Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

Andrew Morton
2017-11-29T23:21:02Z

On Wed, 29 Nov 2017 13:05:04 +1100 "Tobin C. Harding" <me@tobin.cc> wrote:

> printk specifier %p now hashes all addresses before printing. Sometimes
> we need to see the actual unmodified address. This can be achieved using
> %lx but then we face the risk that if in future we want to change the
> way the Kernel handles printing of pointers we will have to grep through
> the already existent 50 000 %lx call sites. Let's add specifier %px as a
> clear, opt-in, way to print a pointer and maintain some level of
> isolation from all the other hex integer output within the Kernel.
>
> Add printk specifier %px to print the actual unmodified address.
>
> ...
>
> +Unmodified Addresses
> +====================
> +
> +::
> +
> + %px 01234567 or 0123456789abcdef
> +
> +For printing pointers when you _really_ want to print the address. Please
> +consider whether or not you are leaking sensitive information about the
> +Kernel layout in memory before printing pointers with %px. %px is
> +functionally equivalent to %lx. %px is preferred to %lx because it is more
> +uniquely grep'able. If, in the future, we need to modify the way the Kernel
> +handles printing pointers it will be nice to be able to find the call
> +sites.
> +

You might want to add a checkpatch rule which emits a stern
do-you-really-want-to-do-this warning when someone uses %px.