x86/boot: Remove run-time relocations from head_{32,64}.S

The BFD linker generates run-time relocations for z_input_len and
z_output_len, even though they are absolute symbols.

This is fixed for binutils-2.35 [1]. Work around this for earlier
versions by defining two variables input_len and output_len in addition
to the symbols, and use them via position-independent references.

This eliminates the last two run-time relocations in the head code and
allows us to drop the -z noreloc-overflow flag to the linker.

Move the -pie and --no-dynamic-linker LDFLAGS to LDFLAGS_vmlinux instead
of KBUILD_LDFLAGS. There shouldn't be anything else getting linked, but
this is the more logical location for these flags, and modversions might
call the linker if an EXPORT_SYMBOL is left over accidentally in one of
the decompressors.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754

Signed-off-by: Arvind Sankar <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Tested-by: Nick Desaulniers <[e-mail]>
Tested-by: Sedat Dilek <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Fangrui Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/boot/compressed/Makefile

···
 KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
 # Compressed kernel should be built as PIE since it may be loaded at any
 # address by the bootloader.
-ifeq ($(CONFIG_X86_32),y)
-KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
-else
-# To build 64-bit compressed kernel as PIE, we disable relocation
-# overflow check to avoid relocation overflow error with a new linker
-# command-line option, -z noreloc-overflow.
-KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
-	&& echo "-z noreloc-overflow -pie --no-dynamic-linker")
-endif
-LDFLAGS_vmlinux := -T
+LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
+LDFLAGS_vmlinux += -T
 
 hostprogs	:= mkpiggy
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include

arch/x86/boot/compressed/head_32.S

···
 /*
  * Do the extraction, and jump to the new kernel..
  */
-				/* push arguments for extract_kernel: */
-	pushl	$z_output_len	/* decompressed length, end of relocs */
+	/* push arguments for extract_kernel: */
 
-	pushl	%ebp		/* output address */
-
-	pushl	$z_input_len	/* input_len */
+	pushl	output_len@GOTOFF(%ebx)	/* decompressed length, end of relocs */
+	pushl	%ebp			/* output address */
+	pushl	input_len@GOTOFF(%ebx)	/* input_len */
 	leal	input_data@GOTOFF(%ebx), %eax
-	pushl	%eax		/* input_data */
+	pushl	%eax			/* input_data */
 	leal	boot_heap@GOTOFF(%ebx), %eax
-	pushl	%eax		/* heap area */
-	pushl	%esi		/* real mode pointer */
-	call	extract_kernel	/* returns kernel location in %eax */
+	pushl	%eax			/* heap area */
+	pushl	%esi			/* real mode pointer */
+	call	extract_kernel		/* returns kernel location in %eax */
 	addl	$24, %esp
 
 /*

arch/x86/boot/compressed/head_64.S

···
 	movq	%rsi, %rdi		/* real mode address */
 	leaq	boot_heap(%rip), %rsi	/* malloc area for uncompression */
 	leaq	input_data(%rip), %rdx  /* input_data */
-	movl	$z_input_len, %ecx	/* input_len */
+	movl	input_len(%rip), %ecx	/* input_len */
 	movq	%rbp, %r8		/* output target address */
-	movl	$z_output_len, %r9d	/* decompressed length, end of relocs */
+	movl	output_len(%rip), %r9d	/* decompressed length, end of relocs */
 	call	extract_kernel		/* returns kernel location in %rax */
 	popq	%rsi
 

arch/x86/boot/compressed/mkpiggy.c

···
 	printf(".incbin \"%s\"\n", argv[1]);
 	printf("input_data_end:\n");
 
+	printf(".section \".rodata\",\"a\",@progbits\n");
+	printf(".globl input_len\n");
+	printf("input_len:\n\t.long %lu\n", ilen);
+	printf(".globl output_len\n");
+	printf("output_len:\n\t.long %lu\n", (unsigned long)olen);
+
 	retval = 0;
 bail:
 	if (f)