linux-kernel - Re: [PATCH v4 0/12] ptrace: cleaning up ptrace

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Date:   Fri, 06 May 2022 09:38:21 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v4 0/12] ptrace: cleaning up ptrace_stop

Oleg Nesterov <oleg@...hat.com> writes:

> On 05/05, Eric W. Biederman wrote:
>>
>> Eric W. Biederman (11): signal: Rename send_signal send_signal_locked
>> signal: Replace __group_send_sig_info with send_signal_locked
>> ptrace/um: Replace PT_DTRACE with TIF_SINGLESTEP ptrace/xtensa:
>> Replace PT_SINGLESTEP with TIF_SINGLESTEP ptrace: Remove
>> arch_ptrace_attach signal: Use lockdep_assert_held instead of
>> assert_spin_locked ptrace: Reimplement PTRACE_KILL by always sending
>> SIGKILL ptrace: Document that wait_task_inactive can't fail ptrace:
>> Admit ptrace_stop can generate spuriuos SIGTRAPs ptrace: Don't change
>> __state ptrace: Always take siglock in ptrace_resume
>>
>> Peter Zijlstra (1):
>>       sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state
>
> I can't comment 5/12. to be honest I didn't even try to look into
> arch/ia64/.

I just looked at arch_ptrace_attach again and I spotted what looks like
a fairly easy analysis that is mostly arch-generic code that shows this
is dead code on ia64.

On ia64 arch_ptrace_attach is ptrace_attach_sync_user_rbs, and does
nothing if __state is not TASK_STOPPED.

When arch_ptrace_attach is called after ptrace_traceme __state is
TASK_RUNNING pretty much by definition as we are running in the
child.  Therefore ptrace_attach_sync_user_rbs does nothing in that case.

When arch_ptrace_attach is called after ptrace_attach __state there
are two possibilities.  If the tracee was already in TASK_STOPPED
before the ptrace_attach, the tracee will be in TASK_TRACED.
Otherwise the tracee will be in TASK_TRACED or on it's way to stopping
in TASK_TRACED.

Unless I totally misread ptrace_attach.  There is no way that after
a successful ptrace_attach for the tracee to be in TASK_STOPPED.
This makes ptrace_attach_sync_user_rbs a big noop, AKA dead code.
So it can be removed.

> But other than that I see no problems in this version. However, I'd
> like to actually apply the whole series and read the changed code
> carefully, but sorry, I don't think I can do this before Monday.

No rush.  I don't expect the merge window will open for a while yet.

Eric