[PATCH v1.1 3/3] x86/static_call: Add support for Jcc tail-calls

[Peter Zijlstra]

On Tue, Jan 24, 2023 at 10:07:53AM -0500, Steven Rostedt wrote:
> On Tue, 24 Jan 2023 14:06:49 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Just to confirm, as it's not clear if this is the static call site or one
> > > of the functions that is being called.  
> > 
> > Ah, you've not looked at enough asm then? ;-) Yes this is the static
> > call site, see the __SCT_ target (instruction at 0x35d).
> 
> Yeah, could you specify it a bit more in the change log such that those
> looking back at this don't have to have that requirement of staring at
> enough asm ;-)

How's this then?

---
Subject: x86/static_call: Add support for Jcc tail-calls
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Jan 20 16:40:33 CET 2023

Clang likes to create conditional tail calls like:

0000000000000350 <amd_pmu_add_event>:
350:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1) 351: R_X86_64_NONE      __fentry__-0x4
355:       48 83 bf 20 01 00 00 00         cmpq   $0x0,0x120(%rdi)
35d:       0f 85 00 00 00 00       jne    363 <amd_pmu_add_event+0x13>     35f: R_X86_64_PLT32     __SCT__amd_pmu_branch_add-0x4
363:       e9 00 00 00 00          jmp    368 <amd_pmu_add_event+0x18>     364: R_X86_64_PLT32     __x86_return_thunk-0x4

Where 0x35d is a static call site that's turned into a conditional
tail-call using the Jcc class of instructions.

Teach the in-line static call text patching about this.

Notably, since there is no conditional-ret, in that case patch the Jcc
to point at an empty stub function that does the ret -- or the return
thunk when needed.

Reported-by: "Erhard F." <erhard_f@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 arch/x86/kernel/static_call.c |   50 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -9,6 +9,7 @@ enum insn_type {
 	NOP = 1,  /* site cond-call */
 	JMP = 2,  /* tramp / site tail-call */
 	RET = 3,  /* tramp / site cond-tail-call */
+	JCC = 4,
 };
 
 /*
@@ -25,12 +26,40 @@ static const u8 xor5rax[] = { 0x2e, 0x2e
 
 static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
 
+static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
+{
+	u8 ret = 0;
+
+	if (insn[0] == 0x0f) {
+		u8 tmp = insn[1];
+		if ((tmp & 0xf0) == 0x80)
+			ret = tmp;
+	}
+
+	return ret;
+}
+
+extern void __static_call_return(void);
+
+asm (".global __static_call_return\n\t"
+     ".type __static_call_return, @function\n\t"
+     ASM_FUNC_ALIGN "\n\t"
+     "__static_call_return:\n\t"
+     ANNOTATE_NOENDBR
+     ANNOTATE_RETPOLINE_SAFE
+     "ret; int3\n\t"
+     ".size __static_call_return, . - __static_call_return \n\t");
+
 static void __ref __static_call_transform(void *insn, enum insn_type type,
 					  void *func, bool modinit)
 {
 	const void *emulate = NULL;
 	int size = CALL_INSN_SIZE;
 	const void *code;
+	u8 op, buf[6];
+
+	if ((type == JMP || type == RET) && (op = __is_Jcc(insn)))
+		type = JCC;
 
 	switch (type) {
 	case CALL:
@@ -57,6 +86,20 @@ static void __ref __static_call_transfor
 		else
 			code = &retinsn;
 		break;
+
+	case JCC:
+		if (!func) {
+			func = __static_call_return;
+			if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+				func = x86_return_thunk;
+		}
+
+		buf[0] = 0x0f;
+		__text_gen_insn(buf+1, op, insn+1, func, 5);
+		code = buf;
+		size = 6;
+
+		break;
 	}
 
 	if (memcmp(insn, code, size) == 0)
@@ -68,9 +111,9 @@ static void __ref __static_call_transfor
 	text_poke_bp(insn, code, size, emulate);
 }
 
-static void __static_call_validate(void *insn, bool tail, bool tramp)
+static void __static_call_validate(u8 *insn, bool tail, bool tramp)
 {
-	u8 opcode = *(u8 *)insn;
+	u8 opcode = insn[0];
 
 	if (tramp && memcmp(insn+5, tramp_ud, 3)) {
 		pr_err("trampoline signature fail");
@@ -79,7 +122,8 @@ static void __static_call_validate(void
 
 	if (tail) {
 		if (opcode == JMP32_INSN_OPCODE ||
-		    opcode == RET_INSN_OPCODE)
+		    opcode == RET_INSN_OPCODE ||
+		    __is_Jcc(insn))
 			return;
 	} else {
 		if (opcode == CALL_INSN_OPCODE ||