[PATCH V3] arm64: Unhash early pointer print plus improve comment

[Robin Murphy]

On 2021-12-21 15:52, Guilherme G. Piccoli wrote:
> When facing a really early issue on DT parsing we have currently
> a message that shows both the physical and virtual address of the FDT.
> The printk pointer modifier for the virtual address shows a hashed
> address there unless the user provides "no_hash_pointers" parameter in
> the command-line. The situation in which this message shows-up is a bit
> more serious though: the boot process is broken, nothing can be done
> (even an oops is too much for this early stage) so we have this message
> as a last resort in order to help debug bootloader issues, for example.
> Hence, we hereby change that to "%px" in order to make debugging easy,
> there's not much information leak risk in such early boot failure.
> 
> Also, we tried to improve a bit the commenting on that function, given
> that if kernel fails there, it just hangs forever in a cpu_relax() loop.
> The reason we cannot BUG/panic is that is too early to do so; thanks to
> Mark Brown for pointing that on IRC and thanks Robin Murphy for the good
> pointer hash discussion in the mailing-list.

Reviewed-by: Robin Murphy <robin.murphy at arm.com>

> Cc: Mark Brown <broonie at kernel.org>
> Cc: Robin Murphy <robin.murphy at arm.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli at igalia.com>
> ---
> 
> V3:
> * Improved commit message (thanks Robin!);
> * Fixed comment style.
> 
>   arch/arm64/kernel/setup.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index be5f85b0a24d..a80430550a73 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -189,11 +189,16 @@ static void __init setup_machine_fdt(phys_addr_t dt_phys)
>   
>   	if (!dt_virt || !early_init_dt_scan(dt_virt)) {
>   		pr_crit("\n"
> -			"Error: invalid device tree blob at physical address %pa (virtual address 0x%p)\n"
> +			"Error: invalid device tree blob at physical address %pa (virtual address 0x%px)\n"
>   			"The dtb must be 8-byte aligned and must not exceed 2 MB in size\n"
>   			"\nPlease check your bootloader.",
>   			&dt_phys, dt_virt);
>   
> +		/*
> +		 * Note that in this _really_ early stage we cannot even BUG()
> +		 * or oops, so the least terrible thing to do is cpu_relax(),
> +		 * or else we could end-up printing non-initialized data, etc.
> +		 */
>   		while (true)
>   			cpu_relax();
>   	}