linux-kernel - Re: [PATCH 1/3] mm: Make generic arch_is_kernel_initmem

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Date:   Mon, 27 Sep 2021 23:11:56 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Andrew Morton <akpm@...ux-foundation.org>, arnd@...db.de
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arch@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Subject: Re: [PATCH 1/3] mm: Make generic arch_is_kernel_initmem_freed() do
 what it says

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Commit 7a5da02de8d6 ("locking/lockdep: check for freed initmem in
> static_obj()") added arch_is_kernel_initmem_freed() which is supposed
> to report whether an object is part of already freed init memory.
>
> For the time being, the generic version of arch_is_kernel_initmem_freed()
> always reports 'false', allthough free_initmem() is generically called
> on all architectures.
>
> Therefore, change the generic version of arch_is_kernel_initmem_freed()
> to check whether free_initmem() has been called. If so, then check
> if a given address falls into init memory.
>
> In order to use function init_section_contains(), the fonction is
> moved at the end of asm-generic/section.h
>
> Cc: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  include/asm-generic/sections.h | 31 +++++++++++++++++--------------
>  1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index d16302d3eb59..d1e5bb2c6b72 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -172,4 +158,21 @@ static inline bool is_kernel_rodata(unsigned long addr)
>  	       addr < (unsigned long)__end_rodata;
>  }
>  
> +/*
> + * Check if an address is part of freed initmem. This is needed on architectures
> + * with virt == phys kernel mapping, for code that wants to check if an address
> + * is part of a static object within [_stext, _end]. After initmem is freed,
> + * memory can be allocated from it, and such allocations would then have
> + * addresses within the range [_stext, _end].
> + */
> +#ifndef arch_is_kernel_initmem_freed
> +static inline int arch_is_kernel_initmem_freed(unsigned long addr)
> +{
> +	if (system_state < SYSTEM_RUNNING)
> +		return 0;
> +
> +	return init_section_contains((void *)addr, 1);
> +}
> +#endif

This will return an incorrect result for a short period during boot
won't it?

See init/main.c:

static int __ref kernel_init(void *unused)
{
	...
	free_initmem();			<- memory is freed here
	mark_readonly();

	/*
	 * Kernel mappings are now finalized - update the userspace page-table
	 * to finalize PTI.
	 */
	pti_finalize();

	system_state = SYSTEM_RUNNING;


After free_initmem() we have address ranges that are now freed initmem,
but arch_is_kernel_initmem_freed() continues to return 0 (false) for all
addresses, until we update system_state.

Possibly that doesn't matter for any of the current callers, but it
seems pretty dicey to me.

cheers