From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id F27FA14EC46 for ; Wed, 24 Sep 2025 12:21:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758716510; cv=none; b=WEhrywPNbM58DQSZHnKnSe63CIsBG9xmEvuZHiv7LiNQVknOSdqzZGMypw+ue1WPTOCUG46bSkWBZsMaKnN4WH17lm7Wvflxt2BhpN4ARg+pjhqJT3N81j4i8eJWz5swn3ofF6fcs0ef+qnddjYmW5y00RahAfH9fgKNksV2CE4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758716510; c=relaxed/simple; bh=zIqeJIPDHPZhoZBDJUm3kCZy/RRLG5S3T1DKqqs3p5E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=q/EXOtEOCd9vde/2z7ZVulWs6g0aSB+8e9eGda8ULO4XCWxZJjl01xHkbVChC5af3sl1bGhphKxmUAUMY0oVIBFJSpFccmkAS7+Q4k31qwVFxE+sSseAempTppgPtdxtz2bnjecKxt8/Wof+MQ0Ptr2P3nQd++gA0LDAwVCWeOg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D72C8106F; Wed, 24 Sep 2025 05:21:38 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 13B483F66E; Wed, 24 Sep 2025 05:21:44 -0700 (PDT) Date: Wed, 24 Sep 2025 13:21:37 +0100 From: Mark Rutland To: Mengchen Li Cc: catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64: kgdb: Ensure atomic single-step execution Message-ID: References: <1756972043-12854-1-git-send-email-mengchenli64@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1756972043-12854-1-git-send-email-mengchenli64@gmail.com> On Thu, Sep 04, 2025 at 03:47:23PM +0800, Mengchen Li wrote: > The existing KGDB single-step handling on ARM64 is susceptible to > interference from external interrupts. If an interrupt arrives in the > narrow time window between the execution of the instruction under test > and the generation of the step exception, the CPU will vector to the > interrupt handler (e.g., el1_interrupt -> __el1_irq) instead of > triggering the debug exception immediately. > > When the step exception is finally taken, the context is no longer that > of the original instruction. This causes the debugger to appear "stuck", > as it repeatedly tries to single-step through the interrupt handler's > code (e.g., irq_enter_rcu()) instead of the target code. > > The fix is to make the single-step operation atomic by masking interrupts > for its duration: > 1. Upon receiving a step ('s') request from GDB, save the current > PSTATE and then mask IRQs by setting the PSTATE.I bit. > 2. After the single-step exception is taken, in kgdb_single_step_handler(), > disable the kernel's single-step mechanism and meticulously restore > the original interrupt mask state from the saved PSTATE. I don't think that works: * Anything which reads PSTATE/DAIF will see PSTATE.I set unexpectedly. For example, that will break irqflag tracing, since arch_irqs_disabled_flags() will return true in cases where it is expected to return false. * That will break anything which sets PSTATE.I. For example, if stepping local_irq_disable(), the initial DAIF value might have DAIF.I clear. If that's restored *after* local_irq_disable() has set DAIF.I, then restoring the original DAIF.I value will erroneously unmask interrupts. More generally: * The DAIF.IF bits need to be handled atomically, for platforms where FIQ is used. * Even if the DAIF.IF bits are set, It's still possible to take SError, and potentially other exceptions in future (e.g. PMU, NMI). I don't think we can only think about the DAIF.I or DAIF.IF bits. If we want to do something here, I think we'd need to enlighten the entry code with more comprehensive management of the singlestep state. I'm not too keen on coupling that with KGDB. > This guarantees the instruction is executed without interruption and the > debug exception is taken in the correct context. > > As a result of this new approach, the following cleanups are also made: > - The global `kgdb_single_step` flag is removed, as state is now precisely > managed by `kgdb_cpu_doing_single_step` and the interrupt mask. > - The logic to disable single-step and manage the flag in the 'c'ontinue > case is removed, as it is rendered redundant. > - The call to `kernel_rewind_single_step()` is unnecessary and is removed. > > Tested on OrangePi 3B (RK3566) via serial console (kgdboc); > allows reliable single-stepping with GDB where it previously failed. > > Signed-off-by: Mengchen Li > --- > arch/arm64/kernel/kgdb.c | 49 ++++++++++++++++++++---------------------------- > 1 file changed, 20 insertions(+), 29 deletions(-) > > diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c > index 968324a..ee8a7e3 100644 > --- a/arch/arm64/kernel/kgdb.c > +++ b/arch/arm64/kernel/kgdb.c > @@ -101,6 +101,8 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = { > { "fpcr", 4, -1 }, > }; > > +static DEFINE_PER_CPU(unsigned int, kgdb_pstate); > + > char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs) > { > if (regno >= DBG_MAX_REG_NUM || regno < 0) > @@ -128,25 +130,15 @@ int dbg_set_reg(int regno, void *mem, struct pt_regs *regs) > void > sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task) > { > - struct cpu_context *cpu_context = &task->thread.cpu_context; > + struct pt_regs *thread_regs; > > /* Initialize to zero */ > memset((char *)gdb_regs, 0, NUMREGBYTES); > > - gdb_regs[19] = cpu_context->x19; > - gdb_regs[20] = cpu_context->x20; > - gdb_regs[21] = cpu_context->x21; > - gdb_regs[22] = cpu_context->x22; > - gdb_regs[23] = cpu_context->x23; > - gdb_regs[24] = cpu_context->x24; > - gdb_regs[25] = cpu_context->x25; > - gdb_regs[26] = cpu_context->x26; > - gdb_regs[27] = cpu_context->x27; > - gdb_regs[28] = cpu_context->x28; > - gdb_regs[29] = cpu_context->fp; > - > - gdb_regs[31] = cpu_context->sp; > - gdb_regs[32] = cpu_context->pc; > + thread_regs = task_pt_regs(task); > + memcpy((void *)gdb_regs, (void *)thread_regs->regs, GP_REG_BYTES); > + /* Special case for PSTATE */ > + dbg_get_reg(33, gdb_regs + GP_REG_BYTES, thread_regs); > } The commit message doesn't explain anything about the behaviour for sleeping threads, so it's not clear why this is changed at all. The task_pt_regs() helper returns a pointer to the pt_regs for the *userspace* context of a task. That doesn't represent the kernel context, and that's meaningless for kthreads without a userspace context. Regardless of anything else, this is definitely wrong. It is at best pointless. [...] > + /* mask interrupts while single stepping */ > + __this_cpu_write(kgdb_pstate, linux_regs->pstate); > + linux_regs->pstate |= (1 << 7); As a general note, please don't open-code bit shifts like this. IIUC this is PSR_I_BIT. Mark.