https://sourceware.org/bugzilla/show_bug.cgi?id=28124
Bug ID: 28124
Summary: ld: Make special sections in a section group GCable
Product: binutils
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: i at maskray dot me
Target Milestone: ---
% cat a.s
.globl _start, bbb
_start:
.section .mytext.bbb,"axG",@progbits,bbb,comdat
bbb:
.byte 0
.section .init_array.bbb,"awG",@init_array,bbb,comdat
.byte 0
% cc -c a.s && ld.bfd --gc-sections --print-gc-sections a.o
(no output)
If I rename .init_array.bbb to .myinit_array.bbb, the group can be GCed
% cc -c a.s && ld.bfd --gc-sections --print-gc-sections a.o
ld.bfd: removing unused section '.group' in file 'a.o'
ld.bfd: removing unused section '.mytext.bbb[bbb]' in file 'a.o'
ld.bfd: removing unused section '.myinit_array.bbb[bbb]' in file 'a.o'
It seems favorable to make all special sections GCable if in a section group.
This makes the rule more consistent.
An SHF_ALLOC SHT_NOTE appears to be a GC root outside of a group.
---
Before 13.0,
clang -fsanitize-coverage=... abuses .init_array.2 (SHT_INIT_ARRAY) in a
section group to make the group a GC root, e.g.
% clang -fsanitize-coverage=trace-pc-guard -c -xc++ =(printf 'int main(){}')
-ffunction-sections -fdata-sections -c -o a.o
% readelf -Wg a.o
group section [ 3] `.group' [main] contains 3 sections:
[Index] Name
[ 4] .text.main
[ 5] .rela.text.main
[ 9] __sancov_guards
COMDAT group section [ 6] `.group' [sancov.module_ctor_trace_pc_guard]
contains 4 sections:
[Index] Name
[ 7] .text.sancov.module_ctor_trace_pc_guard
[ 8] .rela.text.sancov.module_ctor_trace_pc_guard
[ 10] .init_array.2
[ 11] .rela.init_array.2
The intention is that .text.sancov.module_ctor_trace_pc_guard will be retained.
I changed 13.0.0 to use SHF_GNU_RETAIN to express the GC root intention
explicitly.
You may consider work around it, but I think not working around it is also
fine.
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