[PATCH 1/2] powerpc: properly negate error in syscall_set_return_value() in sc case

[Dmitry V. Levin]

On Tue, Jan 28, 2025 at 03:59:29PM +0100, Christophe Leroy 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 ?

Yes, also on alpha.  I don't think microblaze should be in this list,
though.

> > 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 ?

That's a good question.  Of course when returning to user space, but,
besides that, at syscall exit tracepoint (trace_sys_exit), ptrace syscall
exit stop (ptrace_report_syscall_exit), and PTRACE_EVENT_SECCOMP stop
(__secure_computing).  There could be other points where this is exposed.
For example, on many architectures the tracer can specify syscall error
return value also at ptrace syscall entry stop
(ptrace_report_syscall_entry), but powerpc does not implement this.

> I'm also a bit lost between regs->orig_r3, regs->gpr[3] and regs->result.

I'm not aware of the role of regs->result as it's not in struct user_pt_regs
and therefore is not exposed to user space.

> 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.

I hope Michael can comment on this.

> > 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 ?

As I wrote earlier, userspace tracers handling PTRACE_EVENT_SECCOMP
are able to set gpr[3] to either -ERRORCODE or ERRORCODE (along with
SO bit) for all these years, and both methods work up to now.


-- 
ldv