From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 97E58C433F5 for ; Sat, 9 Oct 2021 23:41:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7206960F6B for ; Sat, 9 Oct 2021 23:41:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231350AbhJIXnq (ORCPT ); Sat, 9 Oct 2021 19:43:46 -0400 Received: from linux.microsoft.com ([13.77.154.182]:42736 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231411AbhJIXnp (ORCPT ); Sat, 9 Oct 2021 19:43:45 -0400 Received: from [192.168.254.32] (unknown [47.187.212.181]) by linux.microsoft.com (Postfix) with ESMTPSA id 5C3E620B8008; Sat, 9 Oct 2021 16:41:47 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 5C3E620B8008 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1633822908; bh=z3zf34sgNDMstN+SyGxYF9nej/yHt0w7n1f8XPjEbq8=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=VN5D8jo+bXxo4INnos5oTRkSaTrFn0XmDmrnCyOjzknzYmrE78ZNYxlGlF6231D9Q NZ8r6WUmi3LKAZjHNbTKwMyUiYvmv59wA3n6lmRhjo1ocZUdxmvSt65uw2+qQ9MXsa sQmzqrGL4aslJZuLbrCKr0pM25lfLpGr1kHUBQkQ= Subject: Re: [RFC PATCH v8 1/4] arm64: Make all stack walking functions use arch_stack_walk() To: Mark Rutland Cc: broonie@kernel.org, jpoimboe@redhat.com, ardb@kernel.org, nobuta.keiya@fujitsu.com, sjitindarsingh@gmail.com, catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org, pasha.tatashin@soleen.com, jthierry@redhat.com, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210812190603.25326-1-madvenka@linux.microsoft.com> <20210812190603.25326-2-madvenka@linux.microsoft.com> <20210824131344.GE96738@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: <63dc1c80-2e20-66ec-67c1-c2d447797a11@linux.microsoft.com> Date: Sat, 9 Oct 2021 18:41:46 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20210824131344.GE96738@C02TD0UTHF1T.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On 8/24/21 8:13 AM, Mark Rutland wrote: > On Thu, Aug 12, 2021 at 02:06:00PM -0500, madvenka@linux.microsoft.com wrote: >> From: "Madhavan T. Venkataraman" >> >> Currently, there are multiple functions in ARM64 code that walk the >> stack using start_backtrace() and unwind_frame(). Convert all of >> them to use arch_stack_walk(). This makes maintenance easier. > > It would be good to split this into a series of patches as Mark Brown > suggested in v7. > >> Here is the list of functions: >> >> perf_callchain_kernel() >> get_wchan() >> return_address() >> dump_backtrace() >> profile_pc() > > Note that arch_stack_walk() depends on CONFIG_STACKTRACE (which is not in > defconfig), so we'll need to reorganise things such that it's always defined, > or factor out the core of that function and add a wrapper such that we > can always use it. > >> Signed-off-by: Madhavan T. Venkataraman >> --- >> arch/arm64/include/asm/stacktrace.h | 3 --- >> arch/arm64/kernel/perf_callchain.c | 5 +--- >> arch/arm64/kernel/process.c | 39 ++++++++++++++++++----------- >> arch/arm64/kernel/return_address.c | 6 +---- >> arch/arm64/kernel/stacktrace.c | 38 +++------------------------- >> arch/arm64/kernel/time.c | 22 +++++++++------- >> 6 files changed, 43 insertions(+), 70 deletions(-) >> >> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h >> index 8aebc00c1718..e43dea1c6b41 100644 >> --- a/arch/arm64/include/asm/stacktrace.h >> +++ b/arch/arm64/include/asm/stacktrace.h >> @@ -61,9 +61,6 @@ struct stackframe { >> #endif >> }; >> >> -extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame); >> -extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame, >> - bool (*fn)(void *, unsigned long), void *data); >> extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk, >> const char *loglvl); >> >> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c >> index 4a72c2727309..2f289013c9c9 100644 >> --- a/arch/arm64/kernel/perf_callchain.c >> +++ b/arch/arm64/kernel/perf_callchain.c >> @@ -147,15 +147,12 @@ static bool callchain_trace(void *data, unsigned long pc) >> void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, >> struct pt_regs *regs) >> { >> - struct stackframe frame; >> - >> if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) { >> /* We don't support guest os callchain now */ >> return; >> } >> >> - start_backtrace(&frame, regs->regs[29], regs->pc); >> - walk_stackframe(current, &frame, callchain_trace, entry); >> + arch_stack_walk(callchain_trace, entry, current, regs); >> } > > We can also update callchain_trace take the return value of > perf_callchain_store into acount, e.g. > > | static bool callchain_trace(void *data, unsigned long pc) > | { > | struct perf_callchain_entry_ctx *entry = data; > | return perf_callchain_store(entry, pc) == 0; > | } > >> >> unsigned long perf_instruction_pointer(struct pt_regs *regs) >> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c >> index c8989b999250..52c12fd26407 100644 >> --- a/arch/arm64/kernel/process.c >> +++ b/arch/arm64/kernel/process.c >> @@ -544,11 +544,28 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev, >> return last; >> } >> >> +struct wchan_info { >> + unsigned long pc; >> + int count; >> +}; >> + >> +static bool get_wchan_cb(void *arg, unsigned long pc) >> +{ >> + struct wchan_info *wchan_info = arg; >> + >> + if (!in_sched_functions(pc)) { >> + wchan_info->pc = pc; >> + return false; >> + } >> + wchan_info->count--; >> + return !!wchan_info->count; >> +} > > This will terminate one entry earlier than the old logic since we used > to use a post-increment (testing the prior value), and now we're > effectively using a pre-decrement (testing the new value). > > I don't think that matters all that much in practice, but it might be > best to keep the original logic, e.g. initialize `count` to 0 and here > do: > > return wchan_info->count++ < 16; > >> + >> unsigned long get_wchan(struct task_struct *p) >> { >> - struct stackframe frame; >> - unsigned long stack_page, ret = 0; >> - int count = 0; >> + unsigned long stack_page; >> + struct wchan_info wchan_info; >> + >> if (!p || p == current || task_is_running(p)) >> return 0; >> >> @@ -556,20 +573,12 @@ unsigned long get_wchan(struct task_struct *p) >> if (!stack_page) >> return 0; >> >> - start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p)); >> + wchan_info.pc = 0; >> + wchan_info.count = 16; >> + arch_stack_walk(get_wchan_cb, &wchan_info, p, NULL); >> >> - do { >> - if (unwind_frame(p, &frame)) >> - goto out; >> - if (!in_sched_functions(frame.pc)) { >> - ret = frame.pc; >> - goto out; >> - } >> - } while (count++ < 16); >> - >> -out: >> put_task_stack(p); >> - return ret; >> + return wchan_info.pc; >> } > > Other than the comment above, this looks good to me. > >> unsigned long arch_align_stack(unsigned long sp) >> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c >> index a6d18755652f..92a0f4d434e4 100644 >> --- a/arch/arm64/kernel/return_address.c >> +++ b/arch/arm64/kernel/return_address.c >> @@ -35,15 +35,11 @@ NOKPROBE_SYMBOL(save_return_addr); >> void *return_address(unsigned int level) >> {> struct return_address_data data; >> - struct stackframe frame; >> >> data.level = level + 2; >> data.addr = NULL; >> >> - start_backtrace(&frame, >> - (unsigned long)__builtin_frame_address(0), >> - (unsigned long)return_address); >> - walk_stackframe(current, &frame, save_return_addr, &data); >> + arch_stack_walk(save_return_addr, &data, current, NULL); >> >> if (!data.level) >> return data.addr; > > Nor that arch_stack_walk() will start with it's caller, so > return_address() will be included in the trace where it wasn't > previously, which implies we need to skip an additional level. > > That said, I'm not entirely sure why we need to skip 2 levels today; it > might be worth checking that's correct. > > We should also mark return_address() as noinline to avoid surprises with > LTO. > >> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c >> index 8982a2b78acf..1800310f92be 100644 >> --- a/arch/arm64/kernel/stacktrace.c >> +++ b/arch/arm64/kernel/stacktrace.c >> @@ -151,23 +151,21 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame, >> } >> NOKPROBE_SYMBOL(walk_stackframe); >> >> -static void dump_backtrace_entry(unsigned long where, const char *loglvl) >> +static bool dump_backtrace_entry(void *arg, unsigned long where) >> { >> + char *loglvl = arg; >> printk("%s %pSb\n", loglvl, (void *)where); >> + return true; >> } >> >> void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk, >> const char *loglvl) >> { >> - struct stackframe frame; >> - int skip = 0; >> - >> pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); >> >> if (regs) { >> if (user_mode(regs)) >> return; >> - skip = 1; >> } > > We can simplifiy this to: > > if (regs && user_mode(regs)) > return; > >> >> if (!tsk) >> @@ -176,36 +174,8 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk, >> if (!try_get_task_stack(tsk)) >> return; >> >> - if (tsk == current) { >> - start_backtrace(&frame, >> - (unsigned long)__builtin_frame_address(0), >> - (unsigned long)dump_backtrace); >> - } else { >> - /* >> - * task blocked in __switch_to >> - */ >> - start_backtrace(&frame, >> - thread_saved_fp(tsk), >> - thread_saved_pc(tsk)); >> - } >> - >> printk("%sCall trace:\n", loglvl); >> - do { >> - /* skip until specified stack frame */ >> - if (!skip) { >> - dump_backtrace_entry(frame.pc, loglvl); >> - } else if (frame.fp == regs->regs[29]) { >> - skip = 0; >> - /* >> - * Mostly, this is the case where this function is >> - * called in panic/abort. As exception handler's >> - * stack frame does not contain the corresponding pc >> - * at which an exception has taken place, use regs->pc >> - * instead. >> - */ >> - dump_backtrace_entry(regs->pc, loglvl); >> - } >> - } while (!unwind_frame(tsk, &frame)); >> + arch_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs); > > It turns out we currently need this skipping to get the balance the > ftrace call stack, and arch_stack_walk() doesn't currently do the right > thing when starting from regs. That balancing isn't quite right, and > will be wrong in some case when unwinding across exception boundaries; > we could implement HAVE_FUNCTION_GRAPH_RET_ADDR_PTR using the FP to > solve that. > Hi Mark, It seems that the behavior is the same in the old and new code. Do you agree? In the old code when regs is used, the stack trace starts from regs->regs[29] and the first PC displayed is regs->pc. arch_stack_walk() does the same thing. Can you elaborate what change you want me to make here? Madhavan