[Bug libc/27492] New: Make static linking friendly with {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'

[Bug libc/27492] New: Make static linking friendly with {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27492

            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=27451)
% 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 compare
-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 '__libc_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 GCed.
(See "Metadata sections referenced by text sections" in
http://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order )


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/)

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug libc/27492] Make static linking friendly with {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27492

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-
                 CC|                            |fweimer at redhat dot com

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
I expect that genops.c needs __libc_atexit to flush stdio streams in the
statically linked case.  We could likely handle this differently, e.g. call a
weak function symbol in the process shutdown sequence if it is defined.

At present, the weak symbol wouldn't matter because we have a sequence vSDO
lookup → dynamic linker → malloc → vfprintf → libio which pulls in most of
libio anyway.

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug libc/27492] Make static linking friendly with {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27492

--- Comment #2 from Fangrui Song <i at maskray dot me> ---
The quickest fix is probably

- *(__libc_atexit)
+ KEEP(*(__libc_atexit))

A better fix is to add a R_*_NONE relocation (`.reloc ., R_X86_64_NONE,
target`) from the use site to a symbol in the __libc_atexit section.
Unfortunately the .reloc approach needs to hard code the relocation name on
various targets.

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug libc/27492] Make static linking friendly with {ld.bfd,ld.lld} -z start-stop-gc: removing unused section '__libc_atexit'

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27492

Fangrui Song <i at maskray dot me> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unspecified                 |2.33
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Fangrui Song <i at maskray dot me> ---
Fixed by
https://sourceware.org/git/?p=glibc.git;a=commit;h=cd6ae7ea5431c2b8f16201fb0e2c413bf8d2df06

commit cd6ae7ea5431c2b8f16201fb0e2c413bf8d2df06
Author: Fangrui Song <maskray@google.com>
Date:   Fri Apr 16 11:26:39 2021

    Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

    So that text_set_element/data_set_element/bss_set_element defined
    variables will be retained by the linker.

    Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
    will not be optimized out; 'retain' prevents section garbage collection
    if the linker support SHF_GNU_RETAIN.

    GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
    identifier name sections to be GCed even if there are live
    __start_/__stop_ references.

    Without the change, there are some static linking problems, e.g.
    _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
    stdout is not flushed on exit.

    Note: GCC may warning 'retain' attribute ignored while
__has_attribute(retain)
    is 1 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).

    Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

-- 
You are receiving this mail because:
You are on the CC list for the bug.