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Date:   Wed, 17 Jun 2020 15:17:42 +0200
From:   Cyril Hrubis <chrubis@suse.cz>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        kernel test robot <rong.a.chen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, ltp@lists.linux.it
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