, Dmitry V. Levin a écrit : > > According to the Power Architecture Linux system call ABI documented in > > [1], when the syscall is made with the sc instruction, both a value and an > > error condition are returned, where r3 register contains the return value, > > and cr0.SO bit specifies the error condition. When cr0.SO is clear, the > > syscall succeeded and r3 is the return value. When cr0.SO is set, the > > syscall failed and r3 is the error value. This syscall return semantics > > was implemented from the very beginning of Power Architecture on Linux, > > and syscall tracers and debuggers like strace that read or modify syscall > > return information also rely on this ABI. > > I see a quite similar ABI on microblaze, mips, nios2 and sparc. Do they > behave all the same ?
In terms of ABI, yes: https://gitlab.com/strace/strace/-/blob/master/src/linux/sparc/get_error.c https://gitlab.com/strace/strace/-/blob/master/src/linux/mips/get_error.c https://gitlab.com/strace/strace/-/blob/master/src/linux/nios2/get_error.c (cf. https://gitlab.com/strace/strace/-/blob/master/src/linux/powerpc/get_error.c ) I can't find microblaze SyscallV ABI, but at least both strace and glibc expect negated errno in r3: https://gitlab.com/strace/strace/-/blob/master/src/linux/microblaze/get_error.c https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/microblaze/syscall.S;h=6ebf688d800e19c14a140f0cd5a0ba9344fa95a5;hb=HEAD > > r3 and cr0.SO are exposed directly via struct pt_regs where gpr[3] and > > (ccr & 0x10000000) correspond to r3 and cr0.SO, respectively. > > For example, here is an excerpt from check_syscall_restart() that assigns > > these members of struct pt_regs: > > regs->result = -EINTR; > > regs->gpr[3] = EINTR; > > regs->ccr |= 0x10000000; > > In this example, the semantics of negative ERRORCODE that's being used > > virtually everywhere in generic kernel code is translated to powerpc sc > > syscall return ABI which uses positive ERRORCODE and cr0.SO bit. > > At what point are they exposed really ? At what point do they need to > comply with the ABI ? On ptrace stops via pt_regs, for example? > I'm also a bit lost between regs->orig_r3, regs->gpr[3] and regs->result. > > The comment added by commit 1b1a3702a65c ("powerpc: Don't negate error > in syscall_set_return_value()") says that CCR needs to be set because of > signal code. But signal code is invoked by syscall_exit_prepare() > through call to interrupt_exit_user_prepare_main() after setting CR[SO] > and negating syscall result. > > > > > Also, r3 and cr0.SO are exposed indirectly via helpers. > > For example, here is an excerpt from syscall_get_error(): > > /* > > * If the system call failed, > > * regs->gpr[3] contains a positive ERRORCODE. > > */ > > return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0; > > and here is another example, from regs_return_value(): > > if (is_syscall_success(regs)) > > return regs->gpr[3]; > > else > > return -regs->gpr[3]; > > In these examples, the powerpc sc syscall return ABI which uses positive > > ERRORCODE and cr0.SO bit is translated to the semantics of negative > > ERRORCODE that's being used virtually everywhere in generic kernel code. > > > > Up to a certain point in time the kernel managed to implement the powerpc > > sc syscall return ABI in all cases where struct pt_regs was exposed to user > > space. > > > > The situation changed when SECCOMP_RET_TRACE support was introduced. > > At this point the -ERRORCODE semantics that was used under the hood to > > implement seccomp on powerpc became exposed to user space. The tracer > > handling PTRACE_EVENT_SECCOMP is not just able to observe -ENOSYS in gpr[3] > > - this is relatively harmless as at this stage there is no syscall return > > yet so the powerpc sc syscall return ABI does not apply. What's important > > is that the tracer can change the syscall number to -1 thus making the > > syscall fail, and at this point the tracer is also able to specify the > > error value. This has to be done in accordance with the syscall return > > ABI, however, the current implementation of do_seccomp() supports both the > > generic kernel -ERRORCODE return value ABI and the powerpc sc syscall > > return ABI, thanks to syscall_exit_prepare() that converts the former to > > the latter. Consequently, seccomp_bpf selftest passes both with and > > without this change. > > > > Now comes the moment when PTRACE_SET_SYSCALL_INFO is going to be > > introduced. PTRACE_SET_SYSCALL_INFO