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Sun, 22 Sep 2024 10:39:44 -0400 X-MC-Unique: YQ2wLZ_mO7yMR6mnMZ0BdA-1 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (unknown [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AEA9618F655A; Sun, 22 Sep 2024 14:39:42 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.45.224.28]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id 0A1BD3000235; Sun, 22 Sep 2024 14:39:38 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Sun, 22 Sep 2024 16:39:30 +0200 (CEST) Date: Sun, 22 Sep 2024 16:39:25 +0200 From: Oleg Nesterov To: Will Deacon Cc: Catalin Marinas , "Liao, Chang" , mhiramat@kernel.org, peterz@infradead.org, mark.rutland@arm.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm64: uprobes: Optimize cache flushes for xol slot Message-ID: <20240922143858.GD9426@redhat.com> References: <20240919121719.2148361-1-liaochang1@huawei.com> <20240919141824.GB12149@redhat.com> <41fdfc47-4161-d2e4-6528-4079b660424f@huawei.com> <20240920173223.GA20847@redhat.com> <20240922140910.GA31288@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240922140910.GA31288@willie-the-truck> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240922_073952_579911_E481FC0D X-CRM114-Status: GOOD ( 28.81 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 09/22, Will Deacon wrote: > > On Fri, Sep 20, 2024 at 07:32:23PM +0200, Oleg Nesterov wrote: > > On 09/20, Catalin Marinas wrote: > > > > > > On Fri, Sep 20, 2024 at 04:58:31PM +0800, Liao, Chang wrote: > > > > > > > > > > > > 在 2024/9/19 22:18, Oleg Nesterov 写道: > > > > > On 09/19, Liao Chang wrote: > > > > >> > > > > >> --- a/arch/arm64/kernel/probes/uprobes.c > > > > >> +++ b/arch/arm64/kernel/probes/uprobes.c > > > > >> @@ -17,12 +17,16 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr, > > > > >> void *xol_page_kaddr = kmap_atomic(page); > > > > >> void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK); > > > > >> > > > > >> + if (!memcmp(dst, src, len)) > > > > >> + goto done; > > > > > > > > > > can't really comment, I know nothing about arm64... > > > > > > > > > > but don't we need to change __create_xol_area() > > > > > > > > > > - area->page = alloc_page(GFP_HIGHUSER); > > > > > + area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO); > > > > > > > > > > to avoid the false positives? > > > > > > > > Indeed, it would be safer. > > > > > > > > Could we tolerate these false positives? Even if the page are not reset > > > > to zero bits, if the existing bits are the same as the instruction being > > > > copied, it still can execute the correct instruction. > > > > > > Not if the I-cache has stale data. If alloc_page() returns a page with > > > some random data that resembles a valid instruction but there was never > > > a cache flush (sync_icache_aliases() on arm64), it's irrelevant whether > > > the compare (on the D-cache side) succeeds or not. > > > > But shouldn't the page fault paths on arm64 flush I-cache ? > > > > If alloc_page() returns a page with some random data that resembles a valid > > instruction, user-space can't execute this instruction until > > special_mapping_fault() installs the page allocated in __create_xol_area(). > > > > Again, I know nothing about arm64/icache/etc, I am just curious and trying > > to understand... > > We defer the icache maintenance until set_pte_at() time, where we call > __sync_icache_dcache() if we're installing a present, executable user > eintry. And to me this looks as if __sync_icache_dcache() must be called when user space tries to fault-in the page allocated in __create_xol_area() and returned by special_mapping_fault(). So I still don't understand the problem. Oleg.