[PATCH] arm: Define elf_backend_extern_protected_data to 0

[PATCH] arm: Define elf_backend_extern_protected_data to 0

[Fangrui Song]

From: Fangrui Song <i@maskray.me>

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 b68a20d6675f1360ea4db50a9835c073675b9889 changed ld to produce
R_ARM_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/elf32-arm.c                      | 2 +-
 ld/testsuite/ld-arm/protected-data.d | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
index 5b8f5e5a0e6..36f09cb3427 100644
--- a/bfd/elf32-arm.c
+++ b/bfd/elf32-arm.c
@@ -20315,7 +20315,7 @@ elf32_arm_backend_symbol_processing (bfd *abfd, asymbol *sym)
 #define elf_backend_dtrel_excludes_plt 1
 
 #define elf_backend_got_header_size	12
-#define elf_backend_extern_protected_data 1
+#define elf_backend_extern_protected_data 0
 
 #undef	elf_backend_obj_attrs_vendor
 #define elf_backend_obj_attrs_vendor		"aeabi"
diff --git a/ld/testsuite/ld-arm/protected-data.d b/ld/testsuite/ld-arm/protected-data.d
index 3f266dea1b4..7dffe9ab958 100644
--- a/ld/testsuite/ld-arm/protected-data.d
+++ b/ld/testsuite/ld-arm/protected-data.d
@@ -2,4 +2,4 @@
 #ld: -shared
 #readelf: -r -W
 #...
-.* R_ARM_GLOB_DAT .* var.*
+.* R_ARM_RELATIVE .*
-- 
2.36.1.255.ge46751e96f-goog

Re: [PATCH] arm: Define elf_backend_extern_protected_data to 0

[Fangrui Song]

On Wed, Jun 1, 2022 at 9:27 PM Fangrui Song <maskray@google.com> wrote:
>
> From: Fangrui Song <i@maskray.me>
>
> 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 b68a20d6675f1360ea4db50a9835c073675b9889 changed ld to produce
> R_ARM_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/elf32-arm.c                      | 2 +-
>  ld/testsuite/ld-arm/protected-data.d | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
> index 5b8f5e5a0e6..36f09cb3427 100644
> --- a/bfd/elf32-arm.c
> +++ b/bfd/elf32-arm.c
> @@ -20315,7 +20315,7 @@ elf32_arm_backend_symbol_processing (bfd *abfd, asymbol *sym)
>  #define elf_backend_dtrel_excludes_plt 1
>
>  #define elf_backend_got_header_size    12
> -#define elf_backend_extern_protected_data 1
> +#define elf_backend_extern_protected_data 0
>
>  #undef elf_backend_obj_attrs_vendor
>  #define elf_backend_obj_attrs_vendor           "aeabi"
> diff --git a/ld/testsuite/ld-arm/protected-data.d b/ld/testsuite/ld-arm/protected-data.d
> index 3f266dea1b4..7dffe9ab958 100644
> --- a/ld/testsuite/ld-arm/protected-data.d
> +++ b/ld/testsuite/ld-arm/protected-data.d
> @@ -2,4 +2,4 @@
>  #ld: -shared
>  #readelf: -r -W
>  #...
> -.* R_ARM_GLOB_DAT .* var.*
> +.* R_ARM_RELATIVE .*
> --
> 2.36.1.255.ge46751e96f-goog
>

Ping:)

Re: [PATCH] arm: Define elf_backend_extern_protected_data to 0

[Szabolcs Nagy]

The 06/01/2022 21:27, Fangrui Song wrote:
> From: Fangrui Song <i@maskray.me>
> 
> 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 b68a20d6675f1360ea4db50a9835c073675b9889 changed ld to produce
> R_ARM_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/elf32-arm.c                      | 2 +-
>  ld/testsuite/ld-arm/protected-data.d | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
> index 5b8f5e5a0e6..36f09cb3427 100644
> --- a/bfd/elf32-arm.c
> +++ b/bfd/elf32-arm.c
> @@ -20315,7 +20315,7 @@ elf32_arm_backend_symbol_processing (bfd *abfd, asymbol *sym)
>  #define elf_backend_dtrel_excludes_plt 1
>  
>  #define elf_backend_got_header_size	12
> -#define elf_backend_extern_protected_data 1
> +#define elf_backend_extern_protected_data 0
>  
>  #undef	elf_backend_obj_attrs_vendor
>  #define elf_backend_obj_attrs_vendor		"aeabi"
> diff --git a/ld/testsuite/ld-arm/protected-data.d b/ld/testsuite/ld-arm/protected-data.d
> index 3f266dea1b4..7dffe9ab958 100644
> --- a/ld/testsuite/ld-arm/protected-data.d
> +++ b/ld/testsuite/ld-arm/protected-data.d
> @@ -2,4 +2,4 @@
>  #ld: -shared
>  #readelf: -r -W
>  #...
> -.* R_ARM_GLOB_DAT .* var.*
> +.* R_ARM_RELATIVE .*
> -- 
> 2.36.1.255.ge46751e96f-goog
>