From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cyril Hrubis Date: Wed, 17 Jun 2020 15:17:42 +0200 Subject: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail In-Reply-To: <20200616132705.GW2531@hirez.programming.kicks-ass.net> References: <87y2onbdtb.fsf@nanos.tec.linutronix.de> <8E41B15F-D567-4C52-94E9-367015480345@amacapital.net> <20200616132705.GW2531@hirez.programming.kicks-ass.net> Message-ID: <20200617131742.GD8389@yuki.lan> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: ltp@lists.linux.it 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 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