[llvm-dev] lld extra program headers

[Rafael Avila de Espindola via llvm-dev]

Note too that that is just a different default. You can pass
--no-rosegment to lld if you want.

Cheers,
Rafael

Rui Ueyama via llvm-dev <llvm-dev at lists.llvm.org> writes:

> Hi Will,
>
> ld.gold and ld.bfd seem handle PHDR and INTERP as executable segments.
> That's why they use only one page for PHDR, INTERP and the first executable
> page.
>
> lld on the other hand handles them as non-executable segments, as they are
> not really executable code. I think what lld does is more desirable.
>
> On Thu, Jun 22, 2017 at 2:01 PM, Will Song via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello all,
>>
>> The lld linker appears to generate extra ELF program headers and thus
>> causes file sizes to be significantly larger than what would get with
>> ld.gold or ld.bfd.
>>
>> # readelf -l test.bfd
>>
>> Elf file type is EXEC (Executable file)
>> Entry point 0x4000b0
>> There are 2 program headers, starting at offset 64
>>
>> Program Headers:
>>   Type           Offset             VirtAddr           PhysAddr
>>                  FileSiz            MemSiz              Flags  Align
>>   LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
>>                  0x00000000000000c0 0x00000000000000c0  R E    0x200000
>>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>>                  0x0000000000000000 0x0000000000000000  RW     0x10
>>
>>  Section to Segment mapping:
>>   Segment Sections...
>>    00     .text
>>    01
>> # readelf -l test.gold
>>
>> Elf file type is EXEC (Executable file)
>> Entry point 0x4000b0
>> There are 2 program headers, starting at offset 64
>>
>> Program Headers:
>>   Type           Offset             VirtAddr           PhysAddr
>>                  FileSiz            MemSiz              Flags  Align
>>   LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
>>                  0x00000000000000c0 0x00000000000000c0  R E    0x1000
>>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>>                  0x0000000000000000 0x0000000000000000  RW     0x10
>>
>>  Section to Segment mapping:
>>   Segment Sections...
>>    00     .text
>>    01
>> # readelf -l test.lld
>>
>> Elf file type is EXEC (Executable file)
>> Entry point 0x201000
>> There are 4 program headers, starting at offset 64
>>
>> Program Headers:
>>   Type           Offset             VirtAddr           PhysAddr
>>                  FileSiz            MemSiz              Flags  Align
>>   PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
>>                  0x00000000000000e0 0x00000000000000e0  R      0x8
>>   LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
>>                  0x0000000000000120 0x0000000000000120  R      0x1000
>>   LOAD           0x0000000000001000 0x0000000000201000 0x0000000000201000
>>                  0x0000000000000010 0x0000000000000010  R E    0x1000
>>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>>                  0x0000000000000000 0x0000000000000000  RW     0x0
>>
>>  Section to Segment mapping:
>>   Segment Sections...
>>    00
>>    01
>>    02     .text
>>    03
>>
>> Now obviously disk space is relatively cheap these days but if it could
>> be avoided, can we avoid generating these unused memory mappings unless
>> a PT_INTERP section is going to be generated? I could also be
>> misunderstanding things, and it might be normal to map the program's
>> program headers into memory (and thus the entire ELF header).
>>
>> Thanks.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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