From: Fangrui Song <i@maskray.me>
To: binutils <binutils@sourceware.org>
Subject: [ld] section address : ALIGN(align) and the maximum of input section alignments
Date: Wed, 26 Feb 2020 05:23:00 -0000 [thread overview]
Message-ID: <20200226052300.wazzfmivxta63vef@gmail.com> (raw)
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
next reply other threads:[~2020-02-26 5:23 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-26 5:23 Fangrui Song [this message]
2020-02-26 6:31 ` Fangrui Song
2020-03-03 5:46 ` Fangrui Song
2020-03-03 22:39 ` Alan Modra
2020-03-04 0:03 ` Fangrui Song
[not found] ` <BY5PR07MB7316674DD98011D2AD812A63CBE40@BY5PR07MB7316.namprd07.prod.outlook.com>
2020-03-04 5:56 ` Alan Modra
2020-03-04 6:40 ` Fangrui Song
2020-03-04 8:04 ` Alan Modra
2020-03-05 6:41 ` Fangrui Song
2020-03-05 11:21 ` Alan Modra
2020-03-10 19:00 ` Fangrui Song
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