[PATCH 2/3] libctf: Add comment for conditionally def'd sym

[Nick Alcock]

On 2 Mar 2024, Nicholas Vinson uttered the following:

> diff --git a/libctf/configure.ac b/libctf/configure.ac
> index e4e430615bd..0494a537e78 100644
> --- a/libctf/configure.ac
> +++ b/libctf/configure.ac
> @@ -258,7 +258,8 @@ AC_CACHE_CHECK([for linker versioning flags], [ac_cv_libctf_version_script],
>     CFLAGS="$CFLAGS -fPIC"
>     AC_LINK_IFELSE([AC_LANG_SOURCE([[int ctf_foo (void) { return 0; }
>  				    int main (void) { return ctf_foo(); }]])],
> -		  [ac_cv_libctf_version_script="-Wl,--version-script='$srcdir/libctf.ver'"],
> +		  [ac_cv_libctf_version_script="-Wl,--version-script"
> +		   decommented_version_script=t],
>  		  [])
>     LDFLAGS="$old_LDFLAGS"

Not quite. The stanza you changed there is meant for GNU ld, which
supports both undefined symbols in version scripts and /* comments */
and needs none of this hackery.

We have a three-case case statement here, with the final last-ditch
attempt being -export-symbols-regex=, and we need to add another case
for "no -B local, doesn't like nonexistent symbols, supports
--version-script".

Something like the below, perhaps. (Caveat: when I test with LLVM 17 and
LDFLAGS="-fuse-ld=lld -Wl,--no-undefined-version" there is no failure to
link libctf with trunk binutils, though ld building does fail:

ld.lld: error: undefined symbol: ldlex_backup
>>> referenced by ldgram.y:860 (../../ld/ldgram.y:860)
>>>               ldgram.o:(yyparse)
>>> referenced by ldgram.y:1125 (../../ld/ldgram.y:1125)
>>>               ldgram.o:(yyparse)
>>> referenced by ldgram.y:1146 (../../ld/ldgram.y:1146)
>>>               ldgram.o:(yyparse)
>>> referenced 1 more times

ld.lld: error: undefined symbol: ldlex_wild

so this is not really tested and all I can really say is that clang and
lld are still happy to link. Don't trust what I wrote here, please test
it out -- and obviously it still needs things like the removal of the
ctf_label_set symbols that genuinely don't exist, as well. That patch is
fine.)

diff --git a/libctf/configure.ac b/libctf/configure.ac
index e4e430615bd..28f63792826 100644
--- a/libctf/configure.ac
+++ b/libctf/configure.ac
@@ -251,7 +251,7 @@ AC_SUBST(HAVE_TCL_TRY)
 # Use a version script, if possible, or an -export-symbols-regex otherwise.
 decommented_version_script=
 AC_CACHE_CHECK([for linker versioning flags], [ac_cv_libctf_version_script],
-  [echo 'FOO { global: mai*; local: ctf_fo*; };' > conftest.ver
+  [echo 'FOO { global: mai*; nonexistent; local: ctf_fo*; };' > conftest.ver
    old_LDFLAGS="$LDFLAGS"
    old_CFLAGS="$CFLAGS"
    LDFLAGS="$LDFLAGS -shared -Wl,--version-script=conftest.ver"
@@ -262,7 +262,10 @@ AC_CACHE_CHECK([for linker versioning flags], [ac_cv_libctf_version_script],
 		  [])
    LDFLAGS="$old_LDFLAGS"
 
+   # Solaris: -B local, nonexistent symbols prohibited: use preprocessed
+   # version scripts
    if test -z "$ac_cv_libctf_version_script"; then
+     echo 'FOO { global: mai*; local: ctf_fo*; };' > conftest.ver
      LDFLAGS="$LDFLAGS -shared -Wl,-B,local -Wl,-z,gnu-version-script=conftest.ver"
      AC_LINK_IFELSE([AC_LANG_SOURCE([[int ctf_foo (void) { return 0; }
 				      int main (void) { return ctf_foo(); }]])],
@@ -273,6 +276,19 @@ AC_CACHE_CHECK([for linker versioning flags], [ac_cv_libctf_version_script],
    fi
    CFLAGS="$old_CFLAGS"
 
+   # LLD with --no-undefined-version on by default: no -B local, nonexistent
+   # symbols prohibited: same solution as Solaris
+   if test -z "$ac_cv_libctf_version_script"; then
+     LDFLAGS="$LDFLAGS -shared -Wl,--version-script=conftest.ver"
+     AC_LINK_IFELSE([AC_LANG_SOURCE([[int ctf_foo (void) { return 0; }
+				      int main (void) { return ctf_foo(); }]])],
+		    [ac_cv_libctf_version_script="-Wl,--version-script"
+		     decommented_version_script=t],
+		    [])
+     LDFLAGS="$old_LDFLAGS"
+   fi
+   CFLAGS="$old_CFLAGS"
+
    if test -z "$ac_cv_libctf_version_script"; then
      ac_cv_libctf_version_script='-export-symbols-regex ctf_.*'
    fi
diff --git a/libctf/libctf.ver b/libctf/libctf.ver
index c59847d012b..474852c2c84 100644
--- a/libctf/libctf.ver
+++ b/libctf/libctf.ver
@@ -139,7 +139,6 @@ LIBCTF_1.0 {
 
 	ctf_arc_write;
 	ctf_arc_write_fd;
-	ctf_arc_open;
 	ctf_arc_bufopen;
 	ctf_arc_close;
 	ctf_arc_open_by_name;
@@ -165,6 +164,7 @@ LIBCTF_1.0 {
 	ctf_link_shuffle_syms;
 	ctf_link_write;
 
+	ctf_arc_open;                           /* libctf only.  */
 	ctf_fdopen;                             /* libctf only.  */
 	ctf_open;                               /* libctf only.  */
 	ctf_bfdopen;                            /* libctf only.  */

base-commit: 90f8d97c8efa75f7f019b868eca9c626bc35203d
-- 
2.43.0.272.gce700b77fd