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[157.230.128.187]) by smtp.gmail.com with ESMTPSA id p64sm92874pjp.7.2020.04.20.11.44.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2020 11:44:46 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id 825824028E; Mon, 20 Apr 2020 18:44:45 +0000 (UTC) Date: Mon, 20 Apr 2020 18:44:45 +0000 From: Luis Chamberlain To: Greg KH Cc: Bart Van Assche , axboe@kernel.dk, viro@zeniv.linux.org.uk, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 08/10] blktrace: add checks for created debugfs files on setup Message-ID: <20200420184445.GK11244@42.do-not-panic.com> References: <20200419194529.4872-1-mcgrof@kernel.org> <20200419194529.4872-9-mcgrof@kernel.org> <38240225-e48e-3035-0baa-4929948b23a3@acm.org> <20200419230537.GG11244@42.do-not-panic.com> <20200420114038.GE3906674@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420114038.GE3906674@kroah.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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: On Mon, Apr 20, 2020 at 01:40:38PM +0200, Greg KH wrote: > On Sun, Apr 19, 2020 at 04:17:46PM -0700, Bart Van Assche wrote: > > On 4/19/20 4:05 PM, Luis Chamberlain wrote: > > > On Sun, Apr 19, 2020 at 03:57:58PM -0700, Bart Van Assche wrote: > > > > On 4/19/20 12:45 PM, Luis Chamberlain wrote: > > > > > Even though debugfs can be disabled, enabling BLK_DEV_IO_TRACE will > > > > > select DEBUG_FS, and blktrace exposes an API which userspace uses > > > > > relying on certain files created in debugfs. If files are not created > > > > > blktrace will not work correctly, so we do want to ensure that a > > > > > blktrace setup creates these files properly, and otherwise inform > > > > > userspace. > > > > > > > > > > Signed-off-by: Luis Chamberlain > > > > > --- > > > > > kernel/trace/blktrace.c | 8 +++++--- > > > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > > > > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > > > > > index 9cc0153849c3..fc32a8665ce8 100644 > > > > > --- a/kernel/trace/blktrace.c > > > > > +++ b/kernel/trace/blktrace.c > > > > > @@ -552,17 +552,19 @@ static int blk_trace_create_debugfs_files(struct blk_user_trace_setup *buts, > > > > > struct dentry *dir, > > > > > struct blk_trace *bt) > > > > > { > > > > > - int ret = -EIO; > > > > > - > > > > > bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt, > > > > > &blk_dropped_fops); > > > > > + if (!bt->dropped_file) > > > > > + return -ENOMEM; > > > > > bt->msg_file = debugfs_create_file("msg", 0222, dir, bt, &blk_msg_fops); > > > > > + if (!bt->msg_file) > > > > > + return -ENOMEM; > > > > > bt->rchan = relay_open("trace", dir, buts->buf_size, > > > > > buts->buf_nr, &blk_relay_callbacks, bt); > > > > > if (!bt->rchan) > > > > > - return ret; > > > > > + return -EIO; > > > > > return 0; > > > > > } > > > > > > > > I should have had a look at this patch before I replied to the previous > > > > patch. > > > > > > > > Do you agree that the following code can be triggered by > > > > debugfs_create_file() and also that debugfs_create_file() never returns > > > > NULL? > > > > > > If debugfs is enabled, and not that we know it is in this blktrace code, > > > as we select it, it can return ERR_PTR(-ERROR) if an error occurs. > > > > This is what I found in include/linux/debugfs.h in case debugfs is disabled: > > > > static inline struct dentry *debugfs_create_file(const char *name, > > umode_t mode, struct dentry *parent, void *data, > > const struct file_operations *fops) > > { > > return ERR_PTR(-ENODEV); > > } > > > > I have not found any code path that can cause debugfs_create_file() to > > return NULL. Did I perhaps overlook something? If not, it's not clear to me > > why the above patch adds checks that check whether debugfs_create_file() > > returns NULL? > > Short answer, yes, it can return NULL. Correct answer is, you don't > care, don't check the value and don't do anything about it. It's > debugging code, userspace doesn't care, so just keep moving on. Thing is this code *exposes* knobs to userspace for an API that *does* exepect those files to exist. That is, blktrace *relies* on these debugfs files to exist. So the kconfig which enables blktrace CONFIG_BLK_DEV_IO_TRACE selects DEBUG_FS. So typically we don't care if these files were created or not on regular drivers, but in this case this code is only compiled when debugfs is enabled and CONFIG_BLK_DEV_IO_TRACE, and the userspace interaction with debugfs *expects* these files. So what do you recommend? Luis