1:17 p.m.
Hi!
> >> FYI, we noticed the following commit (built with
gcc-9):
> >>
> >> commit: 2bbc68f8373c0631ebf137f376fbea00e8086be7 ("x86/entry: Convert
Debug exception to IDTENTRY_DB")
> >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> >
> > Is the head of linux.git exposing the same problem or is this an
> > intermittent failure, which only affects bisectability?
>
> It sure looks deterministic:
>
> ptrace08.c:62: BROK: Cannot find address of kernel symbol "do_debug"
ROFL
It's nice to have a good laugh, however I would really appreciate if any
of you would help me to fix the test.
The test in question is a regression test for:
commit f67b15037a7a50c57f72e69a6d59941ad90a0f0f
Author: Linus Torvalds <torvalds(a)linux-foundation.org>
Date: Mon Mar 26 15:39:07 2018 -1000
perf/hwbp: Simplify the perf-hwbp code, fix documentation
Annoyingly, modify_user_hw_breakpoint() unnecessarily complicates the
modification of a breakpoint - simplify it and remove the pointless
local variables.
And as far as I can tell it uses ptrace() with PTRACE_POKEUSER in order to
trigger it. But I'm kind of lost on how exactly we trigger the kernel
crash.
What is does is to write:
(void*)1 to u_debugreg[0]
(void*)1 to u_debugreg[7]
do_debug addr to u_debugreg[0]
Looking at the kernel code the write to register 7 enables the breakpoints and
what we attempt here is to change an invalid address to a valid one after we
enabled the breakpoint but that's as far I can go.
So does anyone has an idea how to trigger the bug without the do_debug function
address? Would any valid kernel function address suffice?
--
Cyril Hrubis
chrubis(a)suse.cz