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From: Michael Ellerman <mpe@ellerman.id.au>
To: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
	Kees Cook <keescook@chromium.org>
Cc: linux-kselftest@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] selftests/seccomp: fix ptrace tests on powerpc
Date: Sun, 13 Sep 2020 22:34:23 +1000	[thread overview]
Message-ID: <875z8hrh2o.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20200911180637.GI4002@mussarela>

Thadeu Lima de Souza Cascardo <cascardo@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

  reply	other threads:[~2020-09-13 12:34 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-30 16:47 [PATCH] selftests/seccomp: fix ptrace tests on powerpc Thadeu Lima de Souza Cascardo
2020-09-08 23:18 ` Kees Cook
2020-09-11 18:06   ` Thadeu Lima de Souza Cascardo
2020-09-13 12:34     ` Michael Ellerman [this message]
2020-09-17 22:37       ` Kees Cook
2020-09-17 22:51         ` Thadeu Lima de Souza Cascardo
2020-09-18  6:22           ` Michael Ellerman

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