[PATCH] aarch64: Define elf_backend_extern_protected_data to 0 [PR 18705]

[Fangrui Song]

If an executable has copy relocations for extern protected data, that
can only work if the library containing the definition is built with
assumptions (a) the compiler emits GOT-generating relocations (b) the
linker produces R_*_GLOB_DAT instead of R_*_RELATIVE.  Otherwise the
library uses its own definition directly and the executable accesses a
stale copy.

Commit 32f573bcb3aaa1c9defcad79dbb5851fcc02ae2d changed ld to produce
R_AARCH64_GLOB_DAT but that defeated the purpose of protected visibility
as an optimization.  Restore the previous behavior (which matches
ld.lld) by defining elf_backend_extern_protected_data to 0.
---
 bfd/elfnn-aarch64.c                      | 2 +-
 ld/testsuite/ld-aarch64/protected-data.d | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/bfd/elfnn-aarch64.c b/bfd/elfnn-aarch64.c
index 4926bab9cf2..cb316a928ef 100644
--- a/bfd/elfnn-aarch64.c
+++ b/bfd/elfnn-aarch64.c
@@ -10090,7 +10090,7 @@ const struct elf_size_info elfNN_aarch64_size_info =
 #define elf_backend_dtrel_excludes_plt 1
 #define elf_backend_got_header_size (GOT_ENTRY_SIZE * 3)
 #define elf_backend_default_execstack  0
-#define elf_backend_extern_protected_data 1
+#define elf_backend_extern_protected_data 0
 #define elf_backend_hash_symbol elf_aarch64_hash_symbol
 
 #undef	elf_backend_obj_attrs_section
diff --git a/ld/testsuite/ld-aarch64/protected-data.d b/ld/testsuite/ld-aarch64/protected-data.d
index 5dabf04748c..ee9b4cfc6b5 100644
--- a/ld/testsuite/ld-aarch64/protected-data.d
+++ b/ld/testsuite/ld-aarch64/protected-data.d
@@ -2,4 +2,4 @@
 #ld: -shared
 #readelf: -r -W
 #...
-.* R_AARCH64_GLOB_DAT .* var.*
+.* R_AARCH64_RELATIVE .*
-- 
2.36.1.255.ge46751e96f-goog