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From: "i at maskray dot me" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/27492] New: Make static linking friendly with
 {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'
Date: Mon, 01 Mar 2021 08:37:27 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27492

            Bug ID: 27492
           Summary: Make static linking friendly with {ld.bfd,ld.lld} -z
                    start-stop-gc: removing unused section '__libc_atexit'
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

#include <stdio.h>
#include <stdlib.h>

void hook() {
  puts("hello");
}
int main() {
  atexit(hook);
}

// Require -z start-stop-gc, which is available in trunk ld.lld
(https://reviews.llvm.org/D96914) and GNU ld newer than 2021-03-01
(https://sourceware.org/bugzilla/show_bug.cgi?id=3D27451)
% gcc -static a.c -Wl,--gc-sections -z start-stop-gc
% ./a.out
hello

This appears to work. However, if you use the approach in
https://sourceware.org/pipermail/binutils/2021-February/115561.html to comp=
are
-z nostart-stop-gc and -z start-stop-gc output,
you should notice a difference

% diff 0 1
16a17
> /home/ray/Dev/binutils-gdb/Debug/ld/ld-new: removing unused section '__li=
bc_atexit' in file 'usr/lib/x86_64-linux-gnu/libc.a(genops.o)'


I don't know whether the difference matters. In 2010, this difference caused
bug 11133.
A GNU ld workaround was installed.
Then in 2015-10, the "__start_xx reference from a live input section retain=
 all
xx sections" rule was extended to all translation units.
This is unfortunate because many modern metadata section usage cannot be GC=
ed.
(See "Metadata sections referenced by text sections" in
http://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-ord=
er )


If __libc_atexit wants to be a GC root, use SHF_GNU_RETAIN;
otherwise, use an artificial relocation like R_X86_64_NONE (see
https://lwn.net/Articles/741494/)

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