[glibc] elf: Unconditionally use __ehdr_start

[glibc] elf: Unconditionally use __ehdr_start

[Fangrui Song]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=302247c89121e8d4c7629e589edbb4974fff6edb

commit 302247c89121e8d4c7629e589edbb4974fff6edb
Author: Fangrui Song <maskray@google.com>
Date:   Tue Aug 10 11:04:56 2021 -0700

    elf: Unconditionally use __ehdr_start
    
    We can consider __ehdr_start (from binutils 2.23 onwards)
    unconditionally supported, since configure.ac requires binutils>=2.25.
    
    The configure.ac check is related to an ia64 bug fixed by binutils 2.24.
    See https://sourceware.org/pipermail/libc-alpha/2014-August/053503.html
    
    Tested on x86_64-linux-gnu. Tested build-many-glibcs.py with
    aarch64-linux-gnu and s390x-linux-gnu.
    
    Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

Diff:
---
 config.h.in  |  3 ---
 configure    | 52 ----------------------------------------------------
 configure.ac | 34 ----------------------------------
 elf/rtld.c   | 13 ++++---------
 4 files changed, 4 insertions(+), 98 deletions(-)

diff --git a/config.h.in b/config.h.in
index 8b45a3a61d..0d92504f65 100644
--- a/config.h.in
+++ b/config.h.in
@@ -198,9 +198,6 @@
 /* Define if CC supports attribute retain.  */
 #undef HAVE_GNU_RETAIN
 
-/* Define if the linker defines __ehdr_start.  */
-#undef HAVE_EHDR_START
-
 /* Define to 1 if the assembler needs intermediate aliases to define
    multiple symbol versions for one symbol.  */
 #define SYMVER_NEEDS_ALIAS 0
diff --git a/configure b/configure
index 9619c10991..7272fbf6ea 100755
--- a/configure
+++ b/configure
@@ -6636,58 +6636,6 @@ if test $libc_cv_predef_fortify_source = yes; then
 fi
 
 
-# Some linkers on some architectures support __ehdr_start but with
-# bugs.  Make sure usage of it does not create relocations in the
-# output (as the linker should resolve them all for us).
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the linker provides working __ehdr_start" >&5
-$as_echo_n "checking whether the linker provides working __ehdr_start... " >&6; }
-if ${libc_cv_ehdr_start+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-
-old_CFLAGS="$CFLAGS"
-old_LDFLAGS="$LDFLAGS"
-old_LIBS="$LIBS"
-CFLAGS="$CFLAGS -fPIC"
-LDFLAGS="$LDFLAGS -nostdlib -nostartfiles -shared $no_ssp"
-LIBS=
-cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-
-typedef struct {
-  char foo;
-  long val;
-} Ehdr;
-extern const Ehdr __ehdr_start __attribute__ ((visibility ("hidden")));
-long ehdr (void) { return __ehdr_start.val; }
-
-_ACEOF
-if ac_fn_c_try_link "$LINENO"; then :
-  if $READELF -r conftest | grep -F __ehdr_start >/dev/null; then
-		  libc_cv_ehdr_start=broken
-		else
-		  libc_cv_ehdr_start=yes
-		fi
-else
-  libc_cv_ehdr_start=no
-fi
-rm -f core conftest.err conftest.$ac_objext \
-    conftest$ac_exeext conftest.$ac_ext
-CFLAGS="$old_CFLAGS"
-LDFLAGS="$old_LDFLAGS"
-LIBS="$old_LIBS"
-
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_ehdr_start" >&5
-$as_echo "$libc_cv_ehdr_start" >&6; }
-if test "$libc_cv_ehdr_start" = yes; then
-  $as_echo "#define HAVE_EHDR_START 1" >>confdefs.h
-
-elif test "$libc_cv_ehdr_start" = broken; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: linker is broken -- you should upgrade" >&5
-$as_echo "$as_me: WARNING: linker is broken -- you should upgrade" >&2;}
-fi
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the assembler requires one version per symbol" >&5
 $as_echo_n "checking whether the assembler requires one version per symbol... " >&6; }
 if ${libc_cv_symver_needs_alias+:} false; then :
diff --git a/configure.ac b/configure.ac
index 34ecbba540..af47cd51e6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1662,40 +1662,6 @@ if test $libc_cv_predef_fortify_source = yes; then
 fi
 AC_SUBST(CPPUNDEFS)
 
-# Some linkers on some architectures support __ehdr_start but with
-# bugs.  Make sure usage of it does not create relocations in the
-# output (as the linker should resolve them all for us).
-AC_CACHE_CHECK([whether the linker provides working __ehdr_start],
-	       libc_cv_ehdr_start, [
-old_CFLAGS="$CFLAGS"
-old_LDFLAGS="$LDFLAGS"
-old_LIBS="$LIBS"
-CFLAGS="$CFLAGS -fPIC"
-LDFLAGS="$LDFLAGS -nostdlib -nostartfiles -shared $no_ssp"
-LIBS=
-AC_LINK_IFELSE([AC_LANG_SOURCE([
-typedef struct {
-  char foo;
-  long val;
-} Ehdr;
-extern const Ehdr __ehdr_start __attribute__ ((visibility ("hidden")));
-long ehdr (void) { return __ehdr_start.val; }
-])],
-	       [if $READELF -r conftest | grep -F __ehdr_start >/dev/null; then
-		  libc_cv_ehdr_start=broken
-		else
-		  libc_cv_ehdr_start=yes
-		fi], [libc_cv_ehdr_start=no])
-CFLAGS="$old_CFLAGS"
-LDFLAGS="$old_LDFLAGS"
-LIBS="$old_LIBS"
-])
-if test "$libc_cv_ehdr_start" = yes; then
-  AC_DEFINE([HAVE_EHDR_START])
-elif test "$libc_cv_ehdr_start" = broken; then
-  AC_MSG_WARN([linker is broken -- you should upgrade])
-fi
-
 dnl Starting with binutils 2.35, GAS can attach multiple symbol versions
 dnl to one symbol (PR 23840).
 AC_CACHE_CHECK(whether the assembler requires one version per symbol,
diff --git a/elf/rtld.c b/elf/rtld.c
index d733359eaf..878e6480f4 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1684,21 +1684,16 @@ dl_main (const ElfW(Phdr) *phdr,
   if (GLRO(dl_use_load_bias) == (ElfW(Addr)) -2)
     GLRO(dl_use_load_bias) = main_map->l_addr == 0 ? -1 : 0;
 
-  /* Set up the program header information for the dynamic linker
-     itself.  It is needed in the dl_iterate_phdr callbacks.  */
-  const ElfW(Ehdr) *rtld_ehdr;
-
   /* Starting from binutils-2.23, the linker will define the magic symbol
      __ehdr_start to point to our own ELF header if it is visible in a
      segment that also includes the phdrs.  If that's not available, we use
      the old method that assumes the beginning of the file is part of the
      lowest-addressed PT_LOAD segment.  */
-#ifdef HAVE_EHDR_START
   extern const ElfW(Ehdr) __ehdr_start __attribute__ ((visibility ("hidden")));
-  rtld_ehdr = &__ehdr_start;
-#else
-  rtld_ehdr = (void *) GL(dl_rtld_map).l_map_start;
-#endif
+
+  /* Set up the program header information for the dynamic linker
+     itself.  It is needed in the dl_iterate_phdr callbacks.  */
+  const ElfW(Ehdr) *rtld_ehdr = &__ehdr_start;
   assert (rtld_ehdr->e_ehsize == sizeof *rtld_ehdr);
   assert (rtld_ehdr->e_phentsize == sizeof (ElfW(Phdr)));