[glibc/release/2.35/master] Use __ehdr_start rather than _begin in _dl_start_final

[Aurelien Jarno <aurel32@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=55a636430bd382fb9537e6d33cc6bc57bc2660c1

commit 55a636430bd382fb9537e6d33cc6bc57bc2660c1
Author: Alan Modra <amodra@gmail.com>
Date:   Wed Apr 27 14:39:10 2022 +0930

    Use __ehdr_start rather than _begin in _dl_start_final
    
    __ehdr_start is already used in rltld.c:dl_main, and can serve the
    same purpose as _begin.  Besides tidying the code, using linker
    defined section relative symbols rather than "-defsym _begin=0" better
    reflects the intent of _dl_start_final use of _begin, which is to
    refer to the load address of ld.so rather than absolute address zero.
    
    Reviewed-by: Florian Weimer <fweimer@redhat.com>
    (cherry picked from commit 6f043e0ee7e477f50a44024ed0cb579d5e3f511d)

Diff:
---
 elf/Makefile | 3 +--
 elf/rtld.c   | 7 +++----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/elf/Makefile b/elf/Makefile
index 811859654a..bf2550472b 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -1296,8 +1296,7 @@ $(objpfx)ld.so: $(objpfx)librtld.os $(ld-map)
 	$(LINK.o) -nostdlib -nostartfiles -shared -o $@.new		\
 		  $(LDFLAGS-rtld) -Wl,-z,defs $(z-now-$(bind-now))	\
 		  $(filter-out $(map-file),$^) $(load-map-file)		\
-		  -Wl,-soname=$(rtld-installed-name)			\
-		  -Wl,-defsym=_begin=0
+		  -Wl,-soname=$(rtld-installed-name)
 	$(call after-link,$@.new)
 	$(READELF) -s $@.new \
 	  | $(AWK) '($$7 ~ /^UND(|EF)$$/ && $$1 != "0:" && $$4 != "REGISTER") { print; p=1 } END { exit p != 0 }'
diff --git a/elf/rtld.c b/elf/rtld.c
index 9cc9e5fd4c..86354fb0c2 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -441,8 +441,8 @@ static ElfW(Addr) _dl_start_final (void *arg,
 				   struct dl_start_final_info *info);
 #endif
 
-/* These defined magically in the linker script.  */
-extern char _begin[] attribute_hidden;
+/* These are defined magically by the linker.  */
+extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
 extern char _etext[] attribute_hidden;
 extern char _end[] attribute_hidden;
 
@@ -487,7 +487,7 @@ _dl_start_final (void *arg, struct dl_start_final_info *info)
 #endif
   _dl_setup_hash (&GL(dl_rtld_map));
   GL(dl_rtld_map).l_real = &GL(dl_rtld_map);
-  GL(dl_rtld_map).l_map_start = (ElfW(Addr)) _begin;
+  GL(dl_rtld_map).l_map_start = (ElfW(Addr)) &__ehdr_start;
   GL(dl_rtld_map).l_map_end = (ElfW(Addr)) _end;
   GL(dl_rtld_map).l_text_end = (ElfW(Addr)) _etext;
   /* Copy the TLS related data if necessary.  */
@@ -1801,7 +1801,6 @@ dl_main (const ElfW(Phdr) *phdr,
      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.  */
-  extern const ElfW(Ehdr) __ehdr_start __attribute__ ((visibility ("hidden")));
 
   /* Set up the program header information for the dynamic linker
      itself.  It is needed in the dl_iterate_phdr callbacks.  */