On Mon, Apr 20, 2020 at 08:20:46PM +0000, Luis Chamberlain wrote: > On Mon, Apr 20, 2020 at 10:11:01PM +0200, Greg KH wrote: > > On Mon, Apr 20, 2020 at 06:44:45PM +0000, Luis Chamberlain wrote: > > > 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 <mcgrof@xxxxxxxxxx> > > > > > > > > --- > > > > > > > > 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. > > > > That's nice, but again, no kernel code should do anything different > > depending on what debugfs happens to be doing at that point in time. > > So even if the debugfs files were *not* created, and this code executes only > if DEBUG_FS, you don't think we should inform userspace if the blktrace > setup ioctl, which sets up these debugfs, didn't happen? > > The "recovery" here would just be to destroy the blktrace setup, and > inform userspace that the blktrace setup ioctl failed. Hm, ok, but comment the heck out of this saying _why_ you are testing the return value, and how that differs from 99% of the other users of this function in the kernel tree please. Otherwise I will end up removing the checks again with my semi-regular sweep of the tree... thanks, greg k-h