x86/kexec: Annotate embedded data with ELF metadata

x86/kexec: Annotate embedded data with ELF metadata

Scanning for embedded endbranch instructions involves parsing the .text
disassembly.  Data in the kexec trampoline has no ELF metadata, so objdump
treats it as instructions and tries to disassemble.  Convert:

  ffff82d040396108 <compatibility_mode_far>:
  ffff82d040396108:       00 00                   add    %al,(%rax)
  ffff82d04039610a:       00 00                   add    %al,(%rax)
  ffff82d04039610c:       10 00                   adc    %al,(%rax)

  ffff82d04039610e <compat_mode_gdt_desc>:
  ffff82d04039610e:       17                      (bad)
          ...

  ffff82d040396118 <compat_mode_gdt>:
          ...
  ffff82d040396120:       ff                      (bad)
  ffff82d040396121:       ff 00                   incl   (%rax)
  ffff82d040396123:       00 00                   add    %al,(%rax)
  ffff82d040396125:       93                      xchg   %eax,%ebx
  ffff82d040396126:       cf                      iret
  ffff82d040396127:       00 ff                   add    %bh,%bh
  ffff82d040396129:       ff 00                   incl   (%rax)
  ffff82d04039612b:       00 00                   add    %al,(%rax)
  ffff82d04039612d:       9b                      fwait
  ffff82d04039612e:       cf                      iret
          ...

  ffff82d040396130 <compat_mode_idt>:
          ...

  ffff82d0403961b6 <kexec_reloc_size>:
  ffff82d0403961b6:       b6 01                   mov    $0x1,%dh
          ...

to:

  ffff82d040396108 <compatibility_mode_far>:
  ffff82d040396108:       00 00 00 00 10 00                               ......

  ffff82d04039610e <compat_mode_gdt_desc>:
  ffff82d04039610e:       17 00 00 00 00 00 00 00 00 00                   ..........

  ffff82d040396118 <compat_mode_gdt>:
          ...
  ffff82d040396120:       ff ff 00 00 00 93 cf 00 ff ff 00 00 00 9b cf 00 ................

  ffff82d040396130 <compat_mode_idt>:
  ffff82d040396130:       00 00 00 00 00 00                               ......

  ffff82d040396136 <reloc_stack>:
          ...

Most data just gains type and size metadata.

The reloc_stack label is the wrong end of the data block to have a size, so
move it to the lowest address and introduce .Lreloc_stack_base as a
replacement.  Also, fix the fact that it is misaligned by 2 bytes.

While kexec_reloc_size could gain metadata, it's use in the linker
assertion (while correct) is deeply confusing to follow.  Drop it entirely,
using a linker symbol instead to denote the end of the trampoline.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>