RFC: Three load segments patch (PR 30907) [version 2]

[Nick Clifton]

Hi Guys,

  I have been looking into the file size increase when we link binaries
  with '-z separate-code' enabled.  The problem is that we end up
  creating four load segments with page aligning happening between them.
  For example linking a small hello-world type program gives:

  with: -z noseparate-code  (file size: 8544 bytes)

          File Offset  Virt Addr  FileSize Flags  Align
    LOAD      0         400000      6d4     R E    0x1000 
    LOAD    df8         401df8      228     RW     0x1000

  with: -z separate-code  (file size: 16736 bytes)

          File Offset  Virt Addr  FileSize Flags  Align
    LOAD      0         400000      510     R      0x1000
    LOAD   1000         401000      155     R E    0x1000
    LOAD   2000         402000       dc     R      0x1000
    LOAD   2df8         403df8      228     RW     0x1000

  The increase in size is unfortunate, and a problem for distributions,
  where larger binary sizes equates to larger install images and more
  download bandwidth, and the problem is even more accute for containers.

  So attached is a proposed patch to add an option that combines the two
  read-only segments into one.  It moves the first code segment to the
  start of the file, followed by a read only segment and then a read
  write segment.  Thus we end up with a layout like this:

  with: -z separate-code -z one-rosegment  (file size: 8544 bytes)

          File Offset  Virt Addr  FileSize Flags  Align
    LOAD      0         400000      435     R E    0x1000
    LOAD    435         401435      2d7     R      0x1000
    LOAD    df8         402df8      228     RW     0x1000

  The section mapping looks like this:

     02     .init .plt .text .fini 
     03     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .rodata .eh_frame_hdr .eh_frame 
     04     .init_array .fini_array .dynamic .got .got.plt .data .bss 

  The patch is incomplete.  It does not have any test cases or
  full documentation at the moment, but I wanted to see if I am on the
  right track this time, or if I need to go back to the drawing board
  again. :-)

Cheers
  Nick

PS. In addition to adding the new command line option the patch also
updates the ld/ld.int documentation file, adding descriptions of linker
scripts that were missing before.  It also reorganises the
ld/scripttempl/elf.sc linker script template, making it easier to
understand how the sections are laid out.  (At least that is my
opinion). 

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