[RFC/RFT PATCH 17/19] x86/head64: Avoid absolute references in startup asm

Ard Biesheuvel
2026-01-08T09:28:43Z

Replace a couple of occurrences of absolute references with RIP-relative
ones. This removes the need for boot-time fixups. This is a prerequisite
for PIE linking, which only permits 64-bit wide loader-visible absolute
references.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/kernel/head_64.S | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 21816b48537c..2c666c8c4519 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -179,8 +179,9 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
xorl %r15d, %r15d

/* Derive the runtime physical address of init_top_pgt[] */
- movq phys_base(%rip), %rax
- addq $(init_top_pgt - __START_KERNEL_map), %rax
+ leaq init_top_pgt(%rip), %rax
+ subq $__START_KERNEL_map, %rax
+ addq phys_base(%rip), %rax

/*
* Retrieve the modifier (SME encryption mask if SME is active) to be
@@ -232,6 +233,9 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
btsl $X86_CR4_PGE_BIT, %ecx
movq %rcx, %cr4

+ /* Use .text as an anchor to emit PC-relative symbol references */
+ leaq .text(%rip), %rbx
+
#ifdef CONFIG_SMP
/*
* For parallel boot, the APIC ID is read from the APIC, and then
@@ -288,10 +292,9 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
.Llookup_AP:
/* EAX contains the APIC ID of the current CPU */
xorl %ecx, %ecx
- leaq cpuid_to_apicid(%rip), %rbx

.Lfind_cpunr:
- cmpl (%rbx,%rcx,4), %eax
+ cmpl cpuid_to_apicid - .text(%rbx,%rcx,4), %eax
jz .Lsetup_cpu
inc %ecx
#ifdef CONFIG_FORCE_NR_CPUS
@@ -311,7 +314,7 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)

.Lsetup_cpu:
/* Get the per cpu offset for the given CPU# which is in ECX */
- movq __per_cpu_offset(,%rcx,8), %rdx
+ movq __per_cpu_offset - .text(%rbx,%rcx,8), %rdx
#else
xorl %edx, %edx /* zero-extended to clear all of RDX */
#endif /* CONFIG_SMP */
@@ -322,7 +325,7 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
*
* RDX contains the per-cpu offset
*/
- movq current_task(%rdx), %rax
+ movq current_task - .text(%rbx,%rdx), %rax
movq TASK_threadsp(%rax), %rsp

/*
@@ -343,7 +346,7 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
*/
subq $16, %rsp
movw $(GDT_SIZE-1), (%rsp)
- leaq gdt_page(%rdx), %rax
+ leaq gdt_page - .text(%rbx,%rdx), %rax
movq %rax, 2(%rsp)
lgdt (%rsp)
addq $16, %rsp
--
2.47.3