Re: [PATCH 0/2] introduce DUMP_PREFIX_UNHASHED for hex dumps

[Kees Cook <keescook@chromium.org>]

On Tue, Jan 19, 2021 at 07:38:24PM +0900, Sergey Senozhatsky wrote:
> On (21/01/19 01:47), Matthew Wilcox wrote:
> [..]
> > 
> > > So maybe DUMP_PREFIX_UNHASHED can do the unhashed dump only when
> > > CONFIG_DEBUG_KERNEL=y and fallback to DUMP_PREFIX_ADDRESS otherwise?
> > 
> > Distros enable CONFIG_DEBUG_KERNEL.
> 
> Oh, I see.
> 
> > If you want to add CONFIG_DEBUG_LEAK_ADDRESSES, then that's great,
> > and you won't even have to change users, you can just change how %p
> > behaves.
> 
> I like the name. config dependent behaviour of %p wouldn't be new,
> well, to some extent, e.g. XFS does something similar (see below).
> I don't think Linus will be sold on this, however.
> 
> 
> fs/xfs/xfs_linux.h:
> 
> /*
>  * Starting in Linux 4.15, the %p (raw pointer value) printk modifier
>  * prints a hashed version of the pointer to avoid leaking kernel
>  * pointers into dmesg.  If we're trying to debug the kernel we want the
>  * raw values, so override this behavior as best we can.
>  */
> #ifdef DEBUG
> # define PTR_FMT "%px"
> #else
> # define PTR_FMT "%p"
> #endif
> 
> And then they just use it as
> 
> 	xfs_alert(mp, "%s: bad inode magic number, dip = "ptr_fmt",
> 		  dino bp = "ptr_fmt", ino = %ld",
> 		  __func__, dip, bp, in_f->ilf_ino);
> 
> 	-ss

Please no, this is effectively a toggle.

-- 
Kees Cook