https://sourceware.org/bugzilla/show_bug.cgi?id=31652
Bug ID: 31652
Summary: weak def in discarded comdat section becomes
unreferenced undefweak with ld -r
Product: binutils
Version: 2.43 (HEAD)
URL: https://sourceware.org/pipermail/binutils/2024-April/1
33602.html
Status: NEW
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: aoliva at sourceware dot org
Target Milestone: ---
The following, say t.s, is dystilled from libstdc++-v3's floating_to_chars.o:
.section .text.foobar,"axG",@progbits,foo,comdat
.weak foo
.type foo,@function
foo:
# .dc.a undef
.size foo, . - foo
.weak bar
.set bar, foo
The weak definitions were originally implicit instantiations of
to_chars_i<unsigned int128_t>, and the aliases were originally meant for abi
compatibility because of changes in int128_t mangling.
The following, say m.s, is dystilled from a
libstdc++-v3/testsuite/std/time/clock/system/io.cc:
.section .text.foobar,"axG",@progbits,foo,comdat
.weak foo
.type foo,@function
foo:
.size foo, . - foo
.global _start
.set _start,foo
And here's how to trigger the problem:
(simplified from the emails, dropping the unnecessary archive and linking the
object file in it directly; the asm sources were also simplified, dropping bits
that were meant to pull t.o from the archive)
gas/as-new t.s -o t.o && gas/as-new m.s -o m.o && ld/ld-new m.o t.o -o m -r &&
binutils/nm-new m | grep bar
w bar
Tested with 2.38, 2.42, and 2.42.50.20240413, targeting x86_64-linux-gnu, and
also 2.42 targeting multiple vxworks7r2 targets.
This is probably expected behavior: the weakly-defined symbol in t.o lives in a
discarded section, so it decays to weak-undefined, and it most likely goes
unnoticed on systems that support undefweak.
If this were a final link, the bar symbol would be gone. So would the undef
symbol, if the reference to it in t.s were uncommented. But because this is a
relocatable link, both undefined symbols would remain.
Both of them are a problem for e.g. vxworks kernel modules, that never undergo
final linking, and whose loader rejects undefined symbols, even weak ones, if
they're not defined elsewhere.
I hoped --strip-discarded would get rid of undef, if not bar, but no such luck.
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