[PATCH 3/3] x86/ftrace: Use alternative RET encoding

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From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit 1f001e9da6bbf482311e45e48f53c2bd2179e59c upstream.

Use the return thunk in ftrace trampolines, if needed.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
[cascardo: use memcpy(text_gen_insn) as there is no __text_gen_insn]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
---
 arch/x86/kernel/ftrace.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 8160d1dc6ed3..b3c9ef01d6c0 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -309,7 +309,7 @@ union ftrace_op_code_union {
 	} __attribute__((packed));
 };
 
-#define RET_SIZE		1 + IS_ENABLED(CONFIG_SLS)
+#define RET_SIZE		(IS_ENABLED(CONFIG_RETPOLINE) ? 5 : 1 + IS_ENABLED(CONFIG_SLS))
 
 static unsigned long
 create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
@@ -365,7 +365,12 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
 		goto fail;
 
 	ip = trampoline + size;
-	memcpy(ip, retq, RET_SIZE);
+
+	/* The trampoline ends with ret(q) */
+	if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+		memcpy(ip, text_gen_insn(JMP32_INSN_OPCODE, ip, &__x86_return_thunk), JMP32_INSN_SIZE);
+	else
+		memcpy(ip, retq, sizeof(retq));
 
 	/* No need to test direct calls on created trampolines */
 	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
-- 
2.34.1




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