Re: [ld] section address : ALIGN(align) and the maximum of input section alignments

[Fangrui Song <i at maskray dot me>]

On 2020-02-25, Fangrui Song wrote:
> https://sourceware.org/binutils/docs/ld/Output-Section-Description.html
> I have observed some strange rules while playing with the output section address and ALIGN.
>
> cat > a.s <<e
> .globl _start; _start: ret
> .section .data.rel.ro,"aw"; .balign 8; .byte 0
> .data; .byte 0
> .section .data2,"aw"; .balign 8; .byte 0
> .section .data3,"aw"; .balign 32; .byte 0
> .bss; .balign 32; .byte 0
> e
>
> cat > a.x <<e
> SECTIONS {
>   .text 0x10000 : { *(.text) }
>   /* sh_addr is aligned to 16. */
>   .data.rel.ro . : ALIGN(16) { *(.data.rel.ro) }
>
>   .data 0x20000 : { *(.data) }
>   /* The output section address is set without ALIGN. sh_addr is set to Dot, ignoring alignment. */
>   /* sh_addralign is the maximum of input section alignments, 8. */
>   .data2 . : { *(.data2) }
>   /* sh_addr is aligned to 32. */
>   .data3 : ALIGN(16) { *(.data3) }
>   /* sh_addr is aligned to 16, ???????????? */
>   .bss . : ALIGN(16) { *(.bss) }
> }
> e
>
> as a.s -o a.o
> ld.bfd -T a.x a.o -o a
>
> % readelf -WS a
>
> Section Headers:
>  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
>  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
>  [ 1] .text             PROGBITS        0000000000010000 001000 000001 00  AX  0   0  1
>  [ 2] .data.rel.ro      PROGBITS        0000000000010010 001010 000001 00  WA  0   0 16
>  [ 3] .data             PROGBITS        0000000000020000 002000 000001 00  WA  0   0  1
>  [ 4] .data2            PROGBITS        0000000000020008 002008 000001 00  WA  0   0  8
>  [ 5] .data3            PROGBITS        0000000000020020 002020 000001 00  WA  0   0 32
>  [ 6] .bss              NOBITS          0000000000020030 002021 000011 00  WA  0   0 32
> ...
>
>
> Why doesn't `.bss . : ALIGN(16)` respect the maximum of input section alignments (32)?
>
> (I have verified that .bss being SHT_NOBITS is unrelated.)
>
> The rule of sh_addr computation appears to be:
>
> * ADDR is unset, ALIGN is unset => max_input_alignment
> * ADDR is unset, ALIGN is set => max(ALIGN, max_input_alignment)
> * ADDR is set, ALIGN is unset => max_input_alignment
> * ADDR is set, ALIGN is set => ALIGN

When ADDR is set, ALIGN is used while max_input_alignment is ignored.

ld/ldlang.c:5582

                if (os->addr_tree == NULL) {
                    ...
		    newdot = os->region->current;
                    // maximum of ALIGN and input section alignments
		    section_alignment = os->bfd_section->alignment_power;
		  }
		else
                  // ALIGN
		  section_alignment = exp_get_power (os->section_alignment, "section alignment");

Alan, is the `else` branch supposed to use `section_alignment = os->bfd_section->alignment_power;` as well?