From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
commit 59b6359dd92d18f5dc04b14a4c926fa08ab66f7c upstream.
If CONFIG_DEBUG_LOCK_ALLOC=y, the kernel log is spammed with a few
hundred identical messages:
unwind: Unknown symbol address c0800300
unwind: Index not found c0800300
c0800300 is the return address from the last subroutine call (to
__memzero()) in __mmap_switched(). Apparently having this address in
the link register confuses the unwinder.
To fix this, reset the link register to zero before jumping to
start_kernel().
Fixes: 9520b1a1b5f7a348 ("ARM: head-common.S: speed up startup code")
Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Looks like this first landed in v4.15-rc1. Without this, we can't tell
during an unwind initiated from start_kernel() when to stop unwinding,
which for the clang specific implementation of the arm frame pointer
unwinder leads to dereferencing a garbage value, triggering an exception
which has no fixup, triggering a panic, triggering an unwind, triggering
an infinite loop that prevents booting. I have more patches to send
upstream to make the unwinder more resilient, but it's ambiguous as to
when to stop unwinding without this patch.
arch/arm/kernel/head-common.S | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm/kernel/head-common.S b/arch/arm/kernel/head-common.S
index 7e662bdd5cb3..932b2244e709 100644
--- a/arch/arm/kernel/head-common.S
+++ b/arch/arm/kernel/head-common.S
@@ -101,6 +101,7 @@ __mmap_switched:
str r2, [r6] @ Save atags pointer
cmp r7, #0
strne r0, [r7] @ Save control register values
+ mov lr, #0
b start_kernel
ENDPROC(__mmap_switched)
--
2.28.0.rc0.142.g3c755180ce-goog
