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

* [PATCH V3] arm64: Unhash early pointer print plus improve comment
@ 2021-12-21 15:52 Guilherme G. Piccoli
  2021-12-21 16:52 ` Robin Murphy
                   ` (2 more replies)
  0 siblings, 3 replies; 5+ messages in thread
From: Guilherme G. Piccoli @ 2021-12-21 15:52 UTC (permalink / raw)
  To: linux-arm-kernel
  Cc: catalin.marinas, will, broonie, u.kleine-koenig, gpiccoli,
	kernel, robin.murphy

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.

Cc: Mark Brown <broonie@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@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();
 	}
-- 
2.34.1


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