[PATCH] ld: Add -z start-stop-gc to let __start_/__stop_ not retain C identifier name sections

[H.J. Lu]

On Sat, Feb 27, 2021 at 5:50 PM Fangrui Song <i@maskray.me> wrote:
>
> On 2021-02-27, H.J. Lu via Binutils wrote:
> >On Sat, Feb 27, 2021 at 12:52 PM Fangrui Song via Binutils
> ><binutils@sourceware.org> wrote:
> >>
> >> This rule was initially added in PR11133 to work around glibc.  However,
> >> it makes GC on C identifier name sections ineffective.  In particular,
> >> many modern metadata sections (e.g. clang -fsanitize-coverage and
> >> -fprofile-generate) cannot be GCed.
> >>
> >> Add -z start-stop-gc to drop the rule.
> >>
> >>         PR 27451
> >> include/
> >>         * bfdlink.h (struct bfd_link_info): Add start_stop_gc.
> >> ld/
> >>         * emultempl/elf.em: Handle -z start-stop-gc and -z nostart-stop-gc.
> >>         * ldmain.c: Initialize start_stop_gc.
> >>         * lang.c (lang_process): Handle start_stop_gc.
> >>         * testsuite/ld-gc/gc.exp: New test.
> >>         * testsuite/ld-gc/start.s: Add .weak directive.
> >>         * testsuite/ld-gc/start2.d: New test.
> >>         * ld.texi: Mention -z start-stop-gc and -z nostart-stop-gc.
> >>         * NEWS: Mention -z start-stop-gc.
> >
> >Does this option work with static glibc executable?
> >
> >--
> >H.J.
>
> Yes for a simple example.
>
> // https://sourceware.org/bugzilla/show_bug.cgi?id=11133
> #include <stdio.h>
> int main() {
>    printf("Hello World\n");
> }
>
>
> # --reproduce feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=26119
> gcc -fuse-ld=lld hello.cc -static -Wl,--gc-sections -Wl,--reproduce=/tmp/rep.tar
> cd /tmp; tar xf rep.tar; cd rep
> # Delete --chroot from response.txt
> % ~/Dev/binutils-gdb/Debug/ld/ld-new @response.txt --gc-sections --print-gc-sections 2> 0
> % ~/Dev/binutils-gdb/Debug/ld/ld-new @response.txt --gc-sections --print-gc-sections -z start-stop-gc 2> 1
> % ./a.out
> Hello World
>
> There is one section which is now GCed:
>
>
> % 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)'

From:

https://sourceware.org/bugzilla/show_bug.cgi?id=11133

---
Removing __libc_atexit was responsible for this behaviour.
---

This option may not be safe for static glibc executable and other cases.

> I don't know what the expectation is, but if the section is required to be retained, we should use SHF_GNU_RETAIN or other GC root approach
> (see
> https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order "What if all metadata sections are discarded?")



-- 
H.J.