Re: [PATCH] x86/alternatives: gracefully skip unrecognized indirect call instructions

>
> That's not what I asked, now was it. I asked what opcodes it does
> generate. Disassemble an affected function and show the difference
> between the normal -O2 and your -O1 build that causes the failure.
on vmlinux-O1:
(gdb) until arch/x86/kernel/alternative.c:557
alt_replace_call (instr=0xffffffff82f447e5 <early_fixup_exception+133>
"\377S\b\220\220\353\371[A\\]\303\314\314\314\314\303\314\314\314\314f\017\037D",
insn_buff=0xffffffff82c03d78 <init_thread_union+15736>
"\350\263\177\020\377",
    a=0xffffffff831dae0e) at arch/x86/kernel/alternative.c:557
557 pr_err("ALT_FLAG_DIRECT_CALL set for unrecognized indirect call\n");
vmlinux-O1:
  0xffffffff82f447e5 <+133>: ff 53 08           call   QWORD PTR [rbx+0x8]
   0xffffffff82f447e8 <+136>: 90                 nop
   0xffffffff82f447e9 <+137>: 90                 nop
   0xffffffff82f447ea <+138>: eb f9               jmp
0xffffffff82f447e5 <early_fixup_exception+133>
   0xffffffff82f447ec <+140>: 5b                 pop    rbx
   0xffffffff82f447ed <+141>: 41 5c               pop    r12

vmlinux-O2:
  0xffffffff82f3bf4b <+123>: 74 b0               je
0xffffffff82f3befd <early_fixup_exception+45>
   0xffffffff82f3bf4d <+125>: eb 08               jmp
0xffffffff82f3bf57 <early_fixup_exception+135>
   0xffffffff82f3bf4f <+127>: ff 15 33 32 d0 ff   call   QWORD PTR
[rip+0xffffffffffd03233]        # 0xffffffff82c3f188 <pv_ops+8>
   0xffffffff82f3bf55 <+133>: eb f8               jmp
0xffffffff82f3bf4f <early_fixup_exception+127>
   0xffffffff82f3bf57 <+135>: 5b                 pop    rbx
   0xffffffff82f3bf58 <+136>: 41 5c               pop    r12
   0xffffffff82f3bf5a <+138>: 5d                 pop    rbp

I use the patch below to bypass the problem while keeping all logic of
alt_replace_call intact.

From 2538684697e537298aa9082b582bc7dddd29b278 Mon Sep 17 00:00:00 2001
From: Zeliang Li <[email protected]>
Date: Fri, 14 Aug 2026 02:32:19 +0800
Subject: [PATCH] x86/mm: force O2 optimization for early_fixup_exception

When the kernel is compiled with non-standard optimization flags
(e.g., KCFLAGS=-O1), the function early_fixup_exception may be
compiled to an indirect call instruction that is not recognized
by alt_replace_call() (e.g., "call *0x8(%rbx)"). This triggers a
BUG() during early boot, as seen in:

  ALT_FLAG_DIRECT_CALL: original instruction is not a 6-byte ...
  instruction at early_fixup_exception+0x85/0xa0, length=5
  instruction bytes: ff 53 08 90 90

Force this function to be compiled with -O2 optimization level,
which guarantees the standard 6-byte `ff 15 <disp32>` form used
by alternatives patching.

Signed-off-by: Zeliang Li <[email protected]>
---
 arch/x86/mm/extable.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index ceb8d03191ab..5d5c7510a817 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -375,6 +375,7 @@ int fixup_exception(struct pt_regs *regs, int
trapnr, unsigned long error_code,
 extern unsigned int early_recursion_flag;

 /* Restricted version used during very early boot */
+__attribute__((optimize("O2")))
 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 {
  /* Ignore early NMIs. */
-- 
2.53.0

-- 
KISS == Keep it simple,stupid~:-)
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