is a generic ptrace API that > > complements PTRACE_GET_SYSCALL_INFO by letting the ptracer modify > > the details of the system calls the tracee is blocked in. > > > > One of the helpers that have to be used to implement > > PTRACE_SET_SYSCALL_INFO is syscall_set_return_value(). > > This helper complements other two helpers, syscall_get_error() and > > syscall_get_return_value(), that are currently used to implement > > PTRACE_GET_SYSCALL_INFO on syscall return. When syscall_set_return_value() > > is used to set an error code, the caller specifies it as a negative value > > in -ERRORCODE format. > > > > Unfortunately, this does not work well on powerpc since commit 1b1a3702a65c > > ("powerpc: Don't negate error in syscall_set_return_value()") because > > syscall_set_return_value() does not follow the powerpc sc syscall return > > ABI: > > /* > > * In the general case it's not obvious that we must deal with > > * CCR here, as the syscall exit path will also do that for us. > > * However there are some places, eg. the signal code, which > > * check ccr to decide if the value in r3 is actually an error. > > */ > > if (error) { > > regs->ccr |= 0x10000000L; > > regs->gpr[3] = error; > > } else { > > regs->ccr &= ~0x10000000L; > > regs->gpr[3] = val; > > } > > > > The reason why this syscall_set_return_value() implementation was able to > > get away with violating the powerpc sc syscall return ABI is the following: > > Up to now, syscall_set_return_value() on powerpc could be called only from > > do_syscall_trace_enter() via do_seccomp(), there was no way it could be > > called from do_syscall_trace_leave() which is the point where tracers on > > syscall return are activated and the powerpc sc syscall return ABI has > > to be respected. > > > > Introduction of PTRACE_SET_SYSCALL_INFO necessitates a change of > > syscall_set_return_value() to comply with the powerpc sc syscall return > > ABI. Without the change, the upcoming ptrace/set_syscall_info selftest > > fails with the following diagnostics: > > > > # set_syscall_info.c:119:set_syscall_info:Expected exp_exit->rval (-38) > > == info->exit.rval (38) > > # set_syscall_info.c:120:set_syscall_info:wait #4: > > PTRACE_GET_SYSCALL_INFO #2: exit stop mismatch > > > > Note that since backwards compatibility with the current implementation has > > to be provided, the kernel has to continue supporting simultaneously both > > the generic kernel -ERRORCODE return value ABI and the powerpc sc syscall > > return ABI at least for PTRACE_EVENT_SECCOMP tracers. Consequently, since > > the point of __secure_computing() invocation and up to the point of > > conversion in syscall_exit_prepare(), gpr[3] may be set according to either > > of these two ABIs. An attempt to address code inconsistencies in syscall > > error return handling that were introduced as a side effect of the dual > > ABI support follows in a separate patch. > > What do you mean by "backwards compatibility" here ? backwards > compatibility applies only to userspace API doesn't it ? So if there was > no way to trigger the problem previously, what does it mean ? ptrace and seccomp users are pretty much userspace, aren't they? > > > > Link: > > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.kernel.org%2Fdoc%2Fhtml%2Flatest%2Farch%2Fpowerpc%2Fsyscall64-abi.html%23return-value&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cc2cf590281c24fe1478408dd3efe4a3e%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638735984085033893%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=EEP9s6k%2Fs5VfqWgrs6VXi879HEfJ8BYOOJ8InmmVTQA%3D&reserved=0 > > [1] > > Signed-off-by: Dmitry V. Levin <l...@strace.io> > > Reviewed-by: Alexey Gladkov <leg...@kernel.org> > > --- > > arch/powerpc/include/asm/syscall.h | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/arch/powerpc/include/asm/syscall.h > > b/arch/powerpc/include/asm/syscall.h > > index 3dd36c5e334a..422d7735ace6 100644 > > --- a/arch/powerpc/include/asm/syscall.h > > +++ b/arch/powerpc/include/asm/syscall.h > > @@ -82,7 +82,11 @@ static inline void syscall_set_return_value(struct > > task_struct *task, > > */ > > if (error) { > > regs->ccr |= 0x10000000L; > > - regs->gpr[3] = error; > > + /* > > + * In case of an error regs->gpr[3] contains > > + * a positive ERRORCODE. > > + */ > > + regs->gpr[3] = -error; > > } else { > > regs->ccr &= ~0x10000000L; > > regs->gpr[3] = val; > -- Eugene Syromyatnikov mailto:evg...@gmail.com xmpp:esyr@jabber.{ru|org}
