[RFC PATCH] arm64/efi: efistub: reuse EFI mapping for Image if it is lower
Ard Biesheuvel
ard.biesheuvel at linaro.org
Wed Jul 16 13:48:38 PDT 2014
On 16 July 2014 16:10, Mark Salter <msalter at redhat.com> wrote:
> On Tue, 2014-07-15 at 18:50 +0200, Ard Biesheuvel wrote:
>> The EFI loader may load Image at any available offset. This means Image may
>> reside very close to or at the base of DRAM, in which case the relocation
>> done by efi_entry() results in Image being moved up in memory, which is
>> undesirable (memory below the kernel is currently not usable).
>>
>> To work around this, we check in the stub whether the relocated offset is higher
>> than the original offset. If this is the case, the original and relocated Images
>> switch roles, and we move the original Image inside its original lower mapping
>> while executing from the relocated Image.
>>
>> To ensure the original mapping has sufficient size, we add 2 megs + TEXT_OFFSET
>> of padding to the PE/COFF .text section, this should be sufficient to cover
>> rounding and adding TEXT_OFFSET.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>
>> This was tested on a Foundation Model using the patch below, which forces the
>> Image to be loaded at 0x80000000 (base of DRAM).
>>
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 2b7bd5eff776..eeadc073bad3 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -138,12 +138,12 @@ pe_header:
>> .short 0
>> coff_header:
>> .short 0xaa64 // AArch64
>> - .short 2 // nr_sections
>> + .short 1 // nr_sections
>> .long 0 // TimeDateStamp
>> .long 0 // PointerToSymbolTable
>> .long 1 // NumberOfSymbols
>> .short section_table - optional_header // SizeOfOptionalHeader
>> - .short 0x206 // Characteristics.
>> + .short 0x207 // Characteristics.
>> // IMAGE_FILE_DEBUG_STRIPPED |
>> // IMAGE_FILE_EXECUTABLE_IMAGE |
>> // IMAGE_FILE_LINE_NUMS_STRIPPED
>> @@ -158,7 +158,7 @@ optional_header:
>> .long stext_offset // BaseOfCode
>>
>> extra_header_fields:
>> - .quad 0 // ImageBase
>> + .quad 0x80000000 // ImageBase
>> .long 0x20 // SectionAlignment
>> .long 0x8 // FileAlignment
>> .short 0 // MajorOperatingSystemVersion
>> @@ -193,6 +193,7 @@ extra_header_fields:
>> // Section table
>> section_table:
>>
>> +#if 0
>> /*
>> * The EFI application loader requires a relocation section
>> * because EFI applications must be relocatable. This is a
>> @@ -212,6 +213,7 @@ section_table:
>> .long 0x42100040 // Characteristics (section flags)
>>
>>
>> +#endif
>> .ascii ".text"
>> .byte 0
>> .byte 0
>>
>>
>> arch/arm64/kernel/Makefile | 1 +
>> arch/arm64/kernel/efi-entry.S | 48 +++++++++++++++++++++++++++++++++----------
>> arch/arm64/kernel/head.S | 7 ++++---
>> 3 files changed, 42 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
>> index cdaedad3afe5..d55da3cd8a97 100644
>> --- a/arch/arm64/kernel/Makefile
>> +++ b/arch/arm64/kernel/Makefile
>> @@ -6,6 +6,7 @@ CPPFLAGS_vmlinux.lds := -DTEXT_OFFSET=$(TEXT_OFFSET)
>> AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
>> CFLAGS_efi-stub.o := -DTEXT_OFFSET=$(TEXT_OFFSET) \
>> -I$(src)/../../../scripts/dtc/libfdt
>> +AFLAGS_efi-entry.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
>>
>> CFLAGS_REMOVE_ftrace.o = -pg
>> CFLAGS_REMOVE_insn.o = -pg
>> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
>> index a0016d3a17da..d154f3c73937 100644
>> --- a/arch/arm64/kernel/efi-entry.S
>> +++ b/arch/arm64/kernel/efi-entry.S
>> @@ -11,6 +11,7 @@
>> */
>> #include <linux/linkage.h>
>> #include <linux/init.h>
>> +#include <linux/sizes.h>
>>
>> #include <asm/assembler.h>
>>
>> @@ -45,10 +46,13 @@ ENTRY(efi_stub_entry)
>> * efi_system_table_t *sys_table,
>> * unsigned long *image_addr) ;
>> */
>> - adrp x8, _text
>> - add x8, x8, #:lo12:_text
>> + adrp x22, _text
>> + add x22, x22, #:lo12:_text // x22: base of Image
>
> Why change x8 to x22? Seems unnecessary. Not only that, it is
> callee-saved, but you don't save it. This could cause a problem
> if boot fails and we try to return to firmware.
