linux-kernel - [PATCH 13/16] ptrace: Document why ptrace

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Message-Id: <20220518225355.784371-13-ebiederm@xmission.com>
Date:   Wed, 18 May 2022 17:53:52 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     rjw@...ysocki.net, Oleg Nesterov <oleg@...hat.com>,
        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,
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        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
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        Daniel Thompson <daniel.thompson@...aro.org>,
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        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 13/16] ptrace: Document why ptrace_setoptions does not need a lock

The functions that change ->ptrace are: ptrace_attach, ptrace_traceme,
ptrace_init_task, __ptrace_unlink, ptrace_setoptions.

Except for ptrace_setoptions all of the places where ->ptrace is
modified hold tasklist_lock for write, and either the tracee or the
tracer is modifies ->ptrace.

When ptrace_setoptions is called the tracee has been frozen with
ptrace_freeze_traced, and most be explicitly unfrozen by the tracer
before it can do anything.  As ptrace_setoption is run in the tracer
there can be no contention by the simple fact that the tracee can't
run.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/ptrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index d0527b6e2b29..fbadd2f21f09 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -689,7 +689,10 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
 	if (ret)
 		return ret;
 
-	/* Avoid intermediate state when all opts are cleared */
+	/*
+	 * With a frozen tracee, only the tracer modifies ->ptrace.
+	 * Avoid intermediate state when all opts are cleared.
+	 */
 	flags = child->ptrace;
 	flags &= ~(PTRACE_O_MASK << PT_OPT_FLAG_SHIFT);
 	flags |= (data << PT_OPT_FLAG_SHIFT);
-- 
2.35.3