[PATCH] selftests/seccomp: fix ptrace tests on powerpc

[Michael Ellerman]

Thadeu Lima de Souza Cascardo <cascardo at canonical.com> writes:
> On Tue, Sep 08, 2020 at 04:18:17PM -0700, Kees Cook wrote:
>> On Tue, Jun 30, 2020 at 01:47:39PM -0300, Thadeu Lima de Souza Cascardo wrote:
...
>> > @@ -1809,10 +1818,15 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
>> >  	EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
>> >  			: PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
>> >  
>> > -	if (!entry)
>> > +	if (!entry && !syscall_nr)
>> >  		return;
>> >  
>> > -	nr = get_syscall(_metadata, tracee);
>> > +	if (entry)
>> > +		nr = get_syscall(_metadata, tracee);
>> > +	else
>> > +		nr = *syscall_nr;
>> 
>> This is weird? Shouldn't get_syscall() be modified to do the right thing
>> here instead of depending on the extra arg?
>> 
>
> R0 might be clobered. Same documentation mentions it as volatile. So, during
> syscall exit, we can't tell for sure that R0 will have the original syscall
> number. So, we need to grab it during syscall enter, save it somewhere and
> reuse it. I used the test context/args for that.

The user r0 (in regs->gpr[0]) shouldn't be clobbered.

But it is modified if the tracer skips the syscall, by setting the
syscall number to -1. Or if the tracer changes the syscall number.

So if you need the original syscall number in the exit path then I think
you do need to save it at entry.

cheers