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

Fangrui Song i@maskray.me
Tue Mar 3 05:46:00 GMT 2020 
On Tue, Feb 25, 2020 at 10:30 PM Fangrui Song <i@maskray.me> wrote:
>
> 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?

Ping :)
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