[PATCH RFC 2/8] arm64: stacktrace: Add arch_within_stack_frames

[David Laight]

> > Thanks for doing this implementation! One reason usercopy hardening
> > didn't persue doing a "full" stacktrace was because it seemed relatively
> > expensive. Did you do any usercopy-heavily workload testing to see if
> > there was a noticeable performance impact?

Look at anything that uses sendmsg().
It is noticeably more expensive than sendto().
All the extra copy_from_user() cause measurable slow slowdowns.
Using __copy_from_user()(to avoid 'hardending') in the socket code
and when reading the iov[] gives a measurable improvement.

	David

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