[PATCH 3/4] x86/boot: Remove runtime relocations from head_{32,64}.S

From: Arvind Sankar
Date: Sun May 24 2020 - 17:28:26 EST


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

Work around this 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 runtime relocations in the head code and
allows us to drop the -z noreloc-overflow flag to the linker.

Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
---
arch/x86/boot/compressed/Makefile | 8 --------
arch/x86/boot/compressed/head_32.S | 17 ++++++++---------
arch/x86/boot/compressed/head_64.S | 4 ++--
arch/x86/boot/compressed/mkpiggy.c | 6 ++++++
4 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index aa9ed814e5fa..d3e882e855ee 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -49,15 +49,7 @@ UBSAN_SANITIZE :=n
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

hostprogs := mkpiggy
diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 66657bb99aae..064e895bad92 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -193,18 +193,17 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
/*
* 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

/*
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f6ba32cd5702..6e4704b6a94e 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -508,9 +508,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
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

diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
index 7e01248765b2..52aa56cdbacc 100644
--- a/arch/x86/boot/compressed/mkpiggy.c
+++ b/arch/x86/boot/compressed/mkpiggy.c
@@ -60,6 +60,12 @@ int main(int argc, char *argv[])
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)
--
2.26.2