> On Jul 8, 2015, at 10:49 AM, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote:
>
> On 8 July 2015 at 16:57, Olivier Martin <olivier.mar...@arm.com
> <mailto:olivier.mar...@arm.com>> wrote:
>> When EDK2 is built for the small memory model with AArch64 LLVM,
>> some page-relative relocations (R_AARCH64_ADR_PREL_PG_HI21 and its
>> R_AARCH64_LDST(16|32|64|128)_ABS_LO12_NC companions) that were not
>> supported before by EDK2 BaseTools are now needed.
>>
>> This change adds support for these relocations in BaseTools.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.0
>> Signed-off-by: Harry Liebel <harry.lie...@arm.com
>> <mailto:harry.lie...@arm.com>>
>> Reviewed-by: Olivier Martin <olivier.mar...@arm.com
>> <mailto:olivier.mar...@arm.com>>
>
> I think all the relocation arithmetic looks fine.
>
> I do have a concern about the consequences of using the small C model
> in this way: nothing in the metadata of the resulting PE/COFF images
> reflects that the 4 KB alignment needs to be preserved in order to
> successfully execute them, so you are essentially producing corrupt
> images.
Ard,
I’m missing something here. The PE/COFF header contains SectionAlignment and
FileAlignment fields in the optional (for COFF, required for PE/COFF) headers?
SectionAlignment is the alignment of sections when copied to memory.
FileAlignment is the alignment in the PE/COFF file.
Thanks,
Andrew Fish
> Every PE/COFF loader needs to be modified (including the ones
> in GRUB and shim, for instance) to prevent such images from being
> loaded in an incorrect way. (I know that most PE/COFF loaders use
> AllocatePages() which aligns these binaries at 4 KB implicitly, but
> this is not mandatory)
>
> So the correct way to do this would be to only use the small C model
> when this alignment can be guaranteed, i.e., .text and .data are both
> aligned at 4 KB. With the latest GenFw changes in place (which sets
> PE/COFF section alignment to the largest ELF input section alignment
> it encounters), the small C model works correctly since the PE/COFF
> loaders adhere to the PE/COFF section alignment. As a bonus, the ELF
> and PE/COFF images will be laid out identically in memory, so all the
> recalculation arithmetic becomes unnecessary (although it probably
> makes sense to perform some checks here)
>
> The downside, of course, is that 4 KB alignment for both .text and
> .data wastes a lot of space: 3.5 KB only for the padding between the
> PE/COFF header and the start of .text, and 2 KB on average between
> .text and .data. Due to the compression of non-XIP binaries, this is
> not such a penalty, but for XIP it is prohibitive (even if these
> typically don't have a .data section)
>
> I am going to have a look at GenFv to figure out if there is some way
> to work around this, i.e., drop the PE/COFF headers or move them in
> some way.
>
> In the mean time, I think this patch is good except the first hunk.
> The small model relocation arithmetic can be merged separately, I
> think, and the logic and policies around how and when to relocate
> PE/COFF images can be addressed in a followup patch.
>
> --
> Ard.
>
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