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[86.9.19.6]) by smtp.gmail.com with ESMTPSA id x8-v6sm108736952wrd.54.2018.11.06.00.57.53 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 06 Nov 2018 00:57:53 -0800 (PST) Date: Tue, 6 Nov 2018 08:57:51 +0000 From: Daniel Thompson To: Mark Rutland Cc: Zhaoyang Huang , Catalin Marinas , Will Deacon , Dave Martin , Michael Weiser , James Morse , linux-kernel@vger.kernel.org Subject: Re: [PATCH] arch/arm64 : fix error in dump_backtrace Message-ID: <20181106085751.hrp7qkp53cftgew6@holly.lan> References: <1541488775-29610-1-git-send-email-huangzhaoyang@gmail.com> <20181106083901.erezwtcomiijvdrk@salmiak> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181106083901.erezwtcomiijvdrk@salmiak> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 06, 2018 at 08:39:01AM +0000, Mark Rutland wrote: > On Tue, Nov 06, 2018 at 03:19:35PM +0800, Zhaoyang Huang wrote: > > From: Zhaoyang Huang > > > > In some cases, the instruction of "bl foo1" will be the last one of the > > foo2[1], which will cause the lr be the first instruction of the adjacent > > foo3[2]. Hence, the backtrace will show the weird result as bellow[3]. > > The patch will fix it by miner 4 of the lr when dump_backtrace > > This has come up in the past (and a similar patch has been applied, then > reverted). > > In general, we don't know that a function call was made via BL, and therefore > cannot know that LR - 4 is the address of the caller. The caller could set up > the LR as it likes, then B or BR to the callee, and depending on how the basic > blocks get laid out in memory, LR - 4 might point at something completely > different. > > More ideally, the compiler wouldn't end a function with a BL. When does that > happen, and is there some way we could arrange for that to not happen? e.g. > somehow pad a NOP after the BL. It's a consequence of having __noreturn isn't it? __noreturn frees the compiler from the burden of having to produce a valid return stack... so it doesn't and unwinding becomes hard. Daniel. > > [1] > > 0xffffff80081e6b04 : adrp x0, 0xffffff8008ca8000 > > 0xffffff80081e6b08 : add x0, x0, #0x5a8 > > 0xffffff80081e6b0c : bl 0xffffff80081b0ca0 > > 0xffffff80081e6b10 : stp x29, x30, [sp,#-64]! > > 0xffffff80081e6b14 : mov x29, sp > > > > [2] > > crash_arm64> rd ffffffc02eec3bd0 2 > > ffffffc02eec3bd0: ffffffc02eec3cb0 ffffff80081e6b10 > > > > [3] > > wrong: > > [] panic+0xf0/0x24c > > [] access_remote_vm+0x0/0x5c > > [] do_page_fault+0x290/0x3b8 > > [] do_mem_abort+0x64/0xdc > > > > correct: > > [ffffffc02eec3bd0] panic at ffffff80081b0da4 > > [ffffffc02eec3cb0] handle_mm_fault at ffffff80081e6b0c > > [ffffffc02eec3d80] do_page_fault at ffffff800809d7ac > > [ffffffc02eec3df0] do_mem_abort at ffffff800808156c > > > > Signed-off-by: Zhaoyang Huang > > --- > > arch/arm64/kernel/traps.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > > index d399d45..7a097cc 100644 > > --- a/arch/arm64/kernel/traps.c > > +++ b/arch/arm64/kernel/traps.c > > @@ -113,7 +113,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > > > > if (tsk == current) { > > frame.fp = (unsigned long)__builtin_frame_address(0); > > - frame.pc = (unsigned long)dump_backtrace; > > + frame.pc = (unsigned long)dump_backtrace + 4; > > } else { > > /* > > * task blocked in __switch_to > > @@ -130,7 +130,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > > do { > > /* skip until specified stack frame */ > > if (!skip) { > > - dump_backtrace_entry(frame.pc); > > + dump_backtrace_entry(frame.pc - 4); > > } else if (frame.fp == regs->regs[29]) { > > skip = 0; > > /* > > -- > > 1.9.1 > >