>
>> + adrp x24, _edata
>> + add x24, x24, #:lo12:_edata
>> + sub x24, x24, x22 // x24: size of Image
>
> Should use a caller-saved register instead of x24.
>
Yes, you are right, I moved it to x22 so I can reuse the value after
the call to efi_entry().
I will correct it when/if I do a v2, i.e., if there is any demand for this.
Thanks,
--
Ard.
>> add x2, sp, 16
>> - str x8, [x2]
>> + str x22, [x2]
>> bl efi_entry
>> cmn x0, #1
>> b.eq efi_load_fail
>> @@ -61,19 +65,41 @@ ENTRY(efi_stub_entry)
>> */
>> mov x20, x0 // DTB address
>> ldr x0, [sp, #16] // relocated _text address
>> - ldr x21, =stext_offset
>> - add x21, x0, x21
>> + ldr x23, =stext_offset
>> + add x21, x0, x23
>>
>> /*
>> + * Check whether the original allocation done by the EFI loader is more
>> + * favorable (i.e., closer to the base of DRAM) than the new one created
>> + * by efi_entry(). If so, reuse the original one.
>> + */
>> + adr x3, 0f
>> + cmp x3, x0
>> + bgt 1f // this image is loaded higher, so just proceed normally
>> +
>> + /*
>> + * Jump into the relocated image so we can move the original Image to
>> + * a 2 MB boundary + TEXT_OFFSET inside the original mapping without
>> + * overwriting ourselves.
>> + */
>> + sub x3, x3, x22 // subtract current base offset
>> + add x3, x3, x0 // add base offset of relocated image
>> + br x3 // jump to next line, but in relocated image
>> +
>> +0: mov x1, x0 // copy from relocated Image
>> + sub x0, x22, #1 // copy to base of original Image
>> + orr x0, x0, #(SZ_2M - 1) // .. rounded up to 2 MB
>> + ldr x3, =(TEXT_OFFSET + 1) // .. plus TEXT_OFFSET
>> + add x0, x0, x3
>> + mov x2, x24 // copy 'size of Image' bytes
>> + bl memcpy
>> + add x21, x0, x23 // add stext_offset to entry point
>> +1:
>> + /*
>> * Flush dcache covering current runtime addresses
>> * of kernel text/data. Then flush all of icache.
>> */
>> - adrp x1, _text
>> - add x1, x1, #:lo12:_text
>> - adrp x2, _edata
>> - add x2, x2, #:lo12:_edata
>> - sub x1, x2, x1
>> -
>> + mov x1, x24 // size of Image
>> bl __flush_dcache_area
>> ic ialluis
>>
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 5cd1f3491df5..2b7bd5eff776 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -129,6 +129,7 @@ efi_head:
>> #endif
>>
>> #ifdef CONFIG_EFI
>> +#define PE_TEXT_PAD SZ_2M + TEXT_OFFSET
>> .globl stext_offset
>> .set stext_offset, stext - efi_head
>> .align 3
>> @@ -150,7 +151,7 @@ optional_header:
>> .short 0x20b // PE32+ format
>> .byte 0x02 // MajorLinkerVersion
>> .byte 0x14 // MinorLinkerVersion
>> - .long _edata - stext // SizeOfCode
>> + .long _edata - stext + PE_TEXT_PAD // SizeOfCode
>> .long 0 // SizeOfInitializedData
>> .long 0 // SizeOfUninitializedData
>> .long efi_stub_entry - efi_head // AddressOfEntryPoint
>> @@ -168,7 +169,7 @@ extra_header_fields:
>> .short 0 // MinorSubsystemVersion
>> .long 0 // Win32VersionValue
>>
>> - .long _edata - efi_head // SizeOfImage
>> + .long _edata - efi_head + PE_TEXT_PAD // SizeOfImage
>>
>> // Everything before the kernel image is considered part of the header
>> .long stext_offset // SizeOfHeaders
>> @@ -215,7 +216,7 @@ section_table:
>> .byte 0
>> .byte 0
>> .byte 0 // end of 0 padding of section name
>> - .long _edata - stext // VirtualSize
>> + .long _edata - stext + PE_TEXT_PAD // VirtualSize
>> .long stext_offset // VirtualAddress
>> .long _edata - stext // SizeOfRawData
>> .long stext_offset // PointerToRawData
>
>
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