linux-kernel - Re: [RFC PATCH] x86/Kconfig: Disable KASLR on debug builds

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Date:   Mon, 4 Dec 2023 10:42:15 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/Kconfig: Disable KASLR on debug builds

On Thu, Nov 30, 2023 at 01:49:09PM +0100, Borislav Petkov wrote:
> On Thu, Nov 30, 2023 at 01:08:31PM +0100, Peter Zijlstra wrote:
> > Works for me, but I have "nokaslr no_hash_pointers" on all my machines
> > by now. It goes right along with "debug ignore_loglevel
> > sysrq_always_enabled earlyprintk=serial,ttyS0,115200" :-)
> 
> Bah, there's cmdline switches. Lemme add them to all my scripts starting
> guests and grub too. Thanks for the hint!
> 
> I guess we could still do the Kconfig dependency when we're on new
> machines, without the scripts.
> 
> Btw, I'm not the only one who's walked right into this one:
> 
> config KCOV
>         bool "Code coverage for fuzzing"
> 
> 	...
> 
>           If RANDOMIZE_BASE is enabled, PC values will not be stable across
>           different machines and across reboots. If you need stable PC values,
>           disable RANDOMIZE_BASE.

IIUC that's no longer necessary, and the Kconfig.debug wording is stale.

That wording was introduced in March 2016 as part of the original KCOV support
in commit:

  5c9a8750a6409c63 ("kernel: add kcov code coverage")

Later, in December 2016 we made KCOV adjust the address to remove the KASLR
offset in commit:

  4983f0ab7ffaad1e ("kcov: make kcov work properly with KASLR enabled")

... so I think that's been stale for ~7 years, and we just forgot to clean up
the Kconfig.debug text.

Mark.