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

The 08/06/2021 22:58, Fangrui Song via Libc-alpha wrote:
> 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.

this looks good.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>


> ---
>  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)));
>  
> -- 
> 2.32.0.605.g8dce9f2422-goog
>