[PATCH 5.15 5/5] x86/static_call: Add support for Jcc tail-calls

Thadeu Lima de Souza Cascardo
2024-03-13T10:43:38Z

From: Peter Zijlstra <peterz@infradead.org>

commit 923510c88d2b7d947c4217835fd9ca6bd65cc56c upstream.

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@mailbox.org>
Change-Id: I99c8fc3f721e5d1c74f06710b38d4bac5230303a
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/Y9Kdg9QjHkr9G5b5@hirez.programming.kicks-ass.net
[cascardo: __static_call_validate didn't have the bool tramp argument]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
---
arch/x86/kernel/static_call.c | 50 ++++++++++++++++++++++++++++++++---
1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index e25050c7ff1e..0e0221271e72 100644
--- 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, 0x2e, 0x31, 0xc0 };

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:
@@ -56,6 +85,20 @@ static void __ref __static_call_transform(void *insn, enum insn_type type,
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)
@@ -67,13 +110,14 @@ static void __ref __static_call_transform(void *insn, enum insn_type type,
text_poke_bp(insn, code, size, emulate);
}

-static void __static_call_validate(void *insn, bool tail)
+static void __static_call_validate(u8 *insn, bool tail)
{
- u8 opcode = *(u8 *)insn;
+ u8 opcode = insn[0];

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 ||
--
2.34.1