Re: [PATCH] x86/kcfi: Optimize call sequence

[Peter Zijlstra]

On Wed, Jun 17, 2026 at 01:36:37PM +0100, David Laight wrote:

> I did have a local change that generated different nop*3 so I could tell
> what was lfence, stac, clac (etc).
> Trying to check the compiler output was hard when there were blocks of
> 6 nop.

You should use objtool more ;-)

$ defconfig-build/tools/objtool/objtool --disas=rseq_update_user_cs --wide defconfig-build/kernel/rseq.o
rseq_update_user_cs:
   f00:  rseq_update_user_cs+0x0                      mov    %rdi,%rax
   f03:  rseq_update_user_cs+0x3                      mov    0x80(%rsi),%rdi
   f0a:  rseq_update_user_cs+0xa                      mov    %gs:0x0(%rip),%rcx        # 0xf12 <current_task>
   f12:  rseq_update_user_cs+0x12                     testq  $0x8000000,(%rcx)
   f19:  rseq_update_user_cs+0x19                     jne    0xf27 <rseq_update_user_cs+0x27>
   f1b:  rseq_update_user_cs+0x1b                   | <alternative.f1b>           | X86_FEATURE_LA57
   f1b:  rseq_update_user_cs+0x1b                   | movabs $0x7ffffffff000,%r10 | movabs $0xfffffffffff000,%r10
   f25:  rseq_update_user_cs+0x25                     jmp    0xf3f <rseq_update_user_cs+0x3f>
   f27:  rseq_update_user_cs+0x27                     mov    $0xc0000000,%r8d
   f2d:  rseq_update_user_cs+0x2d                     lea    0x3fffe000(%r8),%r10
   f34:  rseq_update_user_cs+0x34                     testb  $0x8,0x5ab(%rcx)
   f3b:  rseq_update_user_cs+0x3b                     cmovne %r8,%r10
   f3f:  rseq_update_user_cs+0x3f                     cmp    %r10,%rdx
   f42:  rseq_update_user_cs+0x42                     jae    0xfda <rseq_update_user_cs+0xda>
   f48:  rseq_update_user_cs+0x48                   | <jump_table.f48>                        | JUMP
   f48:  rseq_update_user_cs+0x48                   | jmp    0xfe9 <rseq_update_user_cs+0xe9> | nop5
   f4d:  rseq_update_user_cs+0x4d                     movabs $0x123456789abcdef,%rcx
   f57:  rseq_update_user_cs+0x57                     cmp    %rcx,%rdx
   f5a:  rseq_update_user_cs+0x5a                     cmova  %rcx,%rdx
   f5e:  rseq_update_user_cs+0x5e                   | <alternative.f5e> | X86_FEATURE_SMAP
   f5e:  rseq_update_user_cs+0x5e                   | nop*3             | stac
   f61:  rseq_update_user_cs+0x61                   | <ex_table.f61>       | EXCEPTION
   f61:  rseq_update_user_cs+0x61                   | mov    0x8(%rdx),%r9 | resume at 0xff1 <rseq_update_user_cs+0xf1>
   f65:  rseq_update_user_cs+0x65                   | <ex_table.f65>        | EXCEPTION
   f65:  rseq_update_user_cs+0x65                   | mov    0x10(%rdx),%r8 | resume at 0xff3 <rseq_update_user_cs+0xf3>
   f69:  rseq_update_user_cs+0x69                     add    $0x18,%rdx
   f6d:  rseq_update_user_cs+0x6d                   | <ex_table.f6d>     | EXCEPTION
   f6d:  rseq_update_user_cs+0x6d                   | mov    (%rdx),%rcx | resume at 0xff5 <rseq_update_user_cs+0xf5>
   f70:  rseq_update_user_cs+0x70                     mov    %rdi,%rdx
   f73:  rseq_update_user_cs+0x73                     sub    %r9,%rdx
   f76:  rseq_update_user_cs+0x76                     cmp    %r8,%rdx
   f79:  rseq_update_user_cs+0x79                     jae    0xfbd <rseq_update_user_cs+0xbd>
   f7b:  rseq_update_user_cs+0x7b                     cmp    %r10,%rcx
   f7e:  rseq_update_user_cs+0x7e                     jae    0xfd7 <rseq_update_user_cs+0xd7>
   f80:  rseq_update_user_cs+0x80                     cmp    $0x4,%rcx
   f84:  rseq_update_user_cs+0x84                     jb     0xfd7 <rseq_update_user_cs+0xd7>
   f86:  rseq_update_user_cs+0x86                   | <ex_table.f86>         | EXCEPTION
   f86:  rseq_update_user_cs+0x86                   | mov    -0x4(%rcx),%edx | resume at 0xff7 <rseq_update_user_cs+0xf7>
   f89:  rseq_update_user_cs+0x89                     cmp    %edx,0xa9c(%rax)
   f8f:  rseq_update_user_cs+0x8f                     jne    0xfd7 <rseq_update_user_cs+0xd7>
   f91:  rseq_update_user_cs+0x91                     mov    0xa90(%rax),%rax
   f98:  rseq_update_user_cs+0x98                   | <ex_table.f98>        | EXCEPTION
   f98:  rseq_update_user_cs+0x98                   | movq   $0x0,0x8(%rax) | resume at 0xff7 <rseq_update_user_cs+0xf7>
   fa0:  rseq_update_user_cs+0xa0                     mov    %rcx,0x80(%rsi)
   fa7:  rseq_update_user_cs+0xa7                   | <alternative.fa7> | X86_FEATURE_SMAP
   fa7:  rseq_update_user_cs+0xa7                   | nop*3             | clac
   faa:  rseq_update_user_cs+0xaa                     mov    %r9,%rsi
   fad:  rseq_update_user_cs+0xad                     mov    %r8,%rdx
   fb0:  rseq_update_user_cs+0xb0                     call   0x150 <rseq_trace_ip_fixup>
   fb5:  rseq_update_user_cs+0xb5                     mov    $0x1,%al
   fb7:  rseq_update_user_cs+0xb7                     cs jmp 0xfbd <__x86_return_thunk>
   fbd:  rseq_update_user_cs+0xbd                     mov    0xa90(%rax),%rax
   fc4:  rseq_update_user_cs+0xc4                   | <ex_table.fc4>        | EXCEPTION
   fc4:  rseq_update_user_cs+0xc4                   | movq   $0x0,0x8(%rax) | resume at 0xff7 <rseq_update_user_cs+0xf7>
   fcc:  rseq_update_user_cs+0xcc                   | <alternative.fcc> | X86_FEATURE_SMAP
   fcc:  rseq_update_user_cs+0xcc                   | nop*3             | clac
   fcf:  rseq_update_user_cs+0xcf                     mov    $0x1,%al
   fd1:  rseq_update_user_cs+0xd1                     cs jmp 0xfd7 <__x86_return_thunk>
   fd7:  rseq_update_user_cs+0xd7                   | <alternative.fd7> | X86_FEATURE_SMAP
   fd7:  rseq_update_user_cs+0xd7                   | nop*3             | clac
   fda:  rseq_update_user_cs+0xda                     movb   $0x1,0xaa6(%rax)
   fe1:  rseq_update_user_cs+0xe1                     xor    %eax,%eax
   fe3:  rseq_update_user_cs+0xe3                     cs jmp 0xfe9 <__x86_return_thunk>
   fe9:  rseq_update_user_cs+0xe9                     mov    %rax,%rdi
   fec:  rseq_update_user_cs+0xec                     jmp    0xff1 <rseq_debug_update_user_cs>
   ff1:  rseq_update_user_cs+0xf1                     jmp    0xff7 <rseq_update_user_cs+0xf7>
   ff3:  rseq_update_user_cs+0xf3                     jmp    0xff7 <rseq_update_user_cs+0xf7>
   ff5:  rseq_update_user_cs+0xf5                     jmp    0xff7 <rseq_update_user_cs+0xf7>
   ff7:  rseq_update_user_cs+0xf7                   | <alternative.ff7> | X86_FEATURE_SMAP
   ff7:  rseq_update_user_cs+0xf7                   | nop*3             | clac
   ffa:  rseq_update_user_cs+0xfa                     xor    %eax,%eax
   ffc:  rseq_update_user_cs+0xfc                     cs jmp 0x1002 <__x86_return_thunk>