[llvm-dev] lld extra program headers

[Rui Ueyama via llvm-dev]

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.
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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