On Tue, Mar 10, 2020 at 12:42:26PM +0000, Mark Brown wrote:

On Mon, Mar 09, 2020 at 09:05:05PM +0000, Mark Brown wrote:

quoted

On Fri, Mar 06, 2020 at 10:27:29AM +0000, Catalin Marinas wrote:

quoted

quoted

Does this series affect uprobes in any way? I.e. can you probe a landing
pad?

quoted

You can't probe a landing pad, uprobes on landing pads will be silently
ignored so the program isn't disrupted, you just don't get the expected
trace from those uprobes.  This isn't new with the BTI support since
the landing pads are generally pointer auth instructions, these already
can't be probed regardless of what's going on with this series.  It's
already on the list to get sorted.

Sorry, I realized thanks to Amit's off-list prompting that I was testing
that I was verifying with the wrong kernel binary here (user error since
it took me a while to sort out uprobes) so this isn't quite right - you
can probe the landing pads with or without this series.

Can we not change aarch64_insn_is_nop() to actually return true only for
NOP and ignore everything else in the hint space? We tend to re-use the
hint instructions for new things in the architecture, so I'd rather
white-list what we know we can safely probe than black-listing only some
of the hint instructions.

I haven't assessed the effort of doing the above (probably not a lot)
but as a short-term workaround we could add the BTI and PAC hint
instructions to the aarch64_insn_is_nop() (though my preferred option is
the white-list one).

-- 
Catalin

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