Re: [PATCH 2/3] slub: Print raw pointer addresses when debugging

From: Petr Mladek
Date: Mon May 24 2021 - 07:32:21 EST

On Wed 2021-05-19 18:35:38, Stephen Boyd wrote:
> Obscuring the pointers that slub shows when debugging makes for some
> confusing slub debug messages:
> Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17
> Those addresses are hashed for kernel security reasons. If we're trying
> to be secure with slub_debug on the commandline we have some big
> problems given that we dump whole chunks of kernel memory to the kernel
> logs. Let's use %px here and dump buffers with the actual address for
> the buffer instead of the hashed version so that the logs are
> meaningful. This also helps if a kernel address is in some slub debug
> report so we can figure out that the object is referencing itself.

Please, do not do this!

Use "no_hash_pointers" commandling option when you want to see
raw pointers. It will make it clear when the kernel logs are save
and when not.

If "slub_debug" is useless with hashed pointers then it might enable
"no_hash_pointers". But make sure that it prints the fat warning.

This patch is the worst approach. We have to keep the number of "%px"
callers at minimum to keep it maintainable. The only safe use-case is
when the system is in panic() [*]. If the pointers might be printed
at any time then users should be warned by the fat message printed
by "no_hash_pointers".

[*] Raw pointers are currently printed also by Oops/WARN messages.
It is from historic reasons. Anyway, they are fat warnings
on its own. The system often need to get reported anyway.

Best Regards,