Messages in this thread | | | Date | Wed, 17 Jun 2020 15:17:42 +0200 | From | Cyril Hrubis <> | Subject | Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail |
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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@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@suse.cz
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