objtool: Add action to check for absence of absolute relocations

The x86 startup code must not use absolute references to code or data,
as it executes before the kernel virtual mapping is up.

Add an action to objtool to check all allocatable sections (with the
exception of __patchable_function_entries, which uses absolute
references for nebulous reasons) and raise an error if any absolute
references are found.

Note that debug sections typically contain lots of absolute references
too, but those are not allocatable so they will be ignored.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/20250828102202.1849035-39-ardb+git@google.com

1 files changed, 12 insertions, 0 deletions

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 98c4713c1b09..0ad5cc70ecbe 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -880,3 +880,15 @@ unsigned int arch_reloc_size(struct reloc *reloc)
 		return 8;
 	}
 }
+
+bool arch_absolute_reloc(struct elf *elf, struct reloc *reloc)
+{
+	switch (reloc_type(reloc)) {
+	case R_X86_64_32:
+	case R_X86_64_32S:
+	case R_X86_64_64:
+		return true;
+	default:
+		return false;
+	}
+}