Re: [PATCH 0/2] use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE_ATTRIBUTE

[Rohit Sarkar]

On Sun, Mar 29, 2020 at 10:38:18AM +0100, Jonathan Cameron wrote:
> On Sat, 28 Mar 2020 12:04:53 +0530
> Rohit Sarkar <rohitsarkar5398@xxxxxxxxx> wrote:
> 
> > The debugfs_create_file_unsafe method does not protect the fops given to
> > it from file removal. It must be used with DEFINE_DEBUGFS_ATTRIBUTE
> > which makes the fops aware of the file lifetime.
> > 
> > Further using DEFINE_DEBUGFS_ATTRIBUTE along with
> > debugfs_create_file_unsafe significantly reduces the overhead introduced by
> > debugfs_create_file which creates a lifetime managing proxy around each
> > fops handed in. Refer [1] for more on this.
> > 
> > Fixes the following warnings reported by coccinelle:
> > drivers/iio/imu//adis16460.c:126:0-23: WARNING: adis16460_flash_count_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> > drivers/iio/imu//adis16460.c:108:0-23: WARNING: adis16460_product_id_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> > drivers/iio/imu//adis16460.c:90:0-23: WARNING: adis16460_serial_number_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> > drivers/iio/imu//adis16400.c:278:0-23: WARNING: adis16400_flash_count_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> > drivers/iio/imu//adis16400.c:261:0-23: WARNING: adis16400_product_id_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> > 
> > [1]: https://lists.gt.net/linux/kernel/2369498
> > 
> > Rohit Sarkar (2):
> >   iio: imu: adis16400: use DEFINE_DEBUGFS_ATTRIBUTE instead of
> >     DEFINE_SIMPLE_ATTRIBUTE
> >   iio: imu: adis16460: use DEFINE_DEBUGFS_ATTRIBUTE instead of
> >     DEFINE_SIMPLE_ATTRIBUTE
> > 
> >  drivers/iio/imu/adis16400.c | 4 ++--
> >  drivers/iio/imu/adis16460.c | 6 +++---
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> > 
> Hi Rohit,
Hey, 
> You've opened a can of worms with this one.  There as a previous series
> posted doing exactly this change back in 2019 by Zhong Jiang (cc'd)
> 
> At the time I did a bit of looking into why this had been universally taken
> up cross tree and turned out there are some potential issues.
> 
> Alexandru added it to the list of things to test, but I guess it got
> buried under other work and is still outstanding.
> 
> https://lkml.org/lkml/2019/10/30/144
Acc. to the patch by Zhong this change kind of comes off as a cosmetic
change as in the commit message he mentions "it is more clear". 

But there is certainly more to it than that:
In the current scenario since we are using debugfs_create_file_unsafe
the file has no protection whatsoever against removal. 
> Has Greg KH raising the point that file reference counting is changed (as you
> mention) but that can cause subtle bugs.  It 'might' be fine but is
> definitely one that needs a tested-by from someone with the hardware.
Sure that makes sense. 
> Jonathan

Thanks,
Rohit