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: Erhard F.
Date: Fri Jan 20 2023 - 10:20:35 EST

On Fri, 20 Jan 2023 15:57:47 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> 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 ||

Success! You nailed it.

Applied your patch on top of v6.2-rc4 and with that I got ye olde Thinkpad A275 with it's borked custom config + clang -Os(stupid) booting again. No side effects to be seen so far..

Many thanks!

Regards,
Erhard