From: Kees Cook <keescook@chromium.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
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: Thu, 17 Sep 2020 15:37:16 -0700 [thread overview]
Message-ID: <202009171536.7FACD19@keescook> (raw)
In-Reply-To: <875z8hrh2o.fsf@mpe.ellerman.id.au>
On Sun, Sep 13, 2020 at 10:34:23PM +1000, Michael Ellerman wrote:
> 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.
... the selftest code wants to test the updated syscall (-1 or
whatever), so this sounds correct. Was this test actually failing on
powerpc? (I'd really rather not split entry/exit if I don't have to.)
--
Kees Cook
next prev parent reply other threads:[~2020-09-17 22:37 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
2020-09-17 22:37 ` Kees Cook [this message]
2020-09-17 22:51 ` Thadeu Lima de Souza Cascardo
2020-09-18 6:22 ` Michael Ellerman
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