x86/kexec: Fix location of relocate_kernel with -ffunction-sections

After commit

  cb33ff9 ("x86/kexec: Move relocate_kernel to kernel .data section"),

kernels configured with an option that uses -ffunction-sections, such as
CONFIG_LTO_CLANG, crash when kexecing because the value of relocate_kernel
does not match the value of __relocate_kernel_start so incorrect code gets
copied via machine_kexec_prepare().

  $ llvm-nm good-vmlinux &| rg relocate_kernel
  ffffffff83280d41 T __relocate_kernel_end
  ffffffff83280b00 T __relocate_kernel_start
  ffffffff83280b00 T relocate_kernel

  $ llvm-nm bad-vmlinux &| rg relocate_kernel
  ffffffff83266100 D __relocate_kernel_end
  ffffffff83266100 D __relocate_kernel_start
  ffffffff8120b0d8 T relocate_kernel

When -ffunction-sections is enabled, TEXT_MAIN matches on
'.text.[0-9a-zA-Z_]*' to coalesce the function specific functions back
into .text during link time after they have been optimized. Due to the
placement of TEXT_TEXT before KEXEC_RELOCATE_KERNEL in the x86 linker
script, the .text.relocate_kernel section ends up in .text instead of
.data.

Use a second dot in the relocate_kernel section name to avoid matching
on TEXT_MAIN, which matches a similar situation that happened in
commit

  79cd2a1 ("x86/retpoline,kprobes: Fix position of thunk sections with CONFIG_LTO_CLANG"),

which allows kexec to function properly.

While .data.relocate_kernel still ends up in the .data section via
DATA_MAIN -> DATA_DATA, ensure it is located with the
.text.relocate_kernel section as intended by performing the same
transformation.

Fixes: cb33ff9 ("x86/kexec: Move relocate_kernel to kernel .data section")
Fixes: 8dbec5c ("x86/kexec: Add data section to relocate_kernel")
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: nathanchance

arch/x86/kernel/relocate_kernel_64.S

@@ -23,11 +23,11 @@
 #define PAGE_ATTR (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | _PAGE_DIRTY)
 
 /*
- * The .text.relocate_kernel and .data.relocate_kernel sections are copied
+ * The .text..relocate_kernel and .data..relocate_kernel sections are copied
  * into the control page, and the remainder of the page is used as the stack.
  */
 
-	.section .data.relocate_kernel,"a";
+	.section .data..relocate_kernel,"a";
 /* Minimal CPU state */
 SYM_DATA_LOCAL(saved_rsp, .quad 0)
 SYM_DATA_LOCAL(saved_cr0, .quad 0)
@@ -39,7 +39,7 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
 
-	.section .text.relocate_kernel,"ax";
+	.section .text..relocate_kernel,"ax";
 	.code64
 SYM_CODE_START_NOALIGN(relocate_kernel)
 	UNWIND_HINT_END_OF_STACK

arch/x86/kernel/vmlinux.lds.S

@@ -100,8 +100,8 @@ const_pcpu_hot = pcpu_hot;
 #define KEXEC_RELOCATE_KERNEL					\
 	. = ALIGN(0x100);					\
 	__relocate_kernel_start = .;				\
-	*(.text.relocate_kernel);				\
-	*(.data.relocate_kernel);				\
+	*(.text..relocate_kernel);				\
+	*(.data..relocate_kernel);				\
 	__relocate_kernel_end = .;
 
 ASSERT(__relocate_kernel_end - __relocate_kernel_start <= KEXEC_CONTROL_CODE_MAX_SIZE,