Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

From: Mark Rutland
Date: Tue Mar 23 2021 - 12:48:46 EST

On Tue, Mar 23, 2021 at 10:26:50AM -0500, Madhavan T. Venkataraman wrote:
> On 3/23/21 9:57 AM, Mark Rutland wrote:
> Thanks for explaining the nesting. It is now clear to me.

No problem!

> So, my next question is - can we define a practical limit for the
> nesting so that any nesting beyond that is fatal? The reason I ask is
> - if there is a max, then we can allocate an array of stack frames out
> of band for the special frames so they are not part of the stack and
> will not likely get corrupted.

I suspect we can't define such a fatal limit without introducing a local
DoS vector on some otherwise legitimate workload, and I fear this will
further complicate the entry/exit logic, so I'd prefer to avoid
introducing a new limit.

What exactly do you mean by a "special frame", and why do those need
additional protection over regular frame records?

> Also, we don't have to do any special detection. If the number of out
> of band frames used is one or more then we have exceptions and the
> stack trace is unreliable.

What is expected to protect against?