Re: [PATCH] x86, relocs: move ELF relocation handling to C

From: Yinghai Lu
Date: Wed Jun 12 2013 - 14:13:23 EST

On Tue, Jun 11, 2013 at 11:58 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Moves the relocation handling into C, after decompression. This requires
> that the decompressed size is passed to the decompression routine as
> well so that relocations can be found. Only kernels that need relocation
> support will use the code.
>
> Based on work by Neill Clift and Michael Davidson.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> arch/x86/Kconfig | 4 +-
> arch/x86/Makefile | 8 ++--
> arch/x86/boot/compressed/head_32.S | 31 ++------------
> arch/x86/boot/compressed/head_64.S | 1 +
> arch/x86/boot/compressed/misc.c | 77 +++++++++++++++++++++++++++++++++-
> arch/x86/include/asm/page_32_types.h | 2 +
> arch/x86/include/asm/page_64_types.h | 5 ---
> arch/x86/include/asm/page_types.h | 6 +++
> 8 files changed, 94 insertions(+), 40 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 685692c..b4797b9 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1695,13 +1695,13 @@ config RELOCATABLE
> it has been loaded at and the compile time physical address
> (CONFIG_PHYSICAL_START) is ignored.
>
> -# Relocation on x86-32 needs some additional build support
> +# Relocation on x86 needs some additional build support
> config X86_NEED_RELOCS
> def_bool y
> depends on X86_32 && RELOCATABLE
>
> config PHYSICAL_ALIGN
> - hex "Alignment value to which kernel should be aligned" if X86_32
> + hex "Alignment value to which kernel should be aligned"
> default "0x1000000"
> range 0x2000 0x1000000
> ---help---
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 5c47726..43f8cef 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -16,6 +16,10 @@ endif
> # e.g.: obj-y += foo_$(BITS).o
> export BITS
>
> +ifdef CONFIG_X86_NEED_RELOCS
> + LDFLAGS_vmlinux := --emit-relocs
> +endif
> +
> ifeq ($(CONFIG_X86_32),y)
> BITS := 32
> UTS_MACHINE := i386
> @@ -25,10 +29,6 @@ ifeq ($(CONFIG_X86_32),y)
> KBUILD_AFLAGS += $(biarch)
> KBUILD_CFLAGS += $(biarch)
>
> - ifdef CONFIG_RELOCATABLE
> - LDFLAGS_vmlinux := --emit-relocs
> - endif
> -
> KBUILD_CFLAGS += -msoft-float -mregparm=3 -freg-struct-return
>
> # Never want PIC in a 32-bit kernel, prevent breakage with GCC built
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 1e3184f..5d6f689 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -181,8 +181,9 @@ relocated:
> /*
> * Do the decompression, and jump to the new kernel..
> */
> - leal z_extract_offset_negative(%ebx), %ebp
> /* push arguments for decompress_kernel: */
> + pushl $z_output_len /* decompressed length */
> + leal z_extract_offset_negative(%ebx), %ebp
> pushl %ebp /* output address */
> pushl $z_input_len /* input_len */
> leal input_data(%ebx), %eax
> @@ -191,33 +192,7 @@ relocated:
> pushl %eax /* heap area */
> pushl %esi /* real mode pointer */
> call decompress_kernel
> - addl $20, %esp
> -
> -#if CONFIG_RELOCATABLE
> -/*
> - * Find the address of the relocations.
> - */
> - leal z_output_len(%ebp), %edi
> -
> -/*
> - * Calculate the delta between where vmlinux was compiled to run
> - * and where it was actually loaded.
> - */
> - movl %ebp, %ebx
> - subl $LOAD_PHYSICAL_ADDR, %ebx
> - jz 2f /* Nothing to be done if loaded at compiled addr. */
> -/*
> - * Process relocations.
> - */
> -
> -1: subl $4, %edi
> - movl (%edi), %ecx
> - testl %ecx, %ecx
> - jz 2f
> - addl %ebx, -__PAGE_OFFSET(%ebx, %ecx)
> - jmp 1b
> -2:
> -#endif
> + addl $24, %esp
>
> /*
> * Jump to the decompressed kernel.
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 16f24e6..6632693 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -340,6 +340,7 @@ relocated:
> leaq input_data(%rip), %rdx /* input_data */
> movl $z_input_len, %ecx /* input_len */
> movq %rbp, %r8 /* output target address */
> + movq $z_output_len, %r9 /* decompressed length */
> call decompress_kernel
> popq %rsi
>
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 7cb56c6..b756a04 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -267,6 +267,79 @@ static void error(char *x)
> asm("hlt");
> }
>
> +#if CONFIG_X86_NEED_RELOCS
> +static void handle_relocations(void *output, unsigned long output_len)
> +{
> + int *reloc;
> + unsigned long delta, map, ptr;
> + unsigned long min_addr = (unsigned long)output;
> + unsigned long max_addr = min_addr + output_len;
> +
> + /*
> + * Calculate the delta between where vmlinux was linked to load
> + * and where it was actually loaded.
> + */
> + delta = min_addr - LOAD_PHYSICAL_ADDR;
> + if (!delta) {
> + debug_putstr("No relocation needed... ");
> + return;
> + }
> + debug_putstr("Performing relocations... ");
> +
> + /*
> + * The kernel contains a table of relocation addresses. Those
> + * addresses have the final load address of the kernel in virtual
> + * memory. We are currently working in the self map. So we need to
> + * create an adjustment for kernel memory addresses to the self map.
> + * This will involve subtracting out the base address of the kernel.
> + */
> + map = delta - __START_KERNEL_map;
> +
> + /*
> + * Process relocations: 32 bit relocations first then 64 bit after.
> + * Two sets of binary relocations are added to the end of the kernel
> + * before compression. Each relocation table entry is the kernel
> + * address of the location which needs to be updated stored as a
> + * 32-bit value which is sign extended to 64 bits.
> + *
> + * Format is:
> + *
> + * kernel bits...
> + * 0 - zero terminator for 64 bit relocations
> + * 64 bit relocation repeated
> + * 0 - zero terminator for 32 bit relocations
> + * 32 bit relocation repeated
> + *
> + * So we work backwards from the end of the decompressed image.
> + */
> + for (reloc = output + output_len - sizeof(*reloc); *reloc; reloc--) {
> + int extended = *reloc;
> + extended += map;
> +
> + ptr = (unsigned long)extended;
> + if (ptr < min_addr || ptr > max_addr)
> + error("32-bit relocation outside of kernel!\n");
> +
> + *(uint32_t *)ptr += delta;
> + }
> +#ifdef CONFIG_X86_64
> + for (reloc--; *reloc; reloc--) {
> + long extended = *reloc;
> + extended += map;
> +
> + ptr = (unsigned long)extended;
> + if (ptr < min_addr || ptr > max_addr)
> + error("64-bit relocation outside of kernel!\n");
> +
> + *(uint64_t *)ptr += delta;
> + }
> +#endif

it seems those lines for x86_64 are never used.
as X86_NEED_RELOCS is not defined for x86_64

> config X86_NEED_RELOCS
> def_bool y
> depends on X86_32 && RELOCATABLE


> +}
> +#else
> +static inline void handle_relocations(void *output, unsigned long output_len)
> +{ }
> +#endif

Can you make it clear that this patch only affect x86_32 in patch title?

Yinghai
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