Re: [bisected] clang 15 built kernel fails to boot, stuck at "Loading Linux 6.1.1 ...", gcc 12 built kernel with same config boots fine
From: Peter Zijlstra
Date: Fri Jan 20 2023 - 09:58:22 EST
On Fri, Jan 20, 2023 at 12:16:26PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 20, 2023 at 11:20:53AM +0530, Sandipan Das wrote:
>
> > With the combination of defconfig+kvm_guest.config+localyesconfig, the only
> > thing that made a difference was the compiler optimization choice. The kernel
> > boots up with CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y but not with
> > CONFIG_CC_OPTIMIZE_FOR_SIZE=y. Both Clang 15 and 16 kernel builds are affected.
>
> *groan*..
>
> 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
>
> and static_call() can't deal with Jcc, I wonder why we've not seen that
> before -- this isn't totally idiotic code-gen and esp. clang is known to
> do this (see the retpoline thing).
>
> Let me see if I can do something about that.
Here, this makes my qemu/kvm happy enough to complain about not having a
root device to mount :-)
Let me go try booting it for real.
---
diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index f4b87f08f5c5..29832c338cdc 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -184,6 +184,37 @@ void int3_emulate_ret(struct pt_regs *regs)
unsigned long ip = int3_emulate_pop(regs);
int3_emulate_jmp(regs, ip);
}
+
+static __always_inline
+void int3_emulate_jcc(struct pt_regs *regs, u8 cc, unsigned long ip, unsigned long disp)
+{
+ static const unsigned long jcc_mask[6] = {
+ [0] = X86_EFLAGS_OF,
+ [1] = X86_EFLAGS_CF,
+ [2] = X86_EFLAGS_ZF,
+ [3] = X86_EFLAGS_CF | X86_EFLAGS_ZF,
+ [4] = X86_EFLAGS_SF,
+ [5] = X86_EFLAGS_PF,
+ };
+
+ bool invert = cc & 1;
+ bool match;
+
+ if (cc < 0xc) {
+ match = regs->flags & jcc_mask[cc >> 1];
+ } else {
+ match = ((regs->flags & X86_EFLAGS_SF) >> X86_EFLAGS_SF_BIT) ^
+ ((regs->flags & X86_EFLAGS_OF) >> X86_EFLAGS_OF_BIT);
+ if (cc >= 0xe)
+ match = match || (regs->flags & X86_EFLAGS_ZF);
+ }
+
+ if ((match && !invert) || (!match && invert))
+ ip += disp;
+
+ int3_emulate_jmp(regs, ip);
+}
+
#endif /* !CONFIG_UML_X86 */
#endif /* _ASM_X86_TEXT_PATCHING_H */
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 7d8c3cbde368..cd632a0171b4 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1772,6 +1772,11 @@ 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;
@@ -1893,6 +1898,10 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
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();
}
@@ -1966,16 +1975,26 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
* 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++;
}
@@ -2003,8 +2022,7 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
* 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) {
@@ -2021,10 +2039,15 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
* 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++;
}
@@ -2042,9 +2065,11 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
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;
@@ -2055,6 +2080,13 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
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:
@@ -2071,7 +2103,6 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
BUG_ON(len != insn.length);
}
-
switch (tp->opcode) {
case INT3_INSN_OPCODE:
case RET_INSN_OPCODE:
@@ -2080,6 +2111,7 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
case CALL_INSN_OPCODE:
case JMP32_INSN_OPCODE:
case JMP8_INSN_OPCODE:
+ case 0x70 ... 0x7f: /* Jcc */
tp->disp = insn.immediate.value;
break;
diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index b36f3c367cb2..f2b2f7545ecb 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -464,50 +464,26 @@ static void kprobe_emulate_call(struct kprobe *p, struct pt_regs *regs)
}
NOKPROBE_SYMBOL(kprobe_emulate_call);
-static nokprobe_inline
-void __kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs, bool cond)
+static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
{
unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
- if (cond)
- ip += p->ainsn.rel32;
+ ip += p->ainsn.rel32;
int3_emulate_jmp(regs, ip);
}
-
-static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
-{
- __kprobe_emulate_jmp(p, regs, true);
-}
NOKPROBE_SYMBOL(kprobe_emulate_jmp);
-static const unsigned long jcc_mask[6] = {
- [0] = X86_EFLAGS_OF,
- [1] = X86_EFLAGS_CF,
- [2] = X86_EFLAGS_ZF,
- [3] = X86_EFLAGS_CF | X86_EFLAGS_ZF,
- [4] = X86_EFLAGS_SF,
- [5] = X86_EFLAGS_PF,
-};
-
static void kprobe_emulate_jcc(struct kprobe *p, struct pt_regs *regs)
{
- bool invert = p->ainsn.jcc.type & 1;
- bool match;
-
- if (p->ainsn.jcc.type < 0xc) {
- match = regs->flags & jcc_mask[p->ainsn.jcc.type >> 1];
- } else {
- match = ((regs->flags & X86_EFLAGS_SF) >> X86_EFLAGS_SF_BIT) ^
- ((regs->flags & X86_EFLAGS_OF) >> X86_EFLAGS_OF_BIT);
- if (p->ainsn.jcc.type >= 0xe)
- match = match || (regs->flags & X86_EFLAGS_ZF);
- }
- __kprobe_emulate_jmp(p, regs, (match && !invert) || (!match && invert));
+ int3_emulate_jcc(regs, p->ainsn.jcc.type,
+ regs->ip - INT3_INSN_SIZE + p->ainsn.size,
+ p->ainsn.rel32);
}
NOKPROBE_SYMBOL(kprobe_emulate_jcc);
static void kprobe_emulate_loop(struct kprobe *p, struct pt_regs *regs)
{
+ unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
bool match;
if (p->ainsn.loop.type != 3) { /* LOOP* */
@@ -535,7 +511,9 @@ static void kprobe_emulate_loop(struct kprobe *p, struct pt_regs *regs)
else if (p->ainsn.loop.type == 1) /* LOOPE */
match = match && (regs->flags & X86_EFLAGS_ZF);
- __kprobe_emulate_jmp(p, regs, match);
+ if (match)
+ ip += p->ainsn.rel32;
+ int3_emulate_jmp(regs, ip);
}
NOKPROBE_SYMBOL(kprobe_emulate_loop);
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index 2ebc338980bc..deb909f2b62f 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 +26,38 @@ 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"
+ "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 +84,20 @@ 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)
@@ -79,7 +120,8 @@ static void __static_call_validate(void *insn, bool tail, bool tramp)
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 ||