[RFC PATCH 1/2] arm64: jump_label: use more precise asm constraints

[Ard Biesheuvel]

In order to set bit #0 of the struct static_key pointer in the the jump
label struct, we currently cast the pointer to char[], and take the
address of either the 0th or 1st array member, depending on the value of
'branch'. This works but creates problems with -fpie code generation:
GCC complains about the constraint being unsatisfiable, and Clang
miscompiles the code in a way that causes stability issues (immediate
panic on 'attempt to kill init')

So instead, pass the struct static_key reference and the 'branch'
immediate individually, in a way that satisfies both GCC and Clang (GCC
wants the 'S' constraint, whereas Clang wants the 'i' constraint for
argument %0)

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/jump_label.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index cea441b6aa5d..f741bbacf175 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -23,9 +23,9 @@ static __always_inline bool arch_static_branch(struct static_key *key,
 		 "	.pushsection	__jump_table, \"aw\"	\n\t"
 		 "	.align		3			\n\t"
 		 "	.long		1b - ., %l[l_yes] - .	\n\t"
-		 "	.quad		%c0 - .			\n\t"
+		 "	.quad		%c0 - . + %1		\n\t"
 		 "	.popsection				\n\t"
-		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
+		 :  :  "Si"(key), "i"(branch) :  : l_yes);
 
 	return false;
 l_yes:
@@ -40,9 +40,9 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key,
 		 "	.pushsection	__jump_table, \"aw\"	\n\t"
 		 "	.align		3			\n\t"
 		 "	.long		1b - ., %l[l_yes] - .	\n\t"
-		 "	.quad		%c0 - .			\n\t"
+		 "	.quad		%c0 - . + %1		\n\t"
 		 "	.popsection				\n\t"
-		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
+		 :  :  "Si"(key), "i"(branch) :  : l_yes);
 
 	return false;
 l_yes:
-- 
2.30.2