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    Subject[PATCH-next v5 6/7] x86/boot: Remove run-time relocations from head_{32,64}.S
    Date
    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

    Reviewed-by: Kees Cook <keescook@chromium.org>
    Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
    Reviewed-by: Fangrui Song <maskray@google.com>
    Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
    ---
    arch/x86/boot/compressed/Makefile | 12 ++----------
    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, 18 insertions(+), 21 deletions(-)

    diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
    index b6d7caaaef9e..4a3953a596e0 100644
    --- a/arch/x86/boot/compressed/Makefile
    +++ b/arch/x86/boot/compressed/Makefile
    @@ -51,16 +51,8 @@ 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
    +LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
    +LDFLAGS_vmlinux += -T

    hostprogs := mkpiggy
    HOST_EXTRACFLAGS += -I$(srctree)/tools/include
    diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
    index 8c1a4f5610f5..659fad53ca82 100644
    --- a/arch/x86/boot/compressed/head_32.S
    +++ b/arch/x86/boot/compressed/head_32.S
    @@ -178,18 +178,17 @@ 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 11429092c224..9e46729cf162 100644
    --- a/arch/x86/boot/compressed/head_64.S
    +++ b/arch/x86/boot/compressed/head_64.S
    @@ -534,9 +534,9 @@ 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
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