[PATCH] ARM: decompressor: fix BSS size calculation for LLVM ld.lld

Ard Biesheuvel ardb at kernel.org
Fri Feb 5 03:52:20 EST 2021

The LLVM ld.lld linker uses a different symbol type for __bss_start,
resulting in the calculation of KBSS_SZ to be thrown off. Up until now,
this has gone unnoticed as it only affects the appended DTB case, but
pending changes for ARM in the way the decompressed kernel is cleaned
from the caches has uncovered this problem.

On a ld.lld build:

  $ nm vmlinux |grep bss_
  c1c22034 D __bss_start
  c1c86e98 B __bss_stop

resulting in

  433: c1c86e98     0 NOTYPE  GLOBAL DEFAULT  ABS _kernel_bss_size

which is obviously incorrect, and may cause the cache clean to access
unmapped memory, or cause the size calculation to wrap, resulting in no
cache clean to be performed at all.

Fix this by updating the sed regex to take D type symbols into account.

Cc: Nick Desaulniers <ndesaulniers at google.com>
Cc: Nathan Chancellor <nathan at kernel.org>
Cc: Guillaume Tucker <guillaume.tucker at collabora.com>
Link: https://lore.kernel.org/linux-arm-kernel/6c65bcef-d4e7-25fa-43cf-2c435bb61bb9@collabora.com/
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
 arch/arm/boot/compressed/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index fb521efcc6c2..54307db7854d 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -115,8 +115,8 @@ asflags-y := -DZIMAGE
 # Supply kernel BSS size to the decompressor via a linker symbol.
 KBSS_SZ = $(shell echo $$(($$($(NM) $(obj)/../../../../vmlinux | \
-		sed -n -e 's/^\([^ ]*\) [AB] __bss_start$$/-0x\1/p' \
-		       -e 's/^\([^ ]*\) [AB] __bss_stop$$/+0x\1/p') )) )
+		sed -n -e 's/^\([^ ]*\) [ABD] __bss_start$$/-0x\1/p' \
+		       -e 's/^\([^ ]*\) [ABD] __bss_stop$$/+0x\1/p') )) )
 LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)
 # Supply ZRELADDR to the decompressor via a linker symbol.

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