[PATCH v5 12/17] x86/alternatives: Teach text_poke_bp() to emulate RET

[Peter Zijlstra]

Future patches will need to poke a RET instruction, provide the
infrastructure required for this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/text-patching.h |   19 +++++++++++++++++++
 arch/x86/kernel/alternative.c        |    5 +++++
 2 files changed, 24 insertions(+)

--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -53,6 +53,9 @@ extern void text_poke_finish(void);
 #define INT3_INSN_SIZE		1
 #define INT3_INSN_OPCODE	0xCC
 
+#define RET_INSN_SIZE		1
+#define RET_INSN_OPCODE		0xC3
+
 #define CALL_INSN_SIZE		5
 #define CALL_INSN_OPCODE	0xE8
 
@@ -73,6 +76,7 @@ static __always_inline int text_opcode_s
 
 	switch(opcode) {
 	__CASE(INT3);
+	__CASE(RET);
 	__CASE(CALL);
 	__CASE(JMP32);
 	__CASE(JMP8);
@@ -141,11 +145,26 @@ void int3_emulate_push(struct pt_regs *r
 }
 
 static __always_inline
+unsigned long int3_emulate_pop(struct pt_regs *regs)
+{
+	unsigned long val = *(unsigned long *)regs->sp;
+	regs->sp += sizeof(unsigned long);
+	return val;
+}
+
+static __always_inline
 void int3_emulate_call(struct pt_regs *regs, unsigned long func)
 {
 	int3_emulate_push(regs, regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE);
 	int3_emulate_jmp(regs, func);
 }
+
+static __always_inline
+void int3_emulate_ret(struct pt_regs *regs)
+{
+	unsigned long ip = int3_emulate_pop(regs);
+	int3_emulate_jmp(regs, ip);
+}
 #endif /* !CONFIG_UML_X86 */
 
 #endif /* _ASM_X86_TEXT_PATCHING_H */
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1099,6 +1099,10 @@ int noinstr poke_int3_handler(struct pt_
 		 */
 		goto out_put;
 
+	case RET_INSN_OPCODE:
+		int3_emulate_ret(regs);
+		break;
+
 	case CALL_INSN_OPCODE:
 		int3_emulate_call(regs, (long)ip + tp->rel32);
 		break;
@@ -1240,6 +1244,7 @@ static void text_poke_loc_init(struct te
 
 	switch (tp->opcode) {
 	case INT3_INSN_OPCODE:
+	case RET_INSN_OPCODE:
 		break;
 
 	case CALL_INSN_OPCODE: