Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

From: Robin Murphy
Date: Wed May 27 2020 - 09:45:41 EST

On 2020-05-26 18:31, Nick Desaulniers wrote:
Custom toolchains that modify the default target to -mthumb cannot
compile the arm64 compat vdso32, as
contains assembly that's invalid in -mthumb. Force the use of -marm,

FWIW, this seems suspicious - the only assembly instructions I see there are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the -march=armv7a baseline that we set.

On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and built a Thumb VDSO quite happily with Ubuntu 19.04's gcc-arm-linux-gnueabihf. What was the actual failure you saw?


Cc: Stephen Boyd <swboyd@xxxxxxxxxx>
Reported-by: Luis Lozano <llozano@xxxxxxxxxx>
Tested-by: Manoj Gupta <manojgupta@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Surgeon General's Warning: changing the compiler defaults is not
recommended and can lead to spooky bugs that are hard to reproduce

arch/arm64/kernel/vdso32/Makefile | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
index 3964738ebbde..c449a293d81e 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -104,6 +104,8 @@ VDSO_CFLAGS += -D__uint128_t='void*'
# (on GCC 4.8 or older, there is unfortunately no way to silence this warning)
VDSO_CFLAGS += $(call cc32-disable-warning,shift-count-overflow)
VDSO_CFLAGS += -Wno-int-to-pointer-cast
+# Force vdso to be compiled in ARM mode, not THUMB.
+VDSO_CFLAGS += -marm