[llvm] [MC,COFF] .safeseh: avoid changeSection (PR #132624)

[LIU Hao via llvm-commits]

lhmouse wrote:

The change looks good to me. Here is a test program which used to crash without this patch.

```s
# gcc -m32 test.S -nostdlib /mingw32/lib/libkernel32.a && ./a.exe
# clang -target i686-w64-mingw32 test.S -nostdlib /mingw32/lib/libkernel32.a && ./a.exe

# CODE SECTION
.intel_syntax noprefix
.text

  # static void __stdcall print(const char* msg)
  .def _print at 4; .scl 3; .type 32; .endef
  _print at 4:
    push esi
    sub esp, 24

    # %esi = GetStdHandle(STD_ERROR_HANDLE)
    mov DWORD PTR [esp], -12
    call _GetStdHandle at 4
    push ecx
    mov esi, eax

    # %eax = lstrlenA(msg)
    mov edx, DWORD PTR [esp + 32]
    mov DWORD PTR [esp], edx
    call _lstrlenA at 4
    push ecx

    # WriteFile(%esi, msg, %eax, ignored, nullptr)
    mov DWORD PTR [esp], esi
    mov edx, DWORD PTR [esp + 32]
    mov DWORD PTR [esp + 4], edx
    mov DWORD PTR [esp + 8], eax
    lea eax, [esp + 20]
    mov DWORD PTR [esp + 12], eax
    mov DWORD PTR [esp + 16], 0
    call _WriteFile at 20
    sub esp, 20

    # return
    add esp, 24
    pop esi
    ret 4

  # EXCEPTION_DISPOSITION my_handler(...)
  .def _my_handler; .scl 2; .type 32; .endef
  .globl _my_handler
#if defined __clang__ || defined _MSC_VER
  .safeseh _my_handler
#endif
  _my_handler:
    sub esp, 12

    # print(msg_002)
    mov DWORD PTR [esp], OFFSET _msg_002
    call _print at 4
    push ecx

    # return ExceptionContinueExecution
    xor eax, eax
    add esp, 12
    ret

  # [[noreturn]] int mainCRTStartup(void)
  .globl _mainCRTStartup
  _mainCRTStartup:
    sub esp, 60

    # EXCEPTION_REGISTRATION_RECORD record
    # record.Next = *%fs:0
    # record.Handler = &my_handler
    # *%fs:0 = &record
    lea ecx, [esp + 52]
    mov eax, DWORD PTR fs:[0]
    mov DWORD PTR [ecx], eax
    mov DWORD PTR [ecx + 4], OFFSET _my_handler
    mov DWORD PTR fs:[0], ecx

    # print(msg_000)
    mov DWORD PTR [esp], OFFSET _msg_000
    call _print at 4
    push ecx

    # RaiseException(0x20616263, 0, 0, NULL)
    mov DWORD PTR [esp], 0x20616263
    mov DWORD PTR [esp + 4], 0
    mov DWORD PTR [esp + 8], 0
    mov DWORD PTR [esp + 12], 0
    call _RaiseException at 16
    sub esp, 16

    # print(msg_001)
    mov DWORD PTR [esp], OFFSET _msg_001
    call _print at 4
    push ecx

    # ExitProcess(0)
    mov DWORD PTR [esp], 0
    call _ExitProcess at 4

# DATA SECTION
.section .rdata, "dr"

  _msg_000:  .asciz "my_handler installed\n"
  _msg_001:  .asciz "exiting\n"
  _msg_002:  .asciz "my_handler invoked\n"
```


https://github.com/llvm/llvm-project/pull/132624