From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 BCF56426687 for ; Mon, 20 Jul 2026 19:39:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784576385; cv=none; b=q7BOEVwZP72VBIzvyrxdvWI9zQAGtbg5NhfdSgnxUekNHkgGauOrKpOC32YnnQ+4aGu1GzdMCIVJx8sa7WvxxIWqLkk0rLeMEyc9iLieebEKmiIGrmKeEHR57mKq+GyXbrjHoZp5p1QlN4VuFzBbONUs2f24+Ga7ARJJ/WaB+68= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784576385; c=relaxed/simple; bh=0AoPYaek1zIdCaBn/7OIXheVcLvsGEkh1REHHktsBMY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MG38KE/1yymY6gpcJ4QIap/Y+hTkWcMbUFyAfYqXu2jr9sFx5tch8cP7to6NHWvfk41fS9YVg742SuP3YDvJyC+Q5/VTgcLtL0h2AAKQDW09gu1XBBJ0mscT29xn2f3dfGwwMyexHdMOHX3TQ3koboprjRA95T7a5mrXK5J6Ba8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=UEQylVYa; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="UEQylVYa" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784576381; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/qp0qmX7/lG+2iV/7px+x4IwYJSrixYtqlMKjlDaa2k=; b=UEQylVYahUPHKpHwcBPg9WnwTJ4SqNhc1/GHwwewzeeQTWeJBDq8dQc33sgPNQ5xVM+gpZ rJOtWPzp/R8wroyp2kpT60GUb6BG9zG8ibFxKCiR1JzYty50EqpxtsmyOXaG2NLaOYuYQx jyRfm1vXIHd4Ko6tOlB2ZVX3pIu23H8= Date: Mon, 20 Jul 2026 20:39:37 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH RFC v3 2/6] riscv/mm: add untagged_addr_remote_unlocked() To: Rik van Riel Cc: linux-kernel@vger.kernel.org, Andrew Morton , kernel-team@meta.com, David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , linux-riscv@lists.infradead.org References: <20260720115741.239657-1-usama.arif@linux.dev> <13ec2603d5cbfdc493939f182b71e5133c26af54.camel@surriel.com> <46dc1b58-f2de-492a-9da5-a5c429c19a10@linux.dev> <51c0025d2eb885ff1f4775e4d9ffcf34f478f378.camel@surriel.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 20/07/2026 20:21, Rik van Riel wrote: > On Mon, 2026-07-20 at 19:46 +0100, Usama Arif wrote: >> >> So my understanding from exploring this code is, and hopefully >> someone >> in CC from riscv can correct me, for example: >> >> Tagged pointer:  0xabcd000012345678 >> PMLEN 16:        0x0000000012345678 >> PMLEN 7:         0xffcd000012345678 >> >> The target VMA might be at 0x12345678, but applying PMLEN 7 >> to that tagged pointer does not produce that address. >> >> Previously, the order was: >> >> Take mmap read lock. >> Read pmlen. >> Untag the address. >> Look up the VMA. >> >> Changing PMLEN takes the mmap write lock. The read and write >> operations were therefore serialized. >> >> The new order is: >> >> Read pmlen without mmap lock. >> Untag the address. >> Attempt the per-VMA lookup. >> Possibly take mmap lock later. >> Continue using the already-untagged address. >> >> A concurrent PMLEN change can occur between