[PATCH] elf: Remove the 1-page gap before the RELRO segment

[Fangrui Song]

On 2022-01-10, H.J. Lu via Binutils wrote:
>The existing RELRO scheme may leave a 1-page gap before the RELRO segment
>and align the end of the RELRO segment to the page size:
>
>  [18] .eh_frame    PROGBITS    408fa0 008fa0 005e80 00   A  0   0  8
>  [19] .init_array  INIT_ARRAY  410de0 00fde0 000008 08  WA  0   0  8
>  [20] .fini_array  FINI_ARRAY  410de8 00fde8 000008 08  WA  0   0  8
>  [21] .dynamic     DYNAMIC     410df0 00fdf0 000200 10  WA  7   0  8
>  [22] .got         PROGBITS    410ff0 00fff0 000010 08  WA  0   0  8
>  [23] .got.plt     PROGBITS    411000 010000 000048 08  WA  0   0  8
>
>Instead, we can remove the 1-page gap if the maximum page size >= the
>maximum section alignment:
>
>  [18] .eh_frame    PROGBITS    408fa0 008fa0 005e80 00   A  0   0  8
>  [19] .init_array  INIT_ARRAY  40fde0 00fde0 000008 08  WA  0   0  8
>  [20] .fini_array  FINI_ARRAY  40fde8 00fde8 000008 08  WA  0   0  8
>  [21] .dynamic     DYNAMIC     40fdf0 00fdf0 000200 10  WA  7   0  8
>  [22] .got         PROGBITS    40fff0 00fff0 000010 08  WA  0   0  8
>  [23] .got.plt     PROGBITS    410000 010000 000048 08  WA  0   0  8
>
>Because the end of the RELRO segment is always aligned to the page size
>and may not be moved, the RELRO segment size may be increased:
>
>  [ 3] .dynstr      STRTAB      000148 000148 000001 00   A  0   0  1
>  [ 4] .eh_frame    PROGBITS    000150 000150 000000 00   A  0   0  8
>  [ 5] .init_array  INIT_ARRAY  200150 000150 000010 08  WA  0   0  1
>  [ 6] .fini_array  FINI_ARRAY  200160 000160 000010 08  WA  0   0  1
>  [ 7] .jcr         PROGBITS    200170 000170 000008 00  WA  0   0  1
>  [ 8] .data.rel.ro PROGBITS    200180 000180 000020 00  WA  0   0 16
>  [ 9] .dynamic     DYNAMIC     2001a0 0001a0 0001c0 10  WA  3   0  8
>  [10] .got         PROGBITS    200360 000360 0002a8 00  WA  0   0  8
>  [11] .bss         NOBITS      201000 000608 000840 00  WA  0   0  1
>
>vs the old section layout:
>
>  [ 3] .dynstr      STRTAB      000148 000148 000001 00   A  0   0  1
>  [ 4] .eh_frame    PROGBITS    000150 000150 000000 00   A  0   0  8
>  [ 5] .init_array  INIT_ARRAY  200b48 000b48 000010 08  WA  0   0  1
>  [ 6] .fini_array  FINI_ARRAY  200b58 000b58 000010 08  WA  0   0  1
>  [ 7] .jcr         PROGBITS    200b68 000b68 000008 00  WA  0   0  1
>  [ 8] .data.rel.ro PROGBITS    200b70 000b70 000020 00  WA  0   0 16
>  [ 9] .dynamic     DYNAMIC     200b90 000b90 0001c0 10  WA  3   0  8
>  [10] .got         PROGBITS    200d50 000d50 0002a8 00  WA  0   0  8
>  [11] .bss         NOBITS      201000 000ff8 000840 00  WA  0   0  1
>
>But there is no 1-page gap.
> [...}

If you want to avoid a max-page-size alignment (at the end of PT_GNU_RELRO), you may adopt ld.lld's design
I implemented:

   LOAD           0x000000 0x0000000000200000 0x0000000000200000 0xcaac4c 0xcaac4c R   0x1000
   LOAD           0xcaac50 0x0000000000eabc50 0x0000000000eabc50 0x208a0c0 0x208a0c0 R E 0x1000
   LOAD           0x2d34d10 0x0000000002f36d10 0x0000000002f36d10 0x1777e8 0x1777e8 RW  0x1000     match
   LOAD           0x2eac500 0x00000000030af500 0x00000000030af500 0x008038 0x064664 RW  0x1000
   TLS            0x2d34d10 0x0000000002f35d10 0x0000000002f35d10 0x000000 0x000018 R   0x8
   DYNAMIC        0x2ea5570 0x00000000030a7570 0x00000000030a7570 0x000240 0x000240 RW  0x8
   GNU_RELRO      0x2d34d10 0x0000000002f36d10 0x0000000002f36d10 0x1777e8 0x1782f0 R   0x1        match
   GNU_EH_FRAME   0x89ce34 0x0000000000a9ce34 0x0000000000a9ce34 0x08586c 0x08586c R   0x4
   GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0
   NOTE           0x0002fc 0x00000000002002fc 0x00000000002002fc 0x000020 0x000020 R   0x4

The idea is to have 2 RW PT_LOAD with one exactly matching PT_GNU_RELRO.