[Bug ld/25458] New: --gc-sections removes _environ symbol since version 2.21

[andrew at ishiboo dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=25458

            Bug ID: 25458
           Summary: --gc-sections removes _environ symbol since version
                    2.21
           Product: binutils
           Version: 2.35 (HEAD)
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: andrew at ishiboo dot com
  Target Milestone: ---

The following issue is observed in all `ld` versions >= 2.21. Version 2.20 does
not exhibit this behavior.

A simple C file:

    #include <stdio.h>
    extern char **environ;
    int main(int argc, char **argv) {
        printf("%p\n", environ);
        return 0;
    }

When compiled, will reference `_environ`:

    $ gcc -o environ environ.c && \
      readelf -s environ | grep " _environ"
         4: 0000000000601040     8 OBJECT  WEAK   DEFAULT   24
_environ@GLIBC_2.2.5 (2)
    $

When compiled passing the linker `--gc-sections`, the `_environ` alias is
dropped, but the `__environ` and `environ` ones remain:

    $ gcc -Wl,--gc-sections -o environ environ.c && \
      readelf -s environ | grep " _environ"
    $

This breaks glibc functionality that looks at `_environ` internally, because it
then points to a different address when the shared object is loaded. This
manifested in glibc not respecting memory sub-system environment variables.

    Code x-reference:
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/arena.c;h=4d0deefe19156a46d7d51a8da5d7b1f8e4e2afdd;hb=6164128f1ca84eea240b66f977054e16b94b3c86#l541

The `gold` linker does not discard the `_environ` symbol when `--gc-sections`
is used:

    $ gcc -fuse-ld=gold -Wl,--gc-sections -o environ environ.c && \
      readelf -s environ | grep " _environ"
         5: 0000000000402040     8 OBJECT  GLOBAL DEFAULT   24
_environ@GLIBC_2.2.5 (2)
        31: 0000000000402040     8 OBJECT  GLOBAL DEFAULT   24 _environ

This was checked against older versions of `ld` and version 2.20 works and
retains the `_environ` symbol. Searching for changes that occurred in the
release of 2.21, it appears this commit for bug 20828 changed the behavior:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=81ff47b3a54633819fac4d973e34f1ff0c65606e
https://www.sourceware.org/ml/binutils/2017-01/msg00263.html

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