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Message-ID: <20251110115758.213813530@infradead.org>
Date: Mon, 10 Nov 2025 12:46:43 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
peterz@...radead.org,
kees@...nel.org,
acarmina@...hat.com,
jpoimboe@...nel.org,
mark.rutland@....com,
torvalds@...uxfoundation.org,
maciej.wieczor-retman@...el.com
Subject: [PATCH v2 10/12] x86_64/bug: Implement __WARN_printf()
The basic idea is to have __WARN_printf() be a vararg function such
that the compiler can do the optimal calling convention for us. This
function body will be a #UD and then set up a va_list in the exception
from pt_regs.
But because the trap will be in a called function, the bug_entry must
be passed in. Have that be the first argument, with the format tucked
away inside the bug_entry.
The comments should clarify the real fun details.
The big downside is that all WARNs will now show:
RIP: 0010:__WARN_trap:+0
One possible solution is to simply discard the top frame when
unwinding. A follow up patch takes care of this slightly differently
by abusing the x86 static_call implementation.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/entry/entry.S | 8 +++
arch/x86/include/asm/bug.h | 60 +++++++++++++++++++++--
arch/x86/kernel/traps.c | 115 ++++++++++++++++++++++++++++++++++++++++-----
3 files changed, 168 insertions(+), 15 deletions(-)
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -32,6 +32,14 @@ SYM_FUNC_END(write_ibpb)
/* For KVM */
EXPORT_SYMBOL_GPL(write_ibpb);
+SYM_FUNC_START(__WARN_trap)
+ ANNOTATE_NOENDBR
+ ANNOTATE_REACHABLE
+ ud1 (%ecx), %_ASM_ARG1
+ RET
+SYM_FUNC_END(__WARN_trap)
+EXPORT_SYMBOL(__WARN_trap)
+
.popsection
/*
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -31,6 +31,7 @@
#define BUG_UD2 0xfffe
#define BUG_UD1 0xfffd
#define BUG_UD1_UBSAN 0xfffc
+#define BUG_UD1_WARN 0xfffb
#define BUG_UDB 0xffd6
#define BUG_LOCK 0xfff0
@@ -58,14 +59,17 @@
#define __BUG_ENTRY_FORMAT(format)
#endif
+#ifdef CONFIG_X86_64
+#define HAVE_ARCH_BUG_FORMAT_ARGS
+#endif
+
#define __BUG_ENTRY(format, file, line, flags) \
__BUG_REL("1b") "\t# bug_entry::bug_addr\n" \
__BUG_ENTRY_FORMAT(format) \
__BUG_ENTRY_VERBOSE(file, line) \
"\t.word " flags "\t# bug_entry::flags\n"
-#define _BUG_FLAGS_ASM(ins, format, file, line, flags, size, extra) \
- "1:\t" ins "\n" \
+#define _BUG_FLAGS_ASM(format, file, line, flags, size, extra) \
".pushsection __bug_table,\"aw\"\n\t" \
ANNOTATE_DATA_SPECIAL \
"2:\n\t" \
@@ -82,7 +86,8 @@
#define _BUG_FLAGS(cond_str, ins, flags, extra) \
do { \
- asm_inline volatile(_BUG_FLAGS_ASM(ins, "%c[fmt]", "%c[file]", \
+ asm_inline volatile("1:\t" ins "\n" \
+ _BUG_FLAGS_ASM("%c[fmt]", "%c[file]", \
"%c[line]", "%c[fl]", \
"%c[size]", extra) \
: : [fmt] "i" (WARN_CONDITION_STR(cond_str)), \
@@ -93,7 +98,8 @@ do { \
} while (0)
#define ARCH_WARN_ASM(file, line, flags, size) \
- _BUG_FLAGS_ASM(ASM_UD2, "NULL", file, line, flags, size, "")
+ "1:\t " ASM_UD2 "\n" \
+ _BUG_FLAGS_ASM("NULL", file, line, flags, size, "")
