[PATCH 1/5] arm64/alternative_cb: move callback reference into replacements section

[Ard Biesheuvel]

In preparation of permitting callback type alternatives patching
routines to use any number of alternative instructions, move the
relative reference to the callback routine out of the primary
'struct alt_instr' data structure, where we are currently overloading
the 'alt_offset' member depending on the feature id. Instead, put
a relative reference to the callback at the start of the alternative
sequence.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 arch/arm64/include/asm/alternative.h | 39 +++++++++-----------
 arch/arm64/kernel/alternative.c      | 11 ++++--
 2 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index 4b650ec1d7dd..77da798e888b 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -24,6 +24,11 @@ struct alt_instr {
 	u8  alt_len;		/* size of new instruction(s), <= orig_len */
 };
 
+struct alt_instr_cb {
+	s32 cb_offset;		/* offset to callback handler */
+	__le32 insn[];		/* sequence of alternative instructions */
+};
+
 typedef void (*alternative_cb_t)(struct alt_instr *alt,
 				 __le32 *origptr, __le32 *updptr, int nr_inst);
 
@@ -35,13 +40,9 @@ void apply_alternatives_module(void *start, size_t length);
 static inline void apply_alternatives_module(void *start, size_t length) { }
 #endif
 
-#define ALTINSTR_ENTRY(feature,cb)					      \
+#define ALTINSTR_ENTRY(feature)						      \
 	" .word 661b - .\n"				/* label           */ \
-	" .if " __stringify(cb) " == 0\n"				      \
 	" .word 663f - .\n"				/* new instruction */ \
-	" .else\n"							      \
-	" .word " __stringify(cb) "- .\n"		/* callback */	      \
-	" .endif\n"							      \
 	" .hword " __stringify(feature) "\n"		/* feature bit     */ \
 	" .byte 662b-661b\n"				/* source len      */ \
 	" .byte 664f-663f\n"				/* replacement len */
@@ -59,36 +60,29 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
  * but most assemblers die if insn1 or insn2 have a .inst. This should
  * be fixed in a binutils release posterior to 2.25.51.0.2 (anything
  * containing commit 4e4d08cf7399b606 or c1baaddf8861).
- *
- * Alternatives with callbacks do not generate replacement instructions.
  */
-#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled, cb)	\
-	".if "__stringify(cfg_enabled)" == 1\n"				\
+#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature)			\
 	"661:\n\t"							\
 	oldinstr "\n"							\
 	"662:\n"							\
 	".pushsection .altinstructions,\"a\"\n"				\
-	ALTINSTR_ENTRY(feature,cb)					\
+	ALTINSTR_ENTRY(feature)						\
 	".popsection\n"							\
-	" .if " __stringify(cb) " == 0\n"				\
 	".pushsection .altinstr_replacement, \"a\"\n"			\
 	"663:\n\t"							\
 	newinstr "\n"							\
 	"664:\n\t"							\
-	".popsection\n\t"						\
+	".popsection\n\t"
+
+#define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...)		\
+	".if "__stringify(IS_ENABLED(cfg))" == 1\n"			\
+	__ALTERNATIVE_CFG(oldinstr, newinstr, feature)			\
 	".org	. - (664b-663b) + (662b-661b)\n\t"			\
 	".org	. - (662b-661b) + (664b-663b)\n"			\
-	".else\n\t"							\
-	"663:\n\t"							\
-	"664:\n\t"							\
-	".endif\n"							\
 	".endif\n"
 
-#define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...)	\
-	__ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
-
 #define ALTERNATIVE_CB(oldinstr, cb) \
-	__ALTERNATIVE_CFG(oldinstr, "NOT_AN_INSTRUCTION", ARM64_CB_PATCH, 1, cb)
+	__ALTERNATIVE_CFG(oldinstr, ".word " __stringify(cb) " - .\n", ARM64_CB_PATCH)
 #else
 
 #include <asm/assembler.h>
@@ -158,8 +152,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
 .macro alternative_cb cb
 	.set .Lasm_alt_mode, 0
 	.pushsection .altinstructions, "a"
-	altinstruction_entry 661f, \cb, ARM64_CB_PATCH, 662f-661f, 0
+	altinstruction_entry 661f, 663f, ARM64_CB_PATCH, 662f-661f, 664f-663f
 	.popsection
+	.pushsection .altinstr_replacement, "ax"
+663:	.word	\cb - .
+664:	.popsection
 661:
 .endm
 
diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index b5d603992d40..a49930843784 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -151,6 +151,7 @@ static void __apply_alternatives(void *alt_region, bool is_module)
 	struct alt_region *region = alt_region;
 	__le32 *origptr, *updptr;
 	alternative_cb_t alt_cb;
+	struct alt_instr_cb *alt_cb_insn;
 
 	for (alt = region->begin; alt < region->end; alt++) {
 		int nr_inst;
@@ -161,7 +162,7 @@ static void __apply_alternatives(void *alt_region, bool is_module)
 			continue;
 
 		if (alt->cpufeature == ARM64_CB_PATCH)
-			BUG_ON(alt->alt_len != 0);
+			BUG_ON(alt->alt_len < sizeof(*alt_cb_insn));
 		else
 			BUG_ON(alt->alt_len != alt->orig_len);
 
@@ -171,10 +172,12 @@ static void __apply_alternatives(void *alt_region, bool is_module)
 		updptr = is_module ? origptr : lm_alias(origptr);
 		nr_inst = alt->orig_len / AARCH64_INSN_SIZE;
 
-		if (alt->cpufeature < ARM64_CB_PATCH)
+		if (alt->cpufeature < ARM64_CB_PATCH) {
 			alt_cb = patch_alternative;
-		else
-			alt_cb  = ALT_REPL_PTR(alt);
+		} else {
+			alt_cb_insn = ALT_REPL_PTR(alt);
+			alt_cb = offset_to_ptr(&alt_cb_insn->cb_offset);
+		}
 
 		alt_cb(alt, origptr, updptr, nr_inst);
 
-- 
2.19.2