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

[Jan Beulich]

On 04.06.2024 19:02, Nick Clifton wrote:
> 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).

While technically this looks okay, isn't this subverting the purpose of
-z separate-code then, reading that one's documentation? You apparently
retain the "wholly disjoint pages" aspect as per ...

>   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

... this, yet with the segment being readable and executable it's
not clear to me whether that then actually gains us anything.

Jan