RFC: PR 30907: BFD linker option to allow read-only data in code segments

[Nick Clifton]

Hi Guys,

  Attached is an experimental patch to add new option the bfd linker:
  -z rodata-in-code.

  If used the option allows read only data to be placed into a code
  segment.  This is only effective if the -z separate-code option is in
  effect.  When used it has the ability to reduce the number of loadable
  segments from 4 to 3, which can have a big effect on the overall size
  of an executable.  (See PR 30907 for more discussion on this).

  Here is an example:

  $ cat test.c
      extern int printf (const char *, ...);
      int i = 42;
      const int * j = & i;
      int main (void) { return printf ("hello world %d\n", * j); }

  $ gcc test.c -Wl,-z,separate-code
  $ readelf -Wl a.out
      [...]
      LOAD     0x000000 0x0000000000400000 0x0000000000400000 0x000510 0x000510 R   0x1000
      LOAD     0x001000 0x0000000000401000 0x0000000000401000 0x000155 0x000155 R E 0x1000
      LOAD     0x002000 0x0000000000402000 0x0000000000402000 0x0000dc 0x0000dc R   0x1000
      LOAD     0x002df8 0x0000000000403df8 0x0000000000403df8 0x000228 0x000230 RW  0x1000
      [...]
      Section to Segment mapping:
      [...]
      02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 
      03     .init .plt .text .fini 
      04     .rodata .eh_frame_hdr .eh_frame 
      05     .init_array .fini_array .dynamic .got .got.plt .data .bss 

  $ gcc test.c -Wl,-z,separate-code -Wl,-z,rodata-in-code
  $ readelf -Wl a.out
      [...]
      LOAD           0x000000 0x0000000000400000 0x0000000000400000 0x0004d8 0x0004d8 R   0x1000
      LOAD           0x001000 0x0000000000401000 0x0000000000401000 0x0010dc 0x0010dc R E 0x1000
      LOAD           0x002df8 0x0000000000403df8 0x0000000000403df8 0x000228 0x000230 RW  0x1000
      [...]
      Section to Segment mapping:
      [...]
      02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 
      03     .init .plt .text .fini .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 documentation or test
  cases yet.  But I wanted to see if the general idea was going to cause
  concern for people, and to see if I had missed anything obvious.

  Thoughts, suggestions ?

Cheers
  Nick

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