[RFC/RFT PATCH 04/19] x86: Make the 64-bit bzImage always physically relocatable

Ard Biesheuvel
2026-01-08T09:28:07Z

On x86_64, the physical placement of the kernel is independent from its
mapping in the 'High Kernel Mapping' range. This means that even a
position dependent kernel built without boot-time relocation support can
run from any suitably aligned physical address, and there is no need to
make this behavior dependent on whether or not the kernel is virtually
relocatable.

On i386, the situation is different, given that the physical and virtual
load offsets must be equal, and so only a relocatable kernel can be
loaded at a physical address that deviates from its build-time default.

Clarify this in Kconfig and in the code, and advertise the 64-bit
bzImage as loadable at any physical offset regardless of whether
CONFIG_RELOCATABLE is set. In practice, this makes little difference,
given that it defaults to 'y' and is a prerequisite for EFI_STUB and
RANDOMIZE_BASE, but it will help with some future refactoring of the
relocation code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/Kconfig | 40 ++++++++++++--------
arch/x86/boot/compressed/head_64.S | 4 --
arch/x86/boot/compressed/misc.c | 8 ++--
arch/x86/boot/header.S | 8 +---
4 files changed, 29 insertions(+), 31 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 80527299f859..bf51e17d5813 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1931,7 +1931,7 @@ config EFI
config EFI_STUB
bool "EFI stub support"
depends on EFI
- select RELOCATABLE
+ select RELOCATABLE if X86_32
help
This kernel feature allows a bzImage to be loaded directly
by EFI firmware without the use of a bootloader.
@@ -2028,8 +2028,9 @@ config PHYSICAL_START
help
This gives the physical address where the kernel is loaded.

- If the kernel is not relocatable (CONFIG_RELOCATABLE=n) then bzImage
- will decompress itself to above physical address and run from there.
+ If the kernel is not relocatable (CONFIG_RELOCATABLE=n) and built for
+ i386, then the bzImage will decompress itself to the above physical
+ address and run from there.
Otherwise, bzImage will run from the address where it has been loaded
by the boot loader. The only exception is if it is loaded below the
above physical address, in which case it will relocate itself there.
@@ -2064,16 +2065,22 @@ config PHYSICAL_START
Don't change this unless you know what you are doing.

config RELOCATABLE
- bool "Build a relocatable kernel"
- default y
+ bool "Build a relocatable kernel" if X86_32
+ default X86_32
help
- This builds a kernel image that retains relocation information
- so it can be loaded someplace besides the default 1MB.
+ This builds a kernel image that retains relocation information so it
+ can be placed someplace besides the default PAGE_OFFSET + 1MB. This
+ is a prerequisite for KASLR.
The relocations tend to make the kernel binary about 10% larger,
but are discarded at runtime.

- One use is for the kexec on panic case where the recovery kernel
- must live at a different physical address than the primary
+ On i386, where the virtual and physical load offset of the kernel
+ must be equal, this also allows the kernel image to be placed at a
+ physical load address that differs from the compile time default. On
+ x86_64, this is always permitted.
+
+ One use is for the kexec on panic case on i386, where the recovery
+ kernel must live at a different physical address than the primary
kernel.

Note: If CONFIG_RELOCATABLE=y, then the kernel runs from the address
@@ -2082,7 +2089,7 @@ config RELOCATABLE

config RANDOMIZE_BASE
bool "Randomize the address of the kernel image (KASLR)"
- depends on RELOCATABLE
+ select RELOCATABLE
default y
help
In support of Kernel Address Space Layout Randomization (KASLR),
@@ -2118,7 +2125,7 @@ config RANDOMIZE_BASE
# Relocation on x86 needs some additional build support
config X86_NEED_RELOCS
def_bool y
- depends on RANDOMIZE_BASE || (X86_32 && RELOCATABLE)
+ depends on RELOCATABLE
select ARCH_VMLINUX_NEEDS_RELOCS

config PHYSICAL_ALIGN
@@ -2131,12 +2138,13 @@ config PHYSICAL_ALIGN
where kernel is loaded and run from. Kernel is compiled for an
address which meets above alignment restriction.

