[PATCH] arm64/efi: set PE/COFF section alignment to 4 KB

[Ard Biesheuvel]

On 10 October 2014 12:33, Mark Rutland <mark.rutland at arm.com> wrote:
> Hi Ard,
>
> On Fri, Oct 10, 2014 at 10:25:24AM +0100, Ard Biesheuvel wrote:
>> Position independent AArch64 code needs to be linked and loaded at the same
>> relative offset from a 4 KB boundary, or adrp/add and adrp/ldr pairs will
>> not work correctly. (This is how PC relative symbol references with a 4 GB
>> reach are emitted)
>>
>> We need to declare this in the PE/COFF header, otherwise the PE/COFF loader
>> may load the Image and invoke the stub at an offset which violates this rule.
>
> Has this been observed happening, or was this just found by inspection?
>

This is also something found by inspection, or rather, by the
discussion going on in the other thread. I am not aware of any PE/COFF
loaders that may choose an offset that is not 4 KB aligned, even if
the header we give it appears to allow it.

>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm64/kernel/head.S | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 0a6e4f924df8..5e83e5b8a9de 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -159,7 +159,7 @@ optional_header:
>>
>>  extra_header_fields:
>>       .quad   0                               // ImageBase
>> -     .long   0x20                            // SectionAlignment
>> +     .long   0x1000                          // SectionAlignment
>>       .long   0x8                             // FileAlignment
>>       .short  0                               // MajorOperatingSystemVersion
>>       .short  0                               // MinorOperatingSystemVersion
>> @@ -226,7 +226,7 @@ section_table:
>>       .short  0               // NumberOfRelocations  (0 for executables)
>>       .short  0               // NumberOfLineNumbers  (0 for executables)
>>       .long   0xe0500020      // Characteristics (section flags)
>> -     .align 5
>> +     .align 12
>
> Can we get a comment explaining why stext needs the additional
> alignment? Something like:
>
>         /*
>          * EFI will load stext onwards at the 4k section alignment
>          * described in the PE/COFF header. To ensure that instruction
>          * sequences using an adrp and a :lo12: immediate will function
>          * correctly at this alignment, we must ensure that stext is
>          * placed at a 4k boundary in the Image to begin with.
>          */
>         .align 12
>

OK

-- 
Ard.