From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-98.freemail.mail.aliyun.com (out30-98.freemail.mail.aliyun.com [115.124.30.98]) (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 AF548265CA8 for ; Tue, 6 Jan 2026 03:47:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.98 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767671272; cv=none; b=VDeOJPLRA5HgM3uxRvXr9A0OQdBghci79J+iD4xOv0IAwIuGtdWB6hDStid85+KMpnplWhZFFX4RJK5qB2sOJXXNOheHGsUVxRPEWhlpCqn84j+HiHbcBXnW+47PkLKIeB4SHtWfQLpjT8xNGtFR90z4KWcJC2JGtolpOZvdUgE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767671272; c=relaxed/simple; bh=Z/ZrYvpWG/19+zWSjmh9g4px3vxYPEVEHZMgiakUTac=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=L2zfBSm4AFd5BbnVlYfVtJB447qRT7WEFC2h3b0ATGM+OTynlnBkhyOJiLhOjnDK7sctQgSzOJ+/D7l6uw3s/3sLBxfCos00gpLMU3/qf4SxXXvzWwxxfqL7B2ZlnJB0lTKZZuTLc+6EsQ/dbIMipBIoTQy7zaVS9gDxyJxI4aw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=N0S7syNx; arc=none smtp.client-ip=115.124.30.98 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="N0S7syNx" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1767671267; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=p/GXGuZ3cPXHcrpAe9cRwt8UMcvtK3NDfHnG89dSZn4=; b=N0S7syNxnONC91vPEjmsAzbQSYWXyg0qyW1BWto4mRxaJJjj55BRW+m5/qZamRRBZTkkm3s1y/PhCE1OmSSWC10BPzasLmuLIgXuzG+R+l0m/gNbCVGvebGk9kiZvnbbMMwJ5Vve1vUiC054c/Fvymcqy9g8LoL5njPc/ycuGsM= Received: from 30.221.149.230(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WwUF.zz_1767671265 cluster:ay36) by smtp.aliyun-inc.com; Tue, 06 Jan 2026 11:47:45 +0800 Message-ID: <2b12f63b-ceab-4d3e-a06f-f41e6b1b2d23@linux.alibaba.com> Date: Tue, 6 Jan 2026 11:47:44 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink To: Brian Foster Cc: Barry Song <21cnbao@gmail.com>, Matthew Wilcox , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , syzbot+178fff6149127421c2cc@syzkaller.appspotmail.com References: <20251224094027.65842-1-21cnbao@gmail.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 1/5/26 9:58 PM, Brian Foster wrote: > On Thu, Dec 25, 2025 at 06:08:08PM +0800, Baolin Wang wrote: >> >> >> On 2025/12/25 12:04, Barry Song wrote: >>> On Thu, Dec 25, 2025 at 6:01 AM Matthew Wilcox wrote: >>>> >>>> On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote: >>>>> From: Barry Song >>>>> >>>>> Uninitialized folio allocated in shmem_symlink() may be accessed >>>>> during swap-out, causing KMSAN BUG: >>>> >>>> This would be an unfortunate way to fix it. The vast majority of >>>> symlinks are short, and we'll never access past the \0 in normal >>>> operation, so we'll be dirtying a lot of cachelines essentially to (1) >>>> shut up an automated tool and (2) optimise a corner case. >>>> >>>> How about this instead which delays zeroing to swapout? >>> >>> Matthew, thank you very much for your review, even during Christmas. >>> I would like to wish you a happy holiday! >>> >>> I am not quite sure, as shm symlinks do not seem very common. Since >>> allocating a folio requires a symname longer than 128 bytes (where >>> 128 == SHORT_SYMLINK_LEN), such cases appear even rarer. >>> >>> BTW, do we need to migrate the owner_2 flag in folio_migrate_flags()? >>> If so, I am not quite sure it is worth changing the hotpath to >>> accommodate this. >> >> +1. At least for me, using the 'PG_owner_2' flag alone to mark this uncommon >> case doesn't seem quite worthwhile. >> > > Also JFYI the post-eof swapout zeroing work (still pending) looks to me > like it would cover the swapout time case [1]. That's just if you wanted > to go that route here; creation time zeroing for the large symlink case > seems reasonable enough to me as well. IMHO, your post-eof zeroing work is intended to zero !uptodate or beyond EOF folios before swap-out, however, the symlink folio is uptodate and not beyond the EOF. I am not sure if it will make your code more complicated when you want to cover this symlink case. Why I prefer Barry's fix: First, the symlink folio is marked Uptodate after copying the symlink name, but the whole folio hasn’t been initialized, which seems unreasonable to me. Second, as I said before, using the 'PG_owner_2' flag to mark this uncommon case doesn’t seem worthwhile. Currently, IIUC the 'PG_owner_2' is only used by btrfs; if we ever want to remove the 'PG_owner_2', this uncommon symlink case shouldn’t block its removal. BTW, as I said before, I've reviewed and tested your post-eof work[1]. Maybe you could resend the patch set so that Hugh can take a look when he has time. > [1] https://lore.kernel.org/linux-mm/20251121152246.1023918-3-bfoster@redhat.com/ > >>>> diff --git a/mm/shmem.c b/mm/shmem.c >>>> index ec6c01378e9d..f3b3be1b50fe 100644 >>>> --- a/mm/shmem.c >>>> +++ b/mm/shmem.c >>>> @@ -1636,6 +1636,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, >>>> folio_mark_uptodate(folio); >>>> } >>>> >>>> + /* Zero out symlink tails to help with compression */ >>>> + if (folio_test_owner_2(folio)) { >>>> + struct inode *inode = folio->mapping->host; >>>> + folio_zero_segment(folio, inode->i_size, folio_size(folio)); >>>> + folio_clear_owner_2(folio); >>>> + } >>>> + >>>> if (!folio_alloc_swap(folio)) { >>>> bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages); >>>> int error; >>>> @@ -4133,6 +4140,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir, >>>> memcpy(folio_address(folio), symname, len); >>>> folio_mark_uptodate(folio); >>>> folio_mark_dirty(folio); >>>> + folio_set_owner_2(folio); >>>> folio_unlock(folio); >>>> folio_put(folio); >>>> } >>> >>> Thanks >>> Barry >> >>