[PATCH] arm64/efi: efistub: jump to 'stext' directly, not through the header

[Ard Biesheuvel]

On 15 July 2014 11:57, Mark Rutland <mark.rutland at arm.com> wrote:
> Hi Ard,
>
> On Tue, Jul 15, 2014 at 10:10:02AM +0100, Ard Biesheuvel wrote:
>> After the EFI stub has done its business, it jumps into the kernel by branching
>> to offset #0 of the loaded Image, which is where it expects to find the header
>> containing a 'branch to stext' instruction.
>> However, the header is not covered by any PE/COFF section, so the header may
>> not actually be loaded at the expected offset. So instead, jump to 'stext'
>> directly, which is at the base of the PE/COFF .text section.
>
> It would be nice to point out in the commit message that the other
> changes in the patch are just cleanup to use stext_offset rather than
> open-coding it.
>

OK

>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm64/kernel/efi-entry.S |  2 +-
>>  arch/arm64/kernel/head.S      | 10 ++++++----
>>  2 files changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
>> index 619b1dd7bcde..6ef541731d9e 100644
>> --- a/arch/arm64/kernel/efi-entry.S
>> +++ b/arch/arm64/kernel/efi-entry.S
>> @@ -61,7 +61,7 @@ ENTRY(efi_stub_entry)
>>        */
>>       mov     x20, x0         // DTB address
>>       ldr     x0, [sp, #16]   // relocated _text address
>> -     mov     x21, x0
>> +     add     x21, x0, #:lo12:stext_offset
>
> I think we can drop the :lo12: here, which will allow us to have a
> warning if stext_offset is unexpectedly large (I believe this will
> currently silently mask bits were that to happen?).
>

There is no alternative lo12 relocation that errors out when the value
does not fit, so it would have to use a literal instead.

> Other than that, this looks like a sensible thing to do given that we
> cannot rely on the header being present.
>

Cheers,
Ard.


>>
>>       /*
>>        * Flush dcache covering current runtime addresses
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index a2c1195abb7f..78ddae28b090 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -137,6 +137,8 @@ efi_head:
>>  #endif
>>
>>  #ifdef CONFIG_EFI
>> +     .globl  stext_offset
>> +     .set    stext_offset, stext - efi_head
>>       .align 3
>>  pe_header:
>>       .ascii  "PE"
>> @@ -160,7 +162,7 @@ optional_header:
>>       .long   0                               // SizeOfInitializedData
>>       .long   0                               // SizeOfUninitializedData
>>       .long   efi_stub_entry - efi_head       // AddressOfEntryPoint
>> -     .long   stext - efi_head                // BaseOfCode
>> +     .long   stext_offset                    // BaseOfCode
>>
>>  extra_header_fields:
>>       .quad   0                               // ImageBase
>> @@ -177,7 +179,7 @@ extra_header_fields:
>>       .long   _edata - efi_head               // SizeOfImage
>>
>>       // Everything before the kernel image is considered part of the header
>> -     .long   stext - efi_head                // SizeOfHeaders
>> +     .long   stext_offset                    // SizeOfHeaders
>>       .long   0                               // CheckSum
>>       .short  0xa                             // Subsystem (EFI application)
>>       .short  0                               // DllCharacteristics
>> @@ -222,9 +224,9 @@ section_table:
>>       .byte   0
>>       .byte   0                       // end of 0 padding of section name
>>       .long   _edata - stext          // VirtualSize
>> -     .long   stext - efi_head        // VirtualAddress
>> +     .long   stext_offset            // VirtualAddress
>>       .long   _edata - stext          // SizeOfRawData
>> -     .long   stext - efi_head        // PointerToRawData
>> +     .long   stext_offset            // PointerToRawData
>>
>>       .long   0               // PointerToRelocations (0 for executables)
>>       .long   0               // PointerToLineNumbers (0 for executables)
>> --
>> 1.8.3.2
>>
>>