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Thu, 28 Mar 2024 13:51:34 -0400 X-MC-Unique: 4zpXVmEIOq2rWiH1sW_bfg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 621ED185A781; Thu, 28 Mar 2024 17:51:34 +0000 (UTC) Received: from bfoster (unknown [10.22.16.57]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D78E84B4405; Thu, 28 Mar 2024 17:51:33 +0000 (UTC) Date: Thu, 28 Mar 2024 13:53:31 -0400 From: Brian Foster To: Kemeng Shi Cc: akpm@linux-foundation.org, willy@infradead.org, jack@suse.cz, tj@kernel.org, dsterba@suse.com, mjguzik@gmail.com, dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v2 1/6] writeback: protect race between bdi release and bdi_debug_stats_show Message-ID: References: <20240327155751.3536-1-shikemeng@huaweicloud.com> <20240327155751.3536-2-shikemeng@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240327155751.3536-2-shikemeng@huaweicloud.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.10 X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 4BC904000E X-Stat-Signature: kn3mo9n1ujnfirbqs83czfp6w9qznp56 X-HE-Tag: 1711648299-297619 X-HE-Meta: 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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 Wed, Mar 27, 2024 at 11:57:46PM +0800, Kemeng Shi wrote: > There is a race between bdi release and bdi_debug_stats_show: > /* get debug info */ /* bdi release */ > bdi_debug_stats_show > bdi = m->private; > wb = &bdi->wb; > bdi_unregister > bdi_put > release_bdi > kfree(bdi) > /* use after free */ > spin_lock(&wb->list_lock); > Maybe I'm missing something, but it looks to me that bdi_unregister_debug() can't complete until active readers of associated debugfs files have completed. For example, see __debugfs_file_removed() and use of ->active_users[_drained]. Once the dentry is unlinked, further reads fail (I think) via debugfs_file_get(). Hm? Brian > Search bdi on bdi_list under rcu lock in bdi_debug_stats_show to ensure > the bdi is not freed to fix the issue. > > Signed-off-by: Kemeng Shi > --- > mm/backing-dev.c | 33 +++++++++++++++++++++++++++++++-- > 1 file changed, 31 insertions(+), 2 deletions(-) > > diff --git a/mm/backing-dev.c b/mm/backing-dev.c > index 5f2be8c8df11..70f02959f3bd 100644 > --- a/mm/backing-dev.c > +++ b/mm/backing-dev.c > @@ -46,16 +46,44 @@ static void bdi_debug_init(void) > bdi_debug_root = debugfs_create_dir("bdi", NULL); > } > > -static int bdi_debug_stats_show(struct seq_file *m, void *v) > +static struct backing_dev_info *lookup_bdi(struct seq_file *m) > { > + const struct file *file = m->file; > struct backing_dev_info *bdi = m->private; > - struct bdi_writeback *wb = &bdi->wb; > + struct backing_dev_info *exist; > + > + list_for_each_entry_rcu(exist, &bdi_list, bdi_list) { > + if (exist != bdi) > + continue; > + > + if (exist->debug_dir == file->f_path.dentry->d_parent) > + return bdi; > + else > + return NULL; > + } > + > + return NULL; > +} > + > + > +static int bdi_debug_stats_show(struct seq_file *m, void *v) > +{ > + struct backing_dev_info *bdi; > + struct bdi_writeback *wb; > unsigned long background_thresh; > unsigned long dirty_thresh; > unsigned long wb_thresh; > unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time; > struct inode *inode; > > + rcu_read_lock(); > + bdi = lookup_bdi(m); > + if (!bdi) { > + rcu_read_unlock(); > + return -EEXIST; > + } > + > + wb = &bdi->wb; > nr_dirty = nr_io = nr_more_io = nr_dirty_time = 0; > spin_lock(&wb->list_lock); > list_for_each_entry(inode, &wb->b_dirty, i_io_list) > @@ -101,6 +129,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v) > nr_dirty_time, > !list_empty(&bdi->bdi_list), bdi->wb.state); > > + rcu_read_unlock(); > return 0; > } > DEFINE_SHOW_ATTRIBUTE(bdi_debug_stats); > -- > 2.30.0 >