linux-kernel - [PATCH] x86/kexec: Fix location of relocate

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Message-Id: <20241213-kexec-fix-section-name-for-ffunction-sections-v1-1-1ae6050f6a15@kernel.org>
Date: Fri, 13 Dec 2024 18:54:53 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
 David Woodhouse <dwmw@...zon.co.uk>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] x86/kexec: Fix location of relocate_kernel with
 -ffunction-sections

After commit cb33ff9e063c ("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 79cd2a11224e ("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: cb33ff9e063c ("x86/kexec: Move relocate_kernel to kernel .data section")
Fixes: 8dbec5c77bc3 ("x86/kexec: Add data section to relocate_kernel")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 arch/x86/kernel/relocate_kernel_64.S | 6 +++---
 arch/x86/kernel/vmlinux.lds.S        | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 484e8a13bf98399107f00dfb057b908e330c1a2d..38c90a14fc1b4c7d423a4decd227c7584bdfcee2 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -24,11 +24,11 @@
 #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)
@@ -40,7 +40,7 @@ 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
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 0c893997f023627dd1f3187200d3609b234f98d1..63ff60a11be58132435e2ca8424ce31747d86c8c 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/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,

---
base-commit: 7e39d667b06c0849ab9cec1c96e2dc253a57310b
change-id: 20241213-kexec-fix-section-name-for-ffunction-sections-204b328d6db5

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>