[RFC][PATCH 6/8] jump_label, x86: Add variable length patching support

[Peter Zijlstra]

This allows the patching to to emit 2 byte JMP/NOP instruction in
addition to the 5 byte JMP/NOP we already did. This allows for more
compact code.

This code is not yet used, as we don't emit shorter code at compile
time yet.


Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/jump_label.h |    6 +-
 arch/x86/include/asm/nops.h       |    1 
 arch/x86/kernel/jump_label.c      |   77 +++++++++++++++++++++++---------------
 3 files changed, 53 insertions(+), 31 deletions(-)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -7,9 +7,11 @@
 #define JUMP_LABEL_NOP_SIZE 5
 
 #ifdef CONFIG_X86_64
-# define STATIC_KEY_INIT_NOP P6_NOP5_ATOMIC
+# define STATIC_KEY_NOP2 P6_NOP2
+# define STATIC_KEY_NOP5 P6_NOP5_ATOMIC
 #else
-# define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC
+# define STATIC_KEY_NOP2 GENERIC_NOP2
+# define STATIC_KEY_NOP5 GENERIC_NOP5_ATOMIC
 #endif
 
 #include <asm/asm.h>
--- a/arch/x86/include/asm/nops.h
+++ b/arch/x86/include/asm/nops.h
@@ -5,6 +5,7 @@
 /*
  * Define nops for use with alternative() and for tracing.
  *
+ * *_NOP2 must be a single instruction
  * *_NOP5_ATOMIC must be a single instruction.
  */
 
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -17,49 +17,67 @@
 #include <asm/text-patching.h>
 
 union jump_code_union {
-	char code[JUMP_LABEL_NOP_SIZE];
+	char code[JMP32_INSN_SIZE];
 	struct {
-		char jump;
-		int offset;
+		char opcode;
+		union {
+			s8	d8;
+			s32	d32;
+		};
 	} __attribute__((packed));
 };
 
-static void __jump_label_set_jump_code(struct jump_entry *entry,
-				       enum jump_label_type type,
-				       union jump_code_union *code,
-				       int init)
+static inline bool __jump_disp_is_byte(s32 disp)
 {
-	const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
-	const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
-	unsigned char *ip = (void *)jump_entry_code(entry);
+	return false;
+}
+
+static int __jump_label_set_jump_code(struct jump_entry *entry,
+				      enum jump_label_type type,
+				      union jump_code_union *code,
+				      int init)
+{
+	static unsigned char default_nop2[] = { STATIC_KEY_NOP2 };
+	static unsigned char default_nop5[] = { STATIC_KEY_NOP5 };
+	s32 disp = jump_entry_target(entry) - jump_entry_code(entry);
+	void *ip = (void *)jump_entry_code(entry);
+	const unsigned char *nop;
 	const void *expect;
-	int line;
+	int line, size;
 
-	code->jump = 0xe9;
-	code->offset = jump_entry_target(entry) -
-		       (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
-
-	if (init) {
-		expect = default_nop; line = __LINE__;
-	} else if (type == JUMP_LABEL_JMP) {
-		expect = ideal_nop; line = __LINE__;
+	if (__jump_disp_is_byte(disp)) {
+		size = JMP8_INSN_SIZE;
+		code->opcode = JMP8_INSN_OPCODE;
+		code->d8 = disp - size;
+		nop = init ? default_nop2 : ideal_nops[2];
+	} else {
+		size = JMP32_INSN_SIZE;
+		code->opcode = JMP32_INSN_OPCODE;
+		code->d32 = disp - size;
+		nop = init ? default_nop5 : ideal_nops[NOP_ATOMIC5];
+	}
+
+	if (init || type == JUMP_LABEL_JMP) {
+		expect = nop; line = __LINE__;
 	} else {
 		expect = code->code; line = __LINE__;
 	}
 
-	if (memcmp(ip, expect, JUMP_LABEL_NOP_SIZE)) {
+	if (memcmp(ip, expect, size)) {
 		/*
 		 * The location is not an op that we were expecting.
 		 * Something went wrong. Crash the box, as something could be
 		 * corrupting the kernel.
 		 */
-		pr_crit("jump_label: Fatal kernel bug, unexpected op at %pS [%p] (%5ph != %5ph)) line:%d init:%d type:%d\n",
-				ip, ip, ip, expect, line, init, type);
+		pr_crit("jump_label: Fatal kernel bug, unexpected op at %pS [%p] (%5ph != %5ph)) line:%d init:%d size:%d type:%d\n",
+			ip, ip, ip, expect, line, init, size, type);
 		BUG();
 	}
 
 	if (type == JUMP_LABEL_NOP)
-		memcpy(code, ideal_nop, JUMP_LABEL_NOP_SIZE);
+		memcpy(code, nop, size);
+
+	return size;
 }
 
 static void __ref __jump_label_transform(struct jump_entry *entry,
@@ -67,8 +85,9 @@ static void __ref __jump_label_transform
 					 int init)
 {
 	union jump_code_union code;
+	int size;
 
-	__jump_label_set_jump_code(entry, type, &code, init);
+	size = __jump_label_set_jump_code(entry, type, &code, init);
 
 	/*
 	 * As long as only a single processor is running and the code is still
@@ -82,12 +101,11 @@ static void __ref __jump_label_transform
 	 * always nop being the 'currently valid' instruction
 	 */
 	if (init || system_state == SYSTEM_BOOTING) {
-		text_poke_early((void *)jump_entry_code(entry), &code,
-				JUMP_LABEL_NOP_SIZE);
+		text_poke_early((void *)jump_entry_code(entry), &code, size);
 		return;
 	}
 
-	text_poke_bp((void *)jump_entry_code(entry), &code, JUMP_LABEL_NOP_SIZE, NULL);
+	text_poke_bp((void *)jump_entry_code(entry), &code, size, NULL);
 }
 
 void arch_jump_label_transform(struct jump_entry *entry,
@@ -107,6 +125,7 @@ bool arch_jump_label_transform_queue(str
 {
 	struct text_poke_loc *tp;
 	void *entry_code;
+	int size;
 
 	if (system_state == SYSTEM_BOOTING) {
 		/*
@@ -143,10 +162,10 @@ bool arch_jump_label_transform_queue(str
 			return false;
 	}
 
-	__jump_label_set_jump_code(entry, type,
+	size = __jump_label_set_jump_code(entry, type,
 				   (union jump_code_union *)&tp->text, 0);
 
-	text_poke_loc_init(tp, entry_code, NULL, JUMP_LABEL_NOP_SIZE, NULL);
+	text_poke_loc_init(tp, entry_code, NULL, size, NULL);
 
 	tp_vec_nr++;