global: define entry point of libgnutls when called as executable

This defines the entry point of the library, which prints the library
information when called as executable:

  $ lib/.libs/libgnutls.so.30.30.0
  GnuTLS 3.7.2
  Copyright (C) 2000-2021 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.
  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE.

  Library configuration:
    fips-module-name: ...
    libnettle-soname: libnettle.so.8
    libhogweed-soname: libhogweed.so.6
    libgmp-soname: libgmp.so.10

  Please send bug reports to: bugs@gnutls.org
Signed-off-by: Daiki Ueno <ueno@gnu.org>

bootstrap.conf

@@ -27,7 +27,7 @@ required_submodules="tests/suite/tls-fuzzer/python-ecdsa tests/suite/tls-fuzzer/
 
 # Those modules are common to lib/ and src/.
 common_modules="
-alloca attribute byteswap c-ctype c-strcase explicit_bzero fopen-gnu func getline gettext-h gettimeofday hash hash-pjw-bare arpa_inet inet_ntop inet_pton intprops lock memmem-simple minmax netdb netinet_in read-file secure_getenv setsockopt snprintf stdint stpcpy strcase strdup-posix strndup strtok_r strverscmp sys_socket sys_stat sys_types threadlib time_r unistd valgrind-tests vasprintf verify vsnprintf xalloc-oversized
+alloca attribute byteswap c-ctype c-strcase explicit_bzero fopen-gnu func getline gettext-h gettimeofday hash hash-pjw-bare arpa_inet inet_ntop inet_pton intprops lock memmem-simple minmax netdb netinet_in noreturn read-file secure_getenv setsockopt snprintf stdint stpcpy strcase strdup-posix strndup strtok_r strverscmp sys_socket sys_stat sys_types threadlib time_r unistd valgrind-tests vasprintf verify vsnprintf xalloc-oversized
 "
 gnulib_modules="
 $common_modules extensions gendocs havelib ldd lib-msvc-compat lib-symbol-versions maintainer-makefile manywarnings pmccabe2html warnings

configure.ac

@@ -767,6 +767,28 @@ gnutls_so=libgnutls.so.`expr "$LT_CURRENT" - "$LT_AGE"`
 AC_DEFINE_UNQUOTED([GNUTLS_LIBRARY_SONAME], ["$gnutls_so"], [The soname of gnutls library])
 AC_SUBST([gnutls_so])
 
+AC_CHECK_TOOL([READELF], [readelf])
+
+AC_MSG_CHECKING([ELF dynamic linker])
+changequote(,)
+extract_interp='s/.*program interpreter: \(.*\)\]/\1/p'
+changequote([,])
+save_LIBS=$LIBS
+LIBS="$LIBS $GMP_LIBS"
+AC_LINK_IFELSE([AC_LANG_PROGRAM([
+               #include <gmp.h>],[
+	       mpz_t n;
+	       mpz_init(n);])],
+	[interp=`$READELF -lW conftest$EXEEXT | sed -n "$extract_interp"`])
+LIBS=$save_LIBS
+if test -n "$interp"; then
+	AC_DEFINE_UNQUOTED([RUNTIME_LINKER], ["$interp"], [The path to ELF dynamic linker])
+else
+	interp=none
+fi
+AM_CONDITIONAL(ENABLE_LIB_ENTRY_POINT, test "$interp" != none)
+AC_MSG_RESULT($interp)
+
 AC_MSG_CHECKING([whether to build libdane])
 AC_ARG_ENABLE(libdane,
     AS_HELP_STRING([--disable-libdane],

lib/Makefile.am

@@ -213,6 +213,10 @@ else
 libgnutls_la_LDFLAGS += -export-symbols-regex '^(gnutls|_gnutls).*'
 endif
 
+if ENABLE_LIB_ENTRY_POINT
+libgnutls_la_LDFLAGS += -Wl,-e,__libgnutls_main
+endif
+
 if ENABLE_MINITASN1
 libgnutls_la_LIBADD += minitasn1/libminitasn1.la
 else

lib/global.c

@@ -38,6 +38,7 @@
 #include <fips.h>
 #include <atfork.h>
 #include <system-keys.h>
+#include <noreturn.h>
 #include "str.h"
 #include "global.h"
 
@@ -568,3 +569,49 @@ gnutls_get_library_config(void)
 {
 	return _gnutls_library_config;
 }
+
+#ifdef RUNTIME_LINKER
+
+static const char banner_header[] =
+	PACKAGE_NAME " " PACKAGE_VERSION "\n"
+	"Copyright (C) 2000-2021 Free Software Foundation, Inc.\n"
+	"This is free software; see the source for copying conditions.\n"
+	"There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A\n"
+	"PARTICULAR PURPOSE.\n\n";
+
+static const char banner_footer[] =
+	"\nPlease send bug reports to: " PACKAGE_BUGREPORT "\n";
+
+const char interpreter[] __attribute__ ((section (".interp"))) =
+	RUNTIME_LINKER;
+
+_GL_NORETURN_FUNC extern void __libgnutls_main (void);
+void
+__libgnutls_main (void)
+{
+	const gnutls_library_config_st *p;
+
+	write(STDOUT_FILENO, banner_header, sizeof(banner_header)-1);
+
+#define MSG "Library configuration:\n"
+	write(STDOUT_FILENO, MSG, sizeof(MSG)-1);
+#undef MSG
+	for (p = _gnutls_library_config; p->name; p++) {
+#define MSG "  "
+		write(STDOUT_FILENO, MSG, sizeof(MSG)-1);
+#undef MSG
+		write(STDOUT_FILENO, p->name, strlen(p->name));
+#define MSG ": "
+		write(STDOUT_FILENO, MSG, sizeof(MSG)-1);
+#undef MSG
+		write(STDOUT_FILENO, p->value, strlen(p->value));
+#define MSG "\n"
+		write(STDOUT_FILENO, MSG, sizeof(MSG)-1);
+#undef MSG
+	}
+
+	write(STDOUT_FILENO, banner_footer, sizeof(banner_footer)-1);
+	_exit(0);
+}
+
+#endif /* RUNTIME_LINKER */

[Florian Weimer]

You could introduce a helper function that combines write and strlen.