Le 18/02/2020 à 13:33, Masami Hiramatsu a écrit :
On Tue, 18 Feb 2020 12:04:41 +0100
Christophe Leroy <christophe.le...@c-s.fr> wrote:
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()
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
