Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode

Le 18/02/2020 à 13:33, Masami Hiramatsu a écrit :

On Tue, 18 Feb 2020 12:04:41 +0100
Christophe Leroy [off-list ref] wrote:

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Nevertheless, if one symbol has been forgotten in the blacklist, I think
it is a problem if it generate Oopses.

There is a long history also on x86 to make a blacklist. Anyway, how did
you get this error on PPC32? Somewhere would you like to probe and
it is a real mode function? Or, it happened unexpectedly?

The first Oops I got was triggered by a WARN_ON() kind of trap in real
mode. The trap exception handler called kprobe_handler() which tried to
read the instruction at the trap address (which was a real-mode address)
so it triggered a Bad Access Fault.

This was initially the purpose of my patch.

OK, then filtering the trap reason in kprobe handler is a bit strange.
It should be done in the previous stage (maybe in trap.c)

See commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.6-rc2&id=6cc89bad60a673a24386f1ada83de8a068a78909

Can we filter it by exception flag or only by checking the instruction
which causes the exception, or needs get_kprobe()...?

The trap instruction used by kprobe is also used for other purposes like 
BUG_ON() or WARN_ON(), so needs get_kprobe()

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After discussion with you, I started looking at what would be the effect
of setting a kprobe event in a function which runs in real mode.

If the kprobe single-stepping (or emulation) works in real mode, just
ignore the kprobes pre/post_handlers and increment nmissed count.

If that doesn't work, we have to call a BUG_ON, because we can not
continue the code execution. And also, you have to find a way to make
a blacklist for real mode code.

Yes, it has to be done function by function (hoppefully there's not more 
than a dozen).
But I'd like something which can fails gracefully for the functions we 
will forget to mark noprobe.

But as a first step I'd really like a bug fix in 5.6 to avoid Oopsing in 
kprobe_handler() at a non-kprobe trap.

Christophe