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Message-ID: <20230123210607.115718513@infradead.org>
Date: Mon, 23 Jan 2023 21:59:17 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...nel.org
Cc: tglx@...utronix.de, bp@...en8.de, dave.hansen@...ux.intel.com,
x86@...nel.org, hpa@...or.com, peterz@...radead.org,
jpoimboe@...nel.org, jbaron@...mai.com, rostedt@...dmis.org,
ardb@...nel.org, linux-kernel@...r.kernel.org,
erhard_f@...lbox.org, ndesaulniers@...gle.com, mhiramat@...nel.org,
sandipan.das@....com
Subject: [PATCH 2/3] x86/alternative: Teach text_poke_bp() to patch Jcc.d32 instructions
In order to re-write Jcc.d32 instructions text_poke_bp() needs to be
taught about them.
The biggest hurdle is that the whole machinery is currently made for 5
byte instructions and extending this would grow struct text_poke_loc
which is currently a nice 16 bytes and used in an array.
However, since text_poke_loc contains a full copy of the (s32)
displacement, it is possible to map the Jcc.d32 2 byte opcodes to
Jcc.d8 1 byte opcode for the int3 emulation.
This then leaves the replacement bytes; fudge that by only storing the
last 5 bytes and adding the rule that 'length == 6' instruction will
be prefixed with a 0x0f byte.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
---
arch/x86/kernel/alternative.c | 62 +++++++++++++++++++++++++++++++-----------
1 file changed, 47 insertions(+), 15 deletions(-)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -338,6 +338,12 @@ void __init_or_module noinline apply_alt
}
}
+static inline bool is_jcc32(struct insn *insn)
+{
+ /* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
+ return insn->opcode.bytes[0] == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80;
+}
+
#if defined(CONFIG_RETPOLINE) && defined(CONFIG_OBJTOOL)
/*
@@ -376,12 +382,6 @@ static int emit_indirect(int op, int reg
return i;
}
-static inline bool is_jcc32(struct insn *insn)
-{
- /* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
- return insn->opcode.bytes[0] == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80;
-}
-
static int emit_call_track_retpoline(void *addr, struct insn *insn, int reg, u8 *bytes)
{
u8 op = insn->opcode.bytes[0];
@@ -1770,6 +1770,11 @@ void text_poke_sync(void)
on_each_cpu(do_sync_core, NULL, 1);
}
+/*
+ * NOTE: crazy scheme to allow patching Jcc.d32 but not increase the size of
+ * this thing. When len == 6 everything is prefixed with 0x0f and we map
+ * opcode to Jcc.d8, using len to distinguish.
+ */
struct text_poke_loc {
/* addr := _stext + rel_addr */
s32 rel_addr;
@@ -1891,6 +1896,10 @@ noinstr int poke_int3_handler(struct pt_
int3_emulate_jmp(regs, (long)ip + tp->disp);
break;
+ case 0x70 ... 0x7f: /* Jcc */
+ int3_emulate_jcc(regs, tp->opcode & 0xf, (long)ip, tp->disp);
+ break;
+
default:
BUG();
}
@@ -1964,16 +1973,26 @@ static void text_poke_bp_batch(struct te
* Second step: update all but the first byte of the patched range.
*/
for (do_sync = 0, i = 0; i < nr_entries; i++) {
- u8 old[POKE_MAX_OPCODE_SIZE] = { tp[i].old, };
+ u8 old[POKE_MAX_OPCODE_SIZE+1] = { tp[i].old, };
+ u8 _new[POKE_MAX_OPCODE_SIZE+1];
+ const u8 *new = tp[i].text;
int len = tp[i].len;
if (len - INT3_INSN_SIZE > 0) {
memcpy(old + INT3_INSN_SIZE,
text_poke_addr(&tp[i]) + INT3_INSN_SIZE,
len - INT3_INSN_SIZE);
+
+ if (len == 6) {
+ _new[0] = 0x0f;
+ memcpy(_new + 1, new, 5);
+ new = _new;
+ }
+
text_poke(text_poke_addr(&tp[i]) + INT3_INSN_SIZE,
- (const char *)tp[i].text + INT3_INSN_SIZE,
+ new + INT3_INSN_SIZE,
len - INT3_INSN_SIZE);
+
do_sync++;
}
@@ -2001,8 +2020,7 @@ static void text_poke_bp_batch(struct te
* The old instruction is recorded so that the event can be
* processed forwards or backwards.
*/
- perf_event_text_poke(text_poke_addr(&tp[i]), old, len,
- tp[i].text, len);
+ perf_event_text_poke(text_poke_addr(&tp[i]), old, len, new, len);
}
if (do_sync) {
@@ -2019,10 +2037,15 @@ static void text_poke_bp_batch(struct te
* replacing opcode.
*/
for (do_sync = 0, i = 0; i < nr_entries; i++) {
- if (tp[i].text[0] == INT3_INSN_OPCODE)
+ u8 byte = tp[i].text[0];
+
+ if (tp[i].len == 6)
+ byte = 0x0f;
+
+ if (byte == INT3_INSN_OPCODE)
continue;
- text_poke(text_poke_addr(&tp[i]), tp[i].text, INT3_INSN_SIZE);
+ text_poke(text_poke_addr(&tp[i]), &byte, INT3_INSN_SIZE);
do_sync++;
}
@@ -2040,9 +2063,11 @@ static void text_poke_loc_init(struct te
const void *opcode, size_t len, const void *emulate)
{
struct insn insn;
- int ret, i;
+ int ret, i = 0;
- memcpy((void *)tp->text, opcode, len);
+ if (len == 6)
+ i = 1;
+ memcpy((void *)tp->text, opcode+i, len-i);
if (!emulate)
emulate = opcode;
@@ -2053,6 +2078,13 @@ static void text_poke_loc_init(struct te
tp->len = len;
tp->opcode = insn.opcode.bytes[0];
+ if (is_jcc32(&insn)) {
+ /*
+ * Map Jcc.d32 onto Jcc.d8 and use len to distinguish.
+ */
+ tp->opcode = insn.opcode.bytes[1] - 0x10;
+ }
+
switch (tp->opcode) {
case RET_INSN_OPCODE:
case JMP32_INSN_OPCODE:
@@ -2069,7 +2101,6 @@ static void text_poke_loc_init(struct te
BUG_ON(len != insn.length);
}
-
switch (tp->opcode) {
case INT3_INSN_OPCODE:
case RET_INSN_OPCODE:
@@ -2078,6 +2109,7 @@ static void text_poke_loc_init(struct te
case CALL_INSN_OPCODE:
case JMP32_INSN_OPCODE:
case JMP8_INSN_OPCODE:
+ case 0x70 ... 0x7f: /* Jcc */
tp->disp = insn.immediate.value;
break;
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