aarch64: add BTI flag to ELF notes (#822)

When a program is loaded and linked, the first ELF file that doesn't
declare that BTI is supported in the GNU NOTES section disables BTI
support.

Example:
readelf -n ./aarch64-unknown-linux-gnu/.libs/libffi.so
Displaying notes found in: .note.gnu.property
  Owner                Data size 	Description
  GNU                  0x00000010	NT_GNU_PROPERTY_TYPE_0
      Properties: AArch64 feature: BTI

Fixes: #823
Signed-off-by: Bill Roberts <bill.roberts@arm.com>

diff --git a/src/aarch64/sysv.S b/src/aarch64/sysv.S
index 90816752d480c63a03303f15ee8b9c91ec569626..fdd0e8b7bba8fd35bdaa9fa13dad501fc6e3b1c9 100644 (file)
--- a/src/aarch64/sysv.S
+++ b/src/aarch64/sysv.S
@@ -66,7 +66,11 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 
 #define BTI_C hint #34
 #define BTI_J hint #36
-
+/*
+ * The ELF Notes section needs to indicate if BTI is supported, as the first ELF loaded that doesn't
+ * declare this support disables it for the whole process.
+ */
+# define GNU_PROPERTY_AARCH64_BTI (1 << 0)         /* Has Branch Target Identification */
        .text
        .align 4
 
@@ -673,5 +677,17 @@ CNAME(ffi_go_closure_SYSV):
 
 #if defined __ELF__ && defined __linux__
        .section .note.GNU-stack,"",%progbits
+
+       .pushsection .note.gnu.property, "a";
+       .balign 8;
+       .long 4;
+       .long 0x10;
+       .long 0x5;
+       .asciz "GNU";
+       .long 0xc0000000; /* GNU_PROPERTY_AARCH64_FEATURE_1_AND */
+       .long 4;
+       .long GNU_PROPERTY_AARCH64_BTI;
+       .long 0;
+       .popsection;
 #endif