Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

On Mon, Apr 12, 2021 at 02:55:35PM -0500, Madhavan T. Venkataraman wrote:

OK. Just so I am clear on the whole picture, let me state my understanding so far.
Correct me if I am wrong.

1. We are hoping that we can convert a significant number of SYM_CODE functions to
   SYM_FUNC functions by providing them with a proper FP prolog and epilog so that
   we can get objtool coverage for them. These don't need any blacklisting.

I wouldn't expect to be converting lots of SYM_CODE to SYM_FUNC.  I'd
expect the overwhelming majority of SYM_CODE to be SYM_CODE because it's
required to be non standard due to some external interface - things like
the exception vectors, ftrace, and stuff around suspend/hibernate.  A
quick grep seems to confirm this.

3. We are going to assume that the reliable unwinder is only for livepatch purposes
   and will only be invoked on a task that is not currently running. The task either

The reliable unwinder can also be invoked on itself.

4. So, the only functions that will need blacklisting are the remaining SYM_CODE functions
   that might give up the CPU voluntarily. At this point, I am not even sure how
   many of these will exist. One hopes that all of these would have ended up as
   SYM_FUNC functions in (1).

There's stuff like ret_from_fork there.

I suggest we do (3) first. Then, review the assembly functions to do (1). Then, review the
remaining ones to see which ones must be blacklisted, if any.

I'm not clear what the concrete steps you're planning to do first are
there - your 3 seems like a statement of assumptions.  For flagging
functions I do think it'd be safer to default to assuming that all
SYM_CODE functions can't be unwound reliably rather than only explicitly
listing ones that cause problems.