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x86_64/bug: Inline the UD1 

(Ab)use the static_call infrastructure to convert all:

  call __WARN_trap

instances into the desired:

  ud1 (%edx), %rdi

eliminating the CALL/RET, but more importantly, fixing the
fact that all WARNs will have:

  RIP: 0010:__WARN_trap+0

Basically, by making it a static_call trampoline call, objtool will
collect the callsites, and then the inline rewrite will hit the
special case and replace the code with the magic instruction.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251110115758.456717741@infradead.org
pull/1354/merge
Peter Zijlstra 2025-06-07 10:06:40 +02:00
parent 11bb4944f0
commit 860238af7a
3 changed files with 19 additions and 3 deletions
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5
arch/x86/include/asm/bug.h
View File

@ -140,6 +140,9 @@ do { \
#ifdef HAVE_ARCH_BUG_FORMAT_ARGS
#ifndef __ASSEMBLY__
#include <linux/static_call_types.h>
DECLARE_STATIC_CALL(WARN_trap, __WARN_trap);
struct pt_regs;
struct sysv_va_list { /* from AMD64 System V ABI */
unsigned int gp_offset;
@ -171,7 +174,7 @@ extern void *__warn_args(struct arch_va_list *args, struct pt_regs *regs);
#define __WARN_print_arg(flags, format, arg...) \
do { \
int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS ; \
__WARN_trap(__WARN_bug_entry(__flags, format), ## arg); \
static_call_mod(WARN_trap)(__WARN_bug_entry(__flags, format), ## arg); \
asm (""); /* inhibit tail-call optimization */ \
} while (0)

13
arch/x86/kernel/static_call.c
View File

@ -26,6 +26,11 @@ static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
/*
* ud1 (%edx),%rdi -- see __WARN_trap() / decode_bug()
*/
static const u8 warninsn[] = { 0x67, 0x48, 0x0f, 0xb9, 0x3a };
static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
{
u8 ret = 0;
@ -69,7 +74,10 @@ static void __ref __static_call_transform(void *insn, enum insn_type type,
emulate = code;
code = &xor5rax;
}
if (func == &__WARN_trap) {
emulate = code;
code = &warninsn;
}
break;
case NOP:
@ -128,7 +136,8 @@ static void __static_call_validate(u8 *insn, bool tail, bool tramp)
} else {
if (opcode == CALL_INSN_OPCODE ||
!memcmp(insn, x86_nops[5], 5) ||
!memcmp(insn, xor5rax, 5))
!memcmp(insn, xor5rax, 5) ||
!memcmp(insn, warninsn, 5))
return;
}

4
arch/x86/kernel/traps.c
View File

@ -31,6 +31,7 @@
#include <linux/kexec.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/static_call.h>
#include <linux/timer.h>
#include <linux/init.h>
#include <linux/bug.h>
@ -215,6 +216,9 @@ static inline unsigned long pt_regs_val(struct pt_regs *regs, int nr)
}
#ifdef HAVE_ARCH_BUG_FORMAT_ARGS
DEFINE_STATIC_CALL(WARN_trap, __WARN_trap);
EXPORT_STATIC_CALL_TRAMP(WARN_trap);
/*
* Create a va_list from an exception context.
*/