[lld] r301287 - Add comments about Widnows .reloc section.

[Rui Ueyama via llvm-commits]

Thank you for pointing it out. I've forgotten that each entry has a type in
its high bits, though the type doesn't make sense as they are always the
same. I'll fix the comment.

On Wed, Apr 26, 2017 at 12:33 PM, Peter Collingbourne <peter at pcc.me.uk>
wrote:

> Thanks for writing this up.
>
> The description does not seem to match the code, though. It looks like
> relocations are grouped into pages of 4096 bytes, not 65536 bytes.
>
> Peter
>
> On Mon, Apr 24, 2017 at 8:31 PM, Rui Ueyama via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: ruiu
>> Date: Mon Apr 24 22:31:10 2017
>> New Revision: 301287
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=301287&view=rev
>> Log:
>> Add comments about Widnows .reloc section.
>>
>> Modified:
>>     lld/trunk/COFF/Chunks.cpp
>>
>> Modified: lld/trunk/COFF/Chunks.cpp
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Chunks.cp
>> p?rev=301287&r1=301286&r2=301287&view=diff
>> ============================================================
>> ==================
>> --- lld/trunk/COFF/Chunks.cpp (original)
>> +++ lld/trunk/COFF/Chunks.cpp Mon Apr 24 22:31:10 2017
>> @@ -319,8 +319,48 @@ void SEHTableChunk::writeTo(uint8_t *Buf
>>    std::sort(Begin, Begin + Cnt);
>>  }
>>
>> -// Windows-specific.
>> -// This class represents a block in .reloc section.
>> +// Windows-specific. This class represents a block in .reloc section.
>> +// The format is described here.
>> +//
>> +// On Windows, each DLL is linked against a fixed base address and
>> +// usually loaded to that address. However, if there's already another
>> +// DLL that overlaps, the loader has to relocate it. To do that, DLLs
>> +// contain .reloc sections which contain offsets that need to be fixed
>> +// up at runtime. If the loader find that a DLL cannot be loaded to its
>> +// desired base address, it loads it to somewhere else, and add <actual
>> +// base address> - <desired base address> to each offset that is
>> +// specified by .reloc section.
>> +//
>> +// In ELF terms, .reloc sections contain arrays of relocation offsets.
>> +// All these offsets in the section are implicitly R_*_RELATIVE, and
>> +// addends are read from section contents (so it is REL as opposed to
>> +// RELA).
>> +//
>> +// This already reduce the size of relocations to 1/3 compared to ELF
>> +// .dynrel, but Windows does more to reduce it (probably because it was
>> +// invented for PCs in the late '80s or early '90s.) Offsets in .reloc
>> +// are grouped by page where page size is 16 bits, and offsets sharing
>> +// the same page address are stored consecutively to represent them with
>> +// less space. This is a very similar to the page table which is grouped
>> +// by (multiple stages of) pages.
>> +//
>> +// For example, let's say we have 0x00030, 0x00500, 0x01000, 0x01100,
>> +// 0x20004, and 0x20008 in a .reloc section. In the section, they are
>> +// represented like this:
>> +//
>> +//   0x00000  -- page address (4 bytes)
>> +//   16       -- size of this block (4 bytes)
>> +//     0x0030 -- entries (2 bytes each)
>> +//     0x0500
>> +//     0x1000
>> +//     0x1100
>> +//   0x20000  -- page address (4 bytes)
>> +//   12       -- size of this block (4 bytes)
>> +//     0x0004 -- entries (2 bytes each)
>> +//     0x0008
>> +//
>> +// Usually we have a lot of relocatinos for each page, so the number of
>> +// bytes for one .reloc entry is close to 2 bytes.
>>  BaserelChunk::BaserelChunk(uint32_t Page, Baserel *Begin, Baserel *End)
>> {
>>    // Block header consists of 4 byte page RVA and 4 byte block size.
>>    // Each entry is 2 byte. Last entry may be padding.
>>
>>
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>>
>
>
>
> --
> --
> Peter
>
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