Messages in this thread Patch in this message | | | Date | Mon, 23 Jan 2023 21:59:18 +0100 | From | Peter Zijlstra <> | Subject | [PATCH 3/3] x86/static_call: Add support for Jcc tail-calls |
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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
Teach the in-line static call text patching about this.
Notably, since there is no conditional-ret, in that caes 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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> --- 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 ||
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