those operations? > > I suppose it could, but what are the possible outcomes here? > > - We fail to untag the address, the vma lookup > fails, and we fail to access memory. > > - The address is already untagged, maps to a > VMA, and the access succeeds. > > Are there any others? > > The VMAs of the process need to be in the bottom > part of the address space, right? The part where > untagged addresses sit. > > For things like /proc//cmdline we should > automatically get an address without any of the > high bits set. > > For things like ptrace peek / poke, BPF process > accesses, and others, I really do not know if > those could get tagged addresses... > > What are the failures we need to protect against? > > What if something comes in with a tagged address, > but the process disables tagging while that > something waits for the mmap_lock? So I think the above question is what needs to be answered. A tagged pointer can become invalid if PMLEN changes before the old mmap-locked lookup too. The mmap lock only defined whether the lookup observed the old or new mode. For VMA as you said, it should be ok. I don't know about others. I think it would be best to get input from RISC-V folks for this. Hopefully its ok.. If it is ok, then all that would be need to be done is to just remove in the commit message that pmlen is stable. > > Does that reproduce the failure case, without > any locking changes? 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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , linux-riscv@lists.infradead.org References: <20260720115741.239657-1-usama.arif@linux.dev> <13ec2603d5cbfdc493939f182b71e5133c26af54.camel@surriel.com> <46dc1b58-f2de-492a-9da5-a5c429c19a10@linux.dev> <51c0025d2eb885ff1f4775e4d9ffcf34f478f378.camel@surriel.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260720_123955_154809_575E7C62 X-CRM114-Status: GOOD ( 25.42 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org CgpPbiAyMC8wNy8yMDI2IDIwOjIxLCBSaWsgdmFuIFJpZWwgd3JvdGU6Cj4gT24gTW9uLCAyMDI2 LTA3LTIwIGF0IDE5OjQ2ICswMTAwLCBVc2FtYSBBcmlmIHdyb3RlOgo+Pgo+PiBTbyBteSB1bmRl cnN0YW5kaW5nIGZyb20gZXhwbG9yaW5nIHRoaXMgY29kZSBpcywgYW5kIGhvcGVmdWxseQo+PiBz b21lb25lCj4+IGluIENDIGZyb20gcmlzY3YgY2FuIGNvcnJlY3QgbWUsIGZvciBleGFtcGxlOgo+ Pgo+PiBUYWdnZWQgcG9pbnRlcjrCoCAweGFiY2QwMDAwMTIzNDU2NzgKPj4gUE1MRU4gMTY6wqDC oMKgwqDCoMKgwqAgMHgwMDAwMDAwMDEyMzQ1Njc4Cj4+IFBNTEVOIDc6wqDCoMKgwqDCoMKgwqDC oCAweGZmY2QwMDAwMTIzNDU2NzgKPj4KPj4gVGhlIHRhcmdldCBWTUEgbWlnaHQgYmUgYXQgMHgx MjM0NTY3OCwgYnV0IGFwcGx5aW5nIFBNTEVOIDcKPj4gdG8gdGhhdCB0YWdnZWQgcG9pbnRlciBk b2VzIG5vdCBwcm9kdWNlIHRoYXQgYWRkcmVzcy4KPj4KPj4gUHJldmlvdXNseSwgdGhlIG9yZGVy IHdhczoKPj4KPj4gVGFrZSBtbWFwIHJlYWQgbG9jay4KPj4gUmVhZCBwbWxlbi4KPj4gVW50YWcg