linux-kernel - Re: [PATCH] powerpc: fix alignment bug whithin the init sections

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Date:   Fri, 18 Dec 2020 16:38:46 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ariel Marcovitch <arielmarcovitch@...il.com>, mpe@...erman.id.au
Cc:     paulus@...ba.org, linuxppc-dev@...ts.ozlabs.org,
        Ariel Marcovitch <ariel.marcovitch@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: fix alignment bug whithin the init sections



Le 13/12/2020 à 19:35, Ariel Marcovitch a écrit :
> This is a bug that can cause early crashes in configurations with a
> .exit.text section smaller than a page and a .init.text section that
> ends in the beginning of a physical page (this is kinda random, which
> might explain why this wasn't really encountered before).

It can cause, or it causes ? Did you encounter the issue ?

> 
> The init sections are ordered like this:
> 	.init.text
> 	.exit.text
> 	.init.data
> 
> Currently, these sections aren't page aligned.
> 
> Because the init code is mapped read-only at runtime and because the
> .init.text section can potentially reside on the same physical page as
> .init.data, the beginning of .init.data might be mapped read-only along
> with .init.text.

init code is mapped PAGE_KERNEL_TEXT.

Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected options.

> 
> Then when the kernel tries to modify a variable in .init.data (like
> kthreadd_done, used in kernel_init()) the kernel panics.
> 
> To avoid this, I made these sections page aligned.

Should write this unpersonal, something like "To avoid this, make these sections page aligned"

> 
> Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> Signed-off-by: Ariel Marcovitch <ariel.marcovitch@...il.com>
> ---
>   arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 326e113d2e45..e3a7c90c03f4 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -179,6 +179,11 @@ SECTIONS
>   #endif
>   	} :text
>   
> +	/* .init.text is made RO and .exit.text is not, so we must
> +	 * ensure these sections reside in separate physical pages.
> +	 */
> +	. = ALIGN(PAGE_SIZE);
> +

In principle, as it is text, it should be made RO as well. But what happens at the begining doesn't 
really matter, anyway .exit.text should never be executed and is discarded together with init text.
So, I think it is OK the live with it as is for the time being.

Making it page aligned makes sense anyway.

Should we make _einittext page aligned instead, just like _etext ?

>   	/* .exit.text is discarded at runtime, not link time,
>   	 * to deal with references from __bug_table
>   	 */
> @@ -186,6 +191,8 @@ SECTIONS
>   		EXIT_TEXT
>   	}
>   
> +	. = ALIGN(PAGE_SIZE);
> +

Here for sure, as you explain in the coming log, this needs to be separated from init text. So 
making it aligned is a must.

>   	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
>   		INIT_DATA
>   	}
> 
> base-commit: 1398820fee515873379809a6415930ad0764b2f6
> 

Christophe