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 698802BF006; Tue, 12 Aug 2025 01:02:37 +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=1754960559; cv=none; b=DZ8yMSh+PcRbi3n4dvQ5SyzSSl2Bz9hdrTZGjes+vJR72YigMSU5cQo0RNrbb0qIkdNPgBI6uOyNp9mRpwNBe46plH9+LvK7C0wam/ksMyWB8nP02VY6Ck1VCkJz/X2zidd85pii/aCBgmkGP16IjLUak0VIMsDPjJ2xAwPYPOw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754960559; c=relaxed/simple; bh=QNGaoGPX+uZm4m3QPtgruO/pYCxYbHieL/q1FOfWg1I=; h=Date:To:From:Subject:Message-Id; b=XkB0JNpgZhH2SLPnlWqRrdLQWCL2j/QF3+y4wd4YzFSsLVEhujs1niC4IAZr6neRETPrMc60dLG0wSk8RF/1oqrQMay3IGBxUcNbDVbNnqlxpPlalzOhLNtuiI9oyxMAuW8QwvExfxkGyoiERYQSXg2AfYLrQhMb8iEsb3bU74c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=eIbmrrKF; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="eIbmrrKF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B52F8C4CEED; Tue, 12 Aug 2025 01:02:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1754960557; bh=QNGaoGPX+uZm4m3QPtgruO/pYCxYbHieL/q1FOfWg1I=; h=Date:To:From:Subject:From; b=eIbmrrKF0ruNLb+b+m0PVtAeN7UMWOe1WOo4RZbd+cUz5SAjQCCcaZFDMeJuU6FpD LUGPKxLT5LAzXwiNB+iJn0yxkfnQ9579cW9+iMpUx24GpnkdpBDopTAZkSmDjqAoaV jsFyIigQPGc3aDgV6ZJvtCY1PUGy1LkXvG4+6ec0= Date: Mon, 11 Aug 2025 18:02:37 -0700 To: mm-commits@vger.kernel.org,viro@zeniv.linux.org.uk,stable@vger.kernel.org,ryan@lahfa.xyz,maximilian@mbosch.me,dhowells@redhat.com,ct@flyingcircus.io,brauner@kernel.org,arnout@bzzt.net,asmadeus@codewreck.org,akpm@linux-foundation.org From: Andrew Morton Subject: + iov_iter-iterate_folioq-fix-handling-of-offset-=-folio-size.patch added to mm-hotfixes-unstable branch Message-Id: <20250812010237.B52F8C4CEED@smtp.kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: iov_iter: iterate_folioq: fix handling of offset >= folio size has been added to the -mm mm-hotfixes-unstable branch. Its filename is iov_iter-iterate_folioq-fix-handling-of-offset-=-folio-size.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/iov_iter-iterate_folioq-fix-handling-of-offset-=-folio-size.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Dominique Martinet Subject: iov_iter: iterate_folioq: fix handling of offset >= folio size Date: Mon, 11 Aug 2025 16:39:05 +0900 So we've had this regression in 9p for.. almost a year, which is way too long, but there was no "easy" reproducer until yesterday (thank you again!!) It turned out to be a bug with iov_iter on folios, iov_iter_get_pages_alloc2() would advance the iov_iter correctly up to the end edge of a folio and the later copy_to_iter() fails on the iterate_folioq() bug. Happy to consider alternative ways of fixing this, now there's a reproducer it's all much clearer; for the bug to be visible we basically need to make and IO with non-contiguous folios in the iov_iter which is not obvious to test with synthetic VMs, with size that triggers a zero-copy read followed by a non-zero-copy read. It's apparently possible to get an iov forwarded all the way up to the end of the current page we're looking at, e.g. (gdb) p *iter $24 = {iter_type = 4 '\004', nofault = false, data_source = false, iov_offset = 4096, {__ubuf_iovec = { iov_base = 0xffff88800f5bc000, iov_len = 655}, {{__iov = 0xffff88800f5bc000, kvec = 0xffff88800f5bc000, bvec = 0xffff88800f5bc000, folioq = 0xffff88800f5bc000, xarray = 0xffff88800f5bc000, ubuf = 0xffff88800f5bc000}, count = 655}}, {nr_segs = 2, folioq_slot = 2 '\002', xarray_start = 2}} Where iov_offset is 4k with 4k-sized folios This should have been because we're only in the 2nd slot and there's another one after this, but iterate_folioq should not try to map a folio that skips the whole size, and more importantly part here does not end up zero (because 'PAGE_SIZE - skip % PAGE_SIZE' ends up PAGE_SIZE and not zero..), so skip forward to the "advance to next folio" code. Link: https://lkml.kernel.org/r/20250811-iot_iter_folio-v1-0-d9c223adf93c@codewreck.org Link: https://lkml.kernel.org/r/20250811-iot_iter_folio-v1-1-d9c223adf93c@codewreck.org Link: https://lkml.kernel.org/r/D4LHHUNLG79Y.12PI0X6BEHRHW@mbosch.me/ Fixes: db0aa2e9566f ("mm: Define struct folio_queue and ITER_FOLIOQ to handle a sequence of folios") Signed-off-by: Dominique Martinet Reported-by: Maximilian Bosch Reported-by: Ryan Lahfa Reported-by: Christian Theune Reported-by: Arnout Engelen Cc: Al Viro Cc: Christian Brauner Cc: David Howells Cc: Ryan Lahfa Cc: [v6.12+] Signed-off-by: Andrew Morton --- include/linux/iov_iter.h | 3 +++ 1 file changed, 3 insertions(+) --- a/include/linux/iov_iter.h~iov_iter-iterate_folioq-fix-handling-of-offset-=-folio-size +++ a/include/linux/iov_iter.h @@ -168,6 +168,8 @@ size_t iterate_folioq(struct iov_iter *i break; fsize = folioq_folio_size(folioq, slot); + if (skip >= fsize) + goto next; base = kmap_local_folio(folio, skip); part = umin(len, PAGE_SIZE - skip % PAGE_SIZE); remain = step(base, progress, part, priv, priv2); @@ -177,6 +179,7 @@ size_t iterate_folioq(struct iov_iter *i progress += consumed; skip += consumed; if (skip >= fsize) { +next: skip = 0; slot++; if (slot == folioq_nr_slots(folioq) && folioq->next) { _ Patches currently in -mm which might be from asmadeus@codewreck.org are iov_iter-iterate_folioq-fix-handling-of-offset-=-folio-size.patch iov_iter-iov_folioq_get_pages-dont-leave-empty-slot-behind.patch