Messages in this thread Patch in this message | | | From | "Eric W. Biederman" <> | Date | Wed, 18 May 2022 17:53:52 -0500 | Subject | [PATCH 13/16] ptrace: Document why ptrace_setoptions does not need a lock |
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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(-)
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
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