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[13/16] ptrace: Document why ptrace_setoptions does not need a lock

Message ID 20220518225355.784371-13-ebiederm@xmission.com
State New
Headers show
Series ptrace: cleanups and calling do_cldstop with only siglock | expand

Commit Message

Eric W. Biederman May 18, 2022, 10:53 p.m. UTC 
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@xmission.com>
---
 kernel/ptrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
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Patch

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);