#else
@@ -126,6 +132,52 @@ do { \
instrumentation_end(); \
} while (0)
+#ifdef HAVE_ARCH_BUG_FORMAT_ARGS
+
+#ifndef __ASSEMBLY__
+struct bug_entry;
+extern void __WARN_trap(struct bug_entry *bug, ...);
+
+struct pt_regs;
+struct sysv_va_list { /* from AMD64 System V ABI */
+ unsigned int gp_offset;
+ unsigned int fp_offset;
+ void *overflow_arg_area;
+ void *reg_save_area;
+};
+struct arch_va_list {
+ unsigned long regs[6];
+ struct sysv_va_list args;
+};
+extern void *__warn_args(struct arch_va_list *args, struct pt_regs *regs);
+#endif /* __ASSEMBLY__ */
+
+#define __WARN_bug_entry(flags, format) ({ \
+ struct bug_entry *bug; \
+ asm_inline volatile("lea (2f)(%%rip), %[addr]\n1:\n" \
+ _BUG_FLAGS_ASM("%c[fmt]", "%c[file]", \
+ "%c[line]", "%c[fl]", \
+ "%c[size]", "") \
+ : [addr] "=r" (bug) \
+ : [fmt] "i" (format), \
+ [file] "i" (__FILE__), \
+ [line] "i" (__LINE__), \
+ [fl] "i" (flags), \
+ [size] "i" (sizeof(struct bug_entry))); \
+ bug; })
+
+#define __WARN_print_arg(flags, format, arg...) \
+do { \
+ int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS ; \
+ __WARN_trap(__WARN_bug_entry(__flags, format), ## arg); \
+ asm (""); /* inhibit tail-call optimization */ \
+} while (0)
+
+#define __WARN_printf(taint, fmt, arg...) \
+ __WARN_print_arg(BUGFLAG_TAINT(taint), fmt, ## arg)
+
+#endif /* HAVE_ARCH_BUG_FORMAT_ARGS */
+
#include <asm-generic/bug.h>
#endif /* _ASM_X86_BUG_H */
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -102,25 +102,37 @@ __always_inline int is_valid_bugaddr(uns
* UBSan{0}: 67 0f b9 00 ud1 (%eax),%eax
* UBSan{10}: 67 0f b9 40 10 ud1 0x10(%eax),%eax
* static_call: 0f b9 cc ud1 %esp,%ecx
+ * __WARN_trap: 67 48 0f b9 39 ud1 (%ecx),%reg
*
- * Notably UBSAN uses EAX, static_call uses ECX.
+ * Notable, since __WARN_trap can use all registers, the distinction between
+ * UD1 users is through R/M.
*/
__always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
{
unsigned long start = addr;
+ u8 v, reg, rm, rex = 0;
+ int type = BUG_UD1;
bool lock = false;
- u8 v;
if (addr < TASK_SIZE_MAX)
return BUG_NONE;
- v = *(u8 *)(addr++);
- if (v == INSN_ASOP)
+ for (;;) {
v = *(u8 *)(addr++);
+ if (v == INSN_ASOP)
+ continue;
- if (v == INSN_LOCK) {
- lock = true;
- v = *(u8 *)(addr++);
+ if (v == INSN_LOCK) {
+ lock = true;
+ continue;
+ }
+
+ if ((v & 0xf0) == 0x40) {
+ rex = v;
+ continue;
+ }
+
+ break;
}
switch (v) {
@@ -156,18 +168,33 @@ __always_inline int decode_bug(unsigned
if (X86_MODRM_MOD(v) != 3 && X86_MODRM_RM(v) == 4)
addr++; /* SIB */
+ reg = X86_MODRM_REG(v) + 8*!!X86_REX_R(rex);
+ rm = X86_MODRM_RM(v) + 8*!!X86_REX_B(rex);
+
/* Decode immediate, if present */
switch (X86_MODRM_MOD(v)) {
case 0: if (X86_MODRM_RM(v) == 5)
- addr += 4; /* RIP + disp32 */
+ addr += 4; /* RIP + disp32 */
+
+ if (rm == 0) /* (%eax) */
+ type = BUG_UD1_UBSAN;
+
+ if (rm == 1) { /* (%ecx) */
+ *imm = reg;
+ type = BUG_UD1_WARN;
+ }
break;
case 1: *imm = *(s8 *)addr;
addr += 1;
