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b=Tu/z/cIvqd6qIe/fvMYIoTbomBHL8lYpR77CTdS6Ny49+5x8wLsC39Hg7FjbZXLmr7vWasGZmam18PmZdov3diiijTSCvhsxyh9N+2VcUaIhTaGc3HYilG/JvnucDIiHmQGTWEXSCxZkyFJpK5LQOO/aiX+RiUN8UdRZqm2k4/s= Received: from 30.74.144.121(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WveUWeH_1766657288 cluster:ay36) by smtp.aliyun-inc.com; Thu, 25 Dec 2025 18:08:09 +0800 Message-ID: Date: Thu, 25 Dec 2025 18:08:08 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink To: Barry Song <21cnbao@gmail.com>, Matthew Wilcox Cc: 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 X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 0564B1C000A X-Stat-Signature: u1cmpjhtjidttkut8w74wkto5ipgt66x X-HE-Tag: 1766657292-232141 X-HE-Meta: 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 SKpB7CzW 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. >> 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