Messages in this thread | | | Subject | Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode | From | Christophe Leroy <> | Date | Tue, 18 Feb 2020 14:58:00 +0100 |
| |
Le 18/02/2020 à 13:33, Masami Hiramatsu a écrit : > On Tue, 18 Feb 2020 12:04:41 +0100 > Christophe Leroy <christophe.leroy@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
| |