+ if (rm == 0) /* (%eax) */
+ type = BUG_UD1_UBSAN;
break;
case 2: *imm = *(s32 *)addr;
addr += 4;
+ if (rm == 0) /* (%eax) */
+ type = BUG_UD1_UBSAN;
break;
case 3: break;
@@ -176,12 +203,73 @@ __always_inline int decode_bug(unsigned
/* record instruction length */
*len = addr - start;
- if (X86_MODRM_REG(v) == 0) /* EAX */
- return BUG_UD1_UBSAN;
+ return type;
+}
- return BUG_UD1;
+static inline unsigned long pt_regs_val(struct pt_regs *regs, int nr)
+{
+ int offset = pt_regs_offset(regs, nr);
+ if (WARN_ON_ONCE(offset < -0))
+ return 0;
+ return *((unsigned long *)((void *)regs + offset));
}
+#ifdef HAVE_ARCH_BUG_FORMAT_ARGS
+/*
+ * Create a va_list from an exception context.
+ */
+void *__warn_args(struct arch_va_list *args, struct pt_regs *regs)
+{
+ /*
+ * Register save area; populate with function call argument registers
+ */
+ args->regs[0] = regs->di;
+ args->regs[1] = regs->si;
+ args->regs[2] = regs->dx;
+ args->regs[3] = regs->cx;
+ args->regs[4] = regs->r8;
+ args->regs[5] = regs->r9;
+
+ /*
+ * From the ABI document:
+ *
+ * @gp_offset - the element holds the offset in bytes from
+ * reg_save_area to the place where the next available general purpose
+ * argument register is saved. In case all argument registers have
+ * been exhausted, it is set to the value 48 (6*8).
+ *
+ * @fp_offset - the element holds the offset in bytes from
+ * reg_save_area to the place where the next available floating point
+ * argument is saved. In case all argument registers have been
+ * exhausted, it is set to the value 176 (6*8 + 8*16)
+ *
+ * @overflow_arg_area - this pointer is used to fetch arguments passed
+ * on the stack. It is initialized with the address of the first
+ * argument passed on the stack, if any, and then always updated to
+ * point to the start of the next argument on the stack.
+ *
+ * @reg_save_area - the element points to the start of the register
+ * save area.
+ *
+ * Notably the vararg starts with the second argument and there are no
+ * floating point arguments in the kernel.
+ */
+ args->args.gp_offset = 1*8;
+ args->args.fp_offset = 6*8 + 8*16;
+ args->args.reg_save_area = &args->regs;
+ args->args.overflow_arg_area = (void *)regs->sp;
+
+ /*
+ * If the exception came from __WARN_trap, there is a return
+ * address on the stack, skip that. This is why any __WARN_trap()
+ * caller must inhibit tail-call optimization.
+ */
+ if ((void *)regs->ip == &__WARN_trap)
+ args->args.overflow_arg_area += 8;
+
+ return &args->args;
+}
+#endif /* HAVE_ARCH_BUG_FORMAT */
static nokprobe_inline int
do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
@@ -334,6 +422,11 @@ static noinstr bool handle_bug(struct pt
raw_local_irq_enable();
switch (ud_type) {
+ case BUG_UD1_WARN:
+ if (report_bug_entry((void *)pt_regs_val(regs, ud_imm), regs) == BUG_TRAP_TYPE_WARN)
+ handled = true;
+ break;
+
case BUG_UD2:
if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
handled = true;
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