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Subject: Re: [RFC PATCH 10/10] powerpc/fsl_booke/kaslr: dump out kernel offset
 information on panic
To:     Jason Yan <yanaijie@huawei.com>, mpe@ellerman.id.au,
        linuxppc-dev@lists.ozlabs.org, diana.craciun@nxp.com,
        benh@kernel.crashing.org, paulus@samba.org, npiggin@gmail.com,
        keescook@chromium.org, kernel-hardening@lists.openwall.com
Cc:     linux-kernel@vger.kernel.org, wangkefeng.wang@huawei.com,
        yebin10@huawei.com, thunder.leizhen@huawei.com,
        jingxiangfeng@huawei.com, fanchengyang@huawei.com
References: <20190717080621.40424-1-yanaijie@huawei.com>
 <20190717080621.40424-11-yanaijie@huawei.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <d10db1e3-6bc4-eaa4-d68e-b7343e35b55f@c-s.fr>
Date:   Mon, 29 Jul 2019 13:43:05 +0200
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Le 17/07/2019 à 10:06, Jason Yan a écrit :
> When kaslr is enabled, the kernel offset is different for every boot.
> This brings some difficult to debug the kernel. Dump out the kernel
> offset when panic so that we can easily debug the kernel.
> 
> Signed-off-by: Jason Yan <yanaijie@huawei.com>
> Cc: Diana Craciun <diana.craciun@nxp.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>   arch/powerpc/include/asm/page.h     |  5 +++++
>   arch/powerpc/kernel/machine_kexec.c |  1 +
>   arch/powerpc/kernel/setup-common.c  | 23 +++++++++++++++++++++++
>   3 files changed, 29 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
> index 60a68d3a54b1..cd3ac530e58d 100644
> --- a/arch/powerpc/include/asm/page.h
> +++ b/arch/powerpc/include/asm/page.h
> @@ -317,6 +317,11 @@ struct vm_area_struct;
>   
>   extern unsigned long kimage_vaddr;
>   
> +static inline unsigned long kaslr_offset(void)
> +{
> +	return kimage_vaddr - KERNELBASE;
> +}
> +
>   #include <asm-generic/memory_model.h>
>   #endif /* __ASSEMBLY__ */
>   #include <asm/slice.h>
> diff --git a/arch/powerpc/kernel/machine_kexec.c b/arch/powerpc/kernel/machine_kexec.c
> index c4ed328a7b96..078fe3d76feb 100644
> --- a/arch/powerpc/kernel/machine_kexec.c
> +++ b/arch/powerpc/kernel/machine_kexec.c
> @@ -86,6 +86,7 @@ void arch_crash_save_vmcoreinfo(void)
>   	VMCOREINFO_STRUCT_SIZE(mmu_psize_def);
>   	VMCOREINFO_OFFSET(mmu_psize_def, shift);
>   #endif
> +	vmcoreinfo_append_str("KERNELOFFSET=%lx\n", kaslr_offset());
>   }
>   
>   /*
> diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
> index 1f8db666468d..49e540c0adeb 100644
> --- a/arch/powerpc/kernel/setup-common.c
> +++ b/arch/powerpc/kernel/setup-common.c
> @@ -715,12 +715,35 @@ static struct notifier_block ppc_panic_block = {
>   	.priority = INT_MIN /* may not return; must be done last */
>   };
>   
> +/*
> + * Dump out kernel offset information on panic.
> + */
> +static int dump_kernel_offset(struct notifier_block *self, unsigned long v,
> +			      void *p)
> +{
> +	const unsigned long offset = kaslr_offset();
> +
> +	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && offset > 0)
> +		pr_emerg("Kernel Offset: 0x%lx from 0x%lx\n",
> +			 offset, KERNELBASE);
> +	else
> +		pr_emerg("Kernel Offset: disabled\n");

Do we really need that else branch ?

Why not just make the below atomic_notifier_chain_register() 
conditionnal to IS_ENABLED(CONFIG_RANDOMIZE_BASE) && offset > 0
and not print anything otherwise ?

Christophe

> +
> +	return 0;
> +}
> +
> +static struct notifier_block kernel_offset_notifier = {
> +	.notifier_call = dump_kernel_offset
> +};
> +
>   void __init setup_panic(void)
>   {
>   	/* PPC64 always does a hard irq disable in its panic handler */
>   	if (!IS_ENABLED(CONFIG_PPC64) && !ppc_md.panic)
>   		return;
>   	atomic_notifier_chain_register(&panic_notifier_list, &ppc_panic_block);
> +	atomic_notifier_chain_register(&panic_notifier_list,
> +				       &kernel_offset_notifier);
>   }
>   
>   #ifdef CONFIG_CHECK_CACHE_COHERENCY
>