..linker-section-attrs-conflict.md

see here for .section directive flags and _type_s

• also see the solaris ELF object file format docs

https://c.godbolt.org/z/T4j9GP91n https://assembly.godbolt.org/z/TGnGMWWWd

conflicting linker section flags?

i.e.:

        .section        .tbss,"awT",@note
// ...
        .section        .tbss,"awT",@nobits

❯ clang -c diff-flags.s -Wno-unused-command-line-argument
diff-flags.s:3:9: error: changed section type for .tbss, expected: 0x8
        .section        .tbss,"awT",@note
        ^
❯ gcc -c diff-flags.s
diff-flags.s: Assembler messages:
diff-flags.s:3: Warning: setting incorrect section type for .tbss
diff-flags.s:10: Warning: ignoring changed section type for .tbss
diff-flags.s:10: Warning: ignoring changed section attributes for .tbss

conflicting linker section types?

i.e.:

        .section        .tbss,"aS",@nobits
// ...
        .section        .tbss,"wx",@nobits

❯ clang -c diff-types.s -Wno-unused-command-line-argument
diff-types.s:3:9: error: changed section flags for .tbss, expected: 0x403
        .section        .tbss,"aS",@nobits
        ^
diff-types.s:10:9: error: changed section flags for .tbss, expected: 0x403
        .section        .tbss,"wx",@nobits
        ^

❯ gcc -c diff-types.s
diff-types.s: Assembler messages:
diff-types.s:3: Warning: bogus SHF_MERGE / SHF_STRINGS for SHT_NOBITS section
diff-types.s:3: Warning: setting incorrect section attributes for .tbss
diff-types.s:10: Warning: ignoring changed section attributes for .tbss

conflicting attrs across sections that are joined? (i.e. -fdata-sections)

with -fdata-sections, separate "objects" are given their own sections (so that the linker is able to garbage collect ones that aren't referenced); i.e.:

__thread int f = 0;
__thread int g = 0;

is lowered as (with -fdata-sections):

        .globl  f
        .section        .tbss.f,"awT",@nobits
        .align 4
        .type   f, @object
        .size   f, 4
f:
        .zero   4

        .globl  g
        .section        .tbss.g,"awT",@nobits // !!!
        .align 4
        .type   g, @object
        .size   g, 4
g:
        .zero   4

instead of:

        .globl  f
        .section        .tbss,"awT",@nobits // note: `.tbss`, not `.tbss.f`
        .align 4
        .type   f, @object
        .size   f, 4
f:
        .zero   4

        .globl  g // note: still in `.tbss`
        .align 4
        .type   g, @object
        .size   g, 4
g:
        .zero   4

the linker ultimately merges these sections:

❯ ld --verbose | rg "tbss\t"
  .tbss           : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }

this begs the question: how does merging handle sections that have conflicting attributes?

# $1: section basename
# $2: `.tbss.f` flags
# $3: `.tbss.g` flags
# $4: `.tbss.f` type
# $5: `.tbss.g` type
test() {
  cat <<EOF | clang -Wno-unused-command-line-argument -x assembler -c - -o a.o
        .section        .$1.f,"$2",@$4
        .align 4
        .type   f, @object
        .size   f, 4
f:
        .zero   4

        .section        .$1.g,"$3",@$5
        .align 4
        .type   g, @object
        .size   g, 4
g:
        .zero   4
EOF
  ld -shared a.o -o b.o
  readelf -We b.o | grep "] .$1"
}

❯ test tbss aS wx progbits nobits
/nix/store/wi0nl3i48dl27dalzrrlq4n9a6mrl63y-binutils-2.44/bin/ld: warning: b.o has a LOAD segment with RWX permissions
  [ 6] .tbss             PROGBITS        0000000000001f08 001f08 000008 00 WAXT  0   0  4

❯ test tbss awT S note note
/nix/store/wi0nl3i48dl27dalzrrlq4n9a6mrl63y-binutils-2.44/bin/ld: BFD (GNU Binutils) 2.44 assertion fail /build/binutils-with-gold-2.44/bfd/elf.c:5405
readelf: Error: 'b.o': No such file

---

ordering (SHN_BEFORE/SHN_AFTER):

• https://docs.oracle.com/cd/E19120-01/open.solaris/819-0690/6n33n7fcj/index.html#chapter6-47976
• https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order
• https://groups.google.com/g/generic-abi/c/hgx_m1aXqUo

.envrc

use nix -p gcc clang

.gitignore

/.direnv
*.o

diff-flags.s

        .section        .tbss,"awT",@note
        .align 4
        .type   f, @object
        .size   f, 4
f:
        .zero   4

        .section        .tbss,"awT",@nobits
        .align 4
        .type   g, @object
        .size   g, 4
g:
        .zero   4

diff-types.s

        .section        .tbss,"aS",@nobits
        .align 4
        .type   f, @object
        .size   f, 4
f:
        .zero   4

        .section        .tbss,"wx",@nobits
        .align 4
        .type   g, @object
        .size   g, 4
g:
        .zero   4