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

[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

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

[Fangrui Song]

On Wed, Jun 1, 2022 at 9:21 PM Fangrui Song <maskray@google.com> wrote:
>
> 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
>

:)

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

[Szabolcs Nagy]

The 06/01/2022 21:20, Fangrui Song wrote:
> 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.

This patch looks good to me.
Thanks.

> ---
>  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
>