Re: [PATCH V3] Documentation: Document the kernel flag bdev_allow_write_mounted

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Aug 23, 2024 at 12:17:54PM -0600, Jens Axboe wrote:
> On 8/23/24 12:05 PM, Guilherme G. Piccoli wrote:
> > Commit ed5cc702d311 ("block: Add config option to not allow writing to mounted
> > devices") added a Kconfig option along with a kernel command-line tuning to
> > control writes to mounted block devices, as a means to deal with fuzzers like
> > Syzkaller, that provokes kernel crashes by directly writing on block devices
> > bypassing the filesystem (so the FS has no awareness and cannot cope with that).
> > 
> > The patch just missed adding such kernel command-line option to the kernel
> > documentation, so let's fix that.
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Darrick J. Wong <djwong@kernel.org>
> > Cc: Jan Kara <jack@suse.cz>
> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> > ---
> > 
> > V3: Dropped reference to page cache (thanks Bart!).
> > 
> > V2 link: https://lore.kernel.org/r/20240823142840.63234-1-gpiccoli@igalia.com
> > 
> > 
> >  Documentation/admin-guide/kernel-parameters.txt | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 09126bb8cc9f..58b9455baf4a 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -517,6 +517,18 @@
> >  			Format: <io>,<irq>,<mode>
> >  			See header of drivers/net/hamradio/baycom_ser_hdx.c.
> >  
> > +	bdev_allow_write_mounted=
> > +			Format: <bool>
> > +			Control the ability of directly writing to mounted block
> 
> Since we're nit picking...
> 
> Control the ability to directly write [...]
> 
> The directly may be a bit confusing ("does it mean O_DIRECT?"), so maybe
> just
> 
> Control the ability to write [...]
> 
> would be better and more clear.

"Control the ability to open a block device for writing."

Since that's what it actually does, right?

--D

> > +			devices, i.e., allow / disallow writes that bypasses the
> 
> Since we're nit picking, s/bypasses/bypass
> 
> > +			FS. This was implemented as a means to prevent fuzzers
> > +			from crashing the kernel by overwriting the metadata
> > +			underneath a mounted FS without its awareness. This
> > +			also prevents destructive formatting of mounted
> > +			filesystems by naive storage tooling that don't use
> > +			O_EXCL. Default is Y and can be changed through the
> > +			Kconfig option CONFIG_BLK_DEV_WRITE_MOUNTED.
> > +
> >  	bert_disable	[ACPI]
> >  			Disable BERT OS support on buggy BIOSes.
> 
> -- 
> Jens Axboe
>