- If bootloader loads the kernel at a non-aligned address and
- CONFIG_RELOCATABLE is set, kernel will move itself to nearest
- address aligned to above value and run from there.
+ If the bootloader loads the kernel at a non-aligned address and it
+ is built for x86_64 or CONFIG_RELOCATABLE is set, the kernel will
+ move itself to the nearest address aligned to above value and run
+ from there.

- If bootloader loads the kernel at a non-aligned address and
- CONFIG_RELOCATABLE is not set, kernel will ignore the run time
+ If the bootloader loads the i386 kernel at a non-aligned address and
+ CONFIG_RELOCATABLE is not set, the kernel will ignore the run time
load address and decompress itself to the address it has been
compiled for and run from there. The address for which kernel is
compiled already meets above alignment restrictions. Hence the
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index d9dab940ff62..8a964a4d45c2 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -143,7 +143,6 @@ SYM_FUNC_START(startup_32)
* for safe in-place decompression.
*/

-#ifdef CONFIG_RELOCATABLE
movl %ebp, %ebx
movl BP_kernel_alignment(%esi), %eax
decl %eax
@@ -152,7 +151,6 @@ SYM_FUNC_START(startup_32)
andl %eax, %ebx
cmpl $LOAD_PHYSICAL_ADDR, %ebx
jae 1f
-#endif
movl $LOAD_PHYSICAL_ADDR, %ebx
1:

@@ -312,7 +310,6 @@ SYM_CODE_START(startup_64)
*/

/* Start with the delta to where the kernel will run at. */
-#ifdef CONFIG_RELOCATABLE
leaq startup_32(%rip) /* - $startup_32 */, %rbp
movl BP_kernel_alignment(%rsi), %eax
decl %eax
@@ -321,7 +318,6 @@ SYM_CODE_START(startup_64)
andq %rax, %rbp
cmpq $LOAD_PHYSICAL_ADDR, %rbp
jae 1f
-#endif
movq $LOAD_PHYSICAL_ADDR, %rbp
1:

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 0f41ca0e52c0..d37569e7ee10 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -314,12 +314,10 @@ static size_t parse_elf(void *output)
if ((phdr->p_align % 0x200000) != 0)
error("Alignment of LOAD segment isn't multiple of 2MB");
#endif
-#ifdef CONFIG_RELOCATABLE
- dest = output;
- dest += (phdr->p_paddr - LOAD_PHYSICAL_ADDR);
-#else
dest = (void *)(phdr->p_paddr);
-#endif
+ if (IS_ENABLED(CONFIG_X86_64) ||
+ IS_ENABLED(CONFIG_RELOCATABLE))
+ dest += (unsigned long)output - LOAD_PHYSICAL_ADDR;
memmove(dest, output + phdr->p_offset, phdr->p_filesz);
break;
default: /* Ignore other PT_* */ break;
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 9bea5a1e2c52..b72e6055e103 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -332,7 +332,7 @@ initrd_addr_max: .long 0x7fffffff
kernel_alignment: .long CONFIG_PHYSICAL_ALIGN #physical addr alignment
#required for protected mode
#kernel
-#ifdef CONFIG_RELOCATABLE
+#if defined(CONFIG_RELOCATABLE) || defined(CONFIG_X86_64)
relocatable_kernel: .byte 1
#else
relocatable_kernel: .byte 0
@@ -342,14 +342,10 @@ min_alignment: .byte MIN_KERNEL_ALIGN_LG2 # minimum alignment
xloadflags:
#ifdef CONFIG_X86_64
# define XLF0 XLF_KERNEL_64 /* 64-bit kernel */
-#else
-# define XLF0 0
-#endif
-
-#if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64)
/* kernel/boot_param/ramdisk could be loaded above 4g */
# define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
#else
+# define XLF0 0
# define XLF1 0
#endif

--
2.47.3