From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CCEAC19D890 for ; Wed, 11 Jun 2025 17:34:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749663259; cv=none; b=RLquHJLOsYcdCoK2UBxGfEIZQHqKpW4d68QaU0dCZ/naGHV/L25YhedPhJaehMZG3pkYK0Xy9IomHm/MOsgkPwW0aAg+TA4gxHpaBJRHveEvqtI6g+mSchMskhiF4z7fJ+c9bMSPhAuu6oFOQgOPvfHvGX64SKs6sVAStpSFX1Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749663259; c=relaxed/simple; bh=arq9sEPBSyiJkrXfXGK5H21SI5ic27DxPn+1UFfC1ow=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Pn5csDNAJye0vcb/rEZ4hw2TVnW2NQhx4aQz5QNbeKNuwwnQCpoM47w1rdS9Z5dzH3l4H1KF/h93x5pdWM1SfLQuhmvVDm+A9X/sW9frF2jdEDJQ0VtyXHlPGDuOj0PH7QpL8g2uaOx7g7qHEyc4atcjwsr2vkGh2R75+SgPZUY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 91E40C4CEE3; Wed, 11 Jun 2025 17:34:18 +0000 (UTC) Date: Wed, 11 Jun 2025 18:34:15 +0100 From: Catalin Marinas To: Mark Brown Cc: Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64/gcs: Don't call gcs_free() during flush_gcs() Message-ID: References: <20250611-arm64-gcs-flush-thread-v1-1-cc26feeddabd@kernel.org> 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: <20250611-arm64-gcs-flush-thread-v1-1-cc26feeddabd@kernel.org> On Wed, Jun 11, 2025 at 05:28:13PM +0100, Mark Brown wrote: > Currently we call gcs_free() during flush_gcs() to reset the thread state > for GCS. This includes unmapping any kernel allocated GCS, but this is > redundant when doing a flush_thread() since we are reinitialisng the thread > memory too. Inline the reinitialisaton of the thread struct. > > Signed-off-by: Mark Brown > --- > arch/arm64/kernel/process.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index a5ca15daeb8a..5954cec19660 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -288,7 +288,9 @@ static void flush_gcs(void) > if (!system_supports_gcs()) > return; > > - gcs_free(current); > + current->thread.gcspr_el0 = 0; > + current->thread.gcs_base = 0; > + current->thread.gcs_size = 0; > current->thread.gcs_el0_mode = 0; > write_sysreg_s(GCSCRE0_EL1_nTR, SYS_GCSCRE0_EL1); > write_sysreg_s(0, SYS_GCSPR_EL0); I think this makes sense. However, I thought there was another slightly misplaced call to gcs_free() via arch_release_task_struct(). I wouldn't touch the user memory with vm_munmap() when releasing a task structure. Is this needed because the shadow stack is allocated automatically on thread creation, so we need something to free it when the thread died? Another caller of gcs_free() is deactivate_mm(). It's not clear to me when we need to free the shadow stack on this path. On the exit_mm() path for example we have mmput() -> exit_mmap() that takes care of unmapping everything. Similarly on the exec_mmap() path. -- Catalin