dGhlIGFkZHJlc3MuCj4+IExvb2sgdXAgdGhlIFZNQS4KPj4KPj4gQ2hhbmdpbmcgUE1MRU4gdGFr ZXMgdGhlIG1tYXAgd3JpdGUgbG9jay4gVGhlIHJlYWQgYW5kIHdyaXRlCj4+IG9wZXJhdGlvbnMg d2VyZSB0aGVyZWZvcmUgc2VyaWFsaXplZC4KPj4KPj4gVGhlIG5ldyBvcmRlciBpczoKPj4KPj4g UmVhZCBwbWxlbiB3aXRob3V0IG1tYXAgbG9jay4KPj4gVW50YWcgdGhlIGFkZHJlc3MuCj4+IEF0 dGVtcHQgdGhlIHBlci1WTUEgbG9va3VwLgo+PiBQb3NzaWJseSB0YWtlIG1tYXAgbG9jayBsYXRl ci4KPj4gQ29udGludWUgdXNpbmcgdGhlIGFscmVhZHktdW50YWdnZWQgYWRkcmVzcy4KPj4KPj4g QSBjb25jdXJyZW50IFBNTEVOIGNoYW5nZSBjYW4gb2NjdXIgYmV0d2VlbiB0aG9zZSBvcGVyYXRp b25zPwo+IAo+IEkgc3VwcG9zZSBpdCBjb3VsZCwgYnV0IHdoYXQgYXJlIHRoZSBwb3NzaWJsZSBv dXRjb21lcyBoZXJlPwo+IAo+IC0gV2UgZmFpbCB0byB1bnRhZyB0aGUgYWRkcmVzcywgdGhlIHZt YSBsb29rdXAKPiAgIGZhaWxzLCBhbmQgd2UgZmFpbCB0byBhY2Nlc3MgbWVtb3J5Lgo+IAo+IC0g VGhlIGFkZHJlc3MgaXMgYWxyZWFkeSB1bnRhZ2dlZCwgbWFwcyB0byBhCj4gICBWTUEsIGFuZCB0 aGUgYWNjZXNzIHN1Y2NlZWRzLgo+IAo+IEFyZSB0aGVyZSBhbnkgb3RoZXJzPwo+IAo+IFRoZSBW TUFzIG9mIHRoZSBwcm9jZXNzIG5lZWQgdG8gYmUgaW4gdGhlIGJvdHRvbQo+IHBhcnQgb2YgdGhl IGFkZHJlc3Mgc3BhY2UsIHJpZ2h0PyBUaGUgcGFydCB3aGVyZQo+IHVudGFnZ2VkIGFkZHJlc3Nl cyBzaXQuCj4gCj4gRm9yIHRoaW5ncyBsaWtlIC9wcm9jLzxwaWQ+L2NtZGxpbmUgd2Ugc2hvdWxk Cj4gYXV0b21hdGljYWxseSBnZXQgYW4gYWRkcmVzcyB3aXRob3V0IGFueSBvZiB0aGUKPiBoaWdo IGJpdHMgc2V0Lgo+IAo+IEZvciB0aGluZ3MgbGlrZSBwdHJhY2UgcGVlayAvIHBva2UsIEJQRiBw cm9jZXNzCj4gYWNjZXNzZXMsIGFuZCBvdGhlcnMsIEkgcmVhbGx5IGRvIG5vdCBrbm93IGlmCj4g dGhvc2UgY291bGQgZ2V0IHRhZ2dlZCBhZGRyZXNzZXMuLi4KPiAKPiBXaGF0IGFyZSB0aGUgZmFp bHVyZXMgd2UgbmVlZCB0byBwcm90ZWN0IGFnYWluc3Q/Cj4gCj4gV2hhdCBpZiBzb21ldGhpbmcg Y29tZXMgaW4gd2l0aCBhIHRhZ2dlZCBhZGRyZXNzLAo+IGJ1dCB0aGUgcHJvY2VzcyBkaXNhYmxl cyB0YWdnaW5nIHdoaWxlIHRoYXQKPiBzb21ldGhpbmcgd2FpdHMgZm9yIHRoZSBtbWFwX2xvY2s/ CgpTbyBJIHRoaW5rIHRoZSBhYm92ZSBxdWVzdGlvbiBpcyB3aGF0IG5lZWRzIHRvIGJlCmFuc3dl cmVkLgpBIHRhZ2dlZCBwb2ludGVyIGNhbiBiZWNvbWUgaW52YWxpZCBpZiBQTUxFTiBjaGFuZ2Vz CmJlZm9yZSB0aGUgb2xkIG1tYXAtbG9ja2VkIGxvb2t1cCB0b28uClRoZSBtbWFwIGxvY2sgb25s eSBkZWZpbmVkIHdoZXRoZXIgdGhlIGxvb2t1cCBvYnNlcnZlZAp0aGUgb2xkIG9yIG5ldyBtb2Rl LgoKRm9yIFZNQSBhcyB5b3Ugc2FpZCwgaXQgc2hvdWxkIGJlIG9rLiBJIGRvbid0IGtub3cgYWJv dXQKb3RoZXJzLiBJIHRoaW5rIGl0IHdvdWxkIGJlIGJlc3QgdG8gZ2V0IGlucHV0IGZyb20KUklT Qy1WIGZvbGtzIGZvciB0aGlzLiBIb3BlZnVsbHkgaXRzIG9rLi4KSWYgaXQgaXMgb2ssIHRoZW4g YWxsIHRoYXQgd291bGQgYmUgbmVlZCB0byBiZSBkb25lCmlzIHRvIGp1c3QgcmVtb3ZlIGluIHRo ZSBjb21taXQgbWVzc2FnZSB0aGF0IHBtbGVuCmlzIHN0YWJsZS4KCj4gCj4gRG9lcyB0aGF0IHJl cHJvZHVjZSB0aGUgZmFpbHVyZSBjYXNlLCB3aXRob3V0Cj4gYW55IGxvY2tpbmcgY2hhbmdlcz8K CgoKCl9fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fCmxpbnV4 LXJpc2N2IG1haWxpbmcgbGlzdApsaW51eC1yaXNjdkBsaXN0cy5pbmZyYWRlYWQub3JnCmh0dHA6 Ly9saXN0cy5pbmZyYWRlYWQub3JnL21haWxtYW4vbGlzdGluZm8vbGludXgtcmlzY3YK