linux-kernel - [PATCH v2] modules: Ensure natural alignment for .altinstructions and __bug

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Date:   Fri, 1 Jul 2022 20:40:02 +0200
From:   Helge Deller <deller@....de>
To:     mcgrof@...nel.org, jeyu@...nel.org, linux-modules@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: [PATCH v2] modules: Ensure natural alignment for .altinstructions
 and __bug_table sections

In the kernel image vmlinux.lds.S linker scripts the .altinstructions
and __bug_table sections are 32- or 64-bit aligned because they hold 32-
and/or 64-bit values.

But for modules the module.lds.S linker script doesn't define a default
alignment yet, so the linker chooses the default byte alignment, which
then leads to unnecessary unaligned memory accesses at runtime.

Usually such unaligned accesses are unnoticed, because either the
hardware (as on x86 CPUs) or in-kernel exception handlers (e.g. on hppa
or sparc) emulate and fix them up at runtime.

On hppa the 32-bit unalignment exception handler was temporarily broken
due another bad commit, and as such wrong values were returned on
unaligned accesses to the altinstructions table. This then led to
undefined behaviour because wrong kernel addresses were patched and we
suddenly faced lots of unrelated bugs, as can be seen in this mail
thread:
https://lore.kernel.org/all/07d91863-dacc-a503-aa2b-05c3b92a1e39@bell.net/T/#mab602dfa32be5e229d5e192ab012af196d04d75d

This patch adds the missing natural alignment for kernel modules to
avoid unnecessary (hard- or software-based) fixups.

Signed-off-by: Helge Deller <deller@....de>
---
 scripts/module.lds.S | 2 ++
 1 file changed, 2 insertions(+)

--
v2: updated commit message

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 1d0e1e4dc3d2..3a3aa2354ed8 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -27,6 +27,8 @@ SECTIONS {
 	.ctors			0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) }
 	.init_array		0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) }

+	.altinstructions	0 : ALIGN(8) { KEEP(*(.altinstructions)) }
+	__bug_table		0 : ALIGN(8) { KEEP(*(__bug_table)) }
 	__jump_table		0 : ALIGN(8) { KEEP(*(__jump_table)) }

 	__patchable_function_entries : { *(__patchable_function_entries) }