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authorPeter Zijlstra2021-01-18 15:12:16 +0100
committerIngo Molnar2021-02-17 14:08:43 +0100
commit3f2a8fc4b15de18644e8a80a09edda168676e22c (patch)
treeecceaeff23b07b1e7e7e611fdd64d9646b04e099 /arch
parent880cfed3a012d7863f42251791cea7fe78c39390 (diff)
static_call/x86: Add __static_call_return0()
Provide a stub function that return 0 and wire up the static call site patching to replace the CALL with a single 5 byte instruction that clears %RAX, the return value register. The function can be cast to any function pointer type that has a single %RAX return (including pointers). Also provide a version that returns an int for convenience. We are clearing the entire %RAX register in any case, whether the return value is 32 or 64 bits, since %RAX is always a scratch register anyway. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lkml.kernel.org/r/20210118141223.123667-2-frederic@kernel.org
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kernel/static_call.c17
1 files changed, 15 insertions, 2 deletions
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index ca9a380d9c0b..9442c4136c38 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -11,14 +11,26 @@ enum insn_type {
RET = 3, /* tramp / site cond-tail-call */
};
+/*
+ * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax
+ * The REX.W cancels the effect of any data16.
+ */
+static const u8 xor5rax[] = { 0x66, 0x66, 0x48, 0x31, 0xc0 };
+
static void __ref __static_call_transform(void *insn, enum insn_type type, void *func)
{
+ const void *emulate = NULL;
int size = CALL_INSN_SIZE;
const void *code;
switch (type) {
case CALL:
code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
+ if (func == &__static_call_return0) {
+ emulate = code;
+ code = &xor5rax;
+ }
+
break;
case NOP:
@@ -41,7 +53,7 @@ static void __ref __static_call_transform(void *insn, enum insn_type type, void
if (unlikely(system_state == SYSTEM_BOOTING))
return text_poke_early(insn, code, size);
- text_poke_bp(insn, code, size, NULL);
+ text_poke_bp(insn, code, size, emulate);
}
static void __static_call_validate(void *insn, bool tail)
@@ -54,7 +66,8 @@ static void __static_call_validate(void *insn, bool tail)
return;
} else {
if (opcode == CALL_INSN_OPCODE ||
- !memcmp(insn, ideal_nops[NOP_ATOMIC5], 5))
+ !memcmp(insn, ideal_nops[NOP_ATOMIC5], 5) ||
+ !memcmp(insn, xor5rax, 5))
return;
}