From: H.J. Lu <hjl.tools@gmail.com>
Date: Sun, 15 Jun 2025 03:38:54 +0000 (+0800)
Subject: elf: Add optimization barrier for __ehdr_start and _end
X-Git-Tag: glibc-2.42~141
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=81467d4b6168c7ce40d951d6b32e387109c0e5ae;p=thirdparty%2Fglibc.git

elf: Add optimization barrier for __ehdr_start and _end

rtld.c has

extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
...
  _dl_rtld_map.l_map_start = (ElfW(Addr)) &__ehdr_start;
  _dl_rtld_map.l_map_end = (ElfW(Addr)) _end;

As

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120653

shows, compiler may generate run-time relocation on __ehdr_start with

	movq	.LC0(%rip), %xmm0
...
	.section	.data.rel.ro.local,"aw"
	.align 8
.LC0:
	.quad	__ehdr_start

This won't work before run-time relocation is finished in rtld.c.  Add
optimization barrier to prevent run-time relocations against __ehdr_start
and _end.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>
---

diff --git a/elf/rtld.c b/elf/rtld.c
index 8e8f0e6253..e6a181dc31 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -477,7 +477,10 @@ _dl_start_final (void *arg, struct dl_start_final_info *info)
   _dl_setup_hash (&_dl_rtld_map);
   _dl_rtld_map.l_real = &_dl_rtld_map;
   _dl_rtld_map.l_map_start = (ElfW(Addr)) &__ehdr_start;
+  /* Prevent run-time relocations against __ehdr_start and _end.  */
+  asm ("" : "+g" (_dl_rtld_map.l_map_start));
   _dl_rtld_map.l_map_end = (ElfW(Addr)) _end;
+  asm ("" : "+g" (_dl_rtld_map.l_map_end));
   /* Copy the TLS related data if necessary.  */
 #ifndef DONT_USE_BOOTSTRAP_MAP
 # if NO_TLS_OFFSET != 0
diff --git a/sysdeps/x86_64/Makefile b/sysdeps/x86_64/Makefile
index 5723ec1847..e8f8a52eea 100644
--- a/sysdeps/x86_64/Makefile
+++ b/sysdeps/x86_64/Makefile
@@ -214,6 +214,24 @@ $(objpfx)tst-plt-rewrite2: $(objpfx)tst-plt-rewritemod2.so
 endif
 
 test-internal-extras += tst-gnu2-tls2mod1
+
+tests-special += $(objpfx)check-rtld.out
+
+$(objpfx)rtld.reloc: $(objpfx)rtld.os
+	@rm -f $@T
+	LC_ALL=C $(READELF) -rW $< > $@T
+	test -s $@T
+	mv -f $@T $@
+common-generated += $(objpfx)rtld.reloc
+
+# Verify that there are no run-time relocations against __ehdr_start nor
+# _end.
+$(objpfx)check-rtld.out: $(objpfx)rtld.reloc
+	LC_ALL=C; \
+	if grep -E "R_X86_64_64.*(__ehdr_start|_end)" $^ > $@; \
+	  then false; else true; fi; \
+	$(evaluate-test)
+generated += check-rtld.out
 endif # $(subdir) == elf
 
 ifeq ($(subdir),csu)