arm64/build: Assert for unwanted sections

In preparation for warning on orphan sections, discard
unwanted non-zero-sized generated sections, and enforce other
expected-to-be-zero-sized sections (since discarding them might hide
problems with them suddenly gaining unexpected entries).
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200821194310.3089815-14-keescook@chromium.org

arch/arm64/kernel/vmlinux.lds.S

@@ -121,6 +121,14 @@ SECTIONS
 		*(.got)			/* Global offset table		*/
 	}
 
+	/*
+	 * Make sure that the .got.plt is either completely empty or it
+	 * contains only the lazy dispatch entries.
+	 */
+	.got.plt : { *(.got.plt) }
+	ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
+	       "Unexpected GOT/PLT entries detected!")
+
 	. = ALIGN(SEGMENT_ALIGN);
 	_etext = .;			/* End of text section */
 
@@ -243,6 +251,18 @@ SECTIONS
 	ELF_DETAILS
 
 	HEAD_SYMBOLS
+
+	/*
+	 * Sections that should stay zero sized, which is safer to
+	 * explicitly check instead of blindly discarding.
+	 */
+	.plt : {
+		*(.plt) *(.plt.*) *(.iplt) *(.igot)
+	}
+	ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure linkages detected!")
+
+	.data.rel.ro : { *(.data.rel.ro) }
+	ASSERT(SIZEOF(.data.rel.ro) == 0, "Unexpected RELRO detected!")
 }
 
 #include "image-vars.h"