[aarch64] Add support for pointer authentication B key

Armv8.3-A has another key used in pointer authentication called the
B-key (other than the A-key that is already supported). In order for
stack unwinders to work it is necessary to be able to identify frames
that have been signed with the B-key rather than the A-key and it was
felt that keeping this as an augmentation character in the CIE was the
best bet. The DWARF extensions for ARM therefore propose to add a new
augmentation character 'B' to the CIE augmentation string and the
corresponding cfi directive ".cfi_b_key_frame". I've made the relevant
changes to GAS and LD to add support for B-key unwinding, which required
modifying LD to check for 'B' in the augmentation string, adding the
".cfi_b_key_frame" directive to GAS and adding a "pauth_key" field to
GAS's fde_entry and cie_entry structs.

The pointer authentication instructions will behave as NOPs on
architectures that don't support them, and so a check for the
architecture being assembled for is not necessary since there will be no
behavioural difference between augmentation strings with and without the
'B' character on such architectures.

2018-12-05  Sam Tebbs  <sam.tebbs@arm.com>

bfd/
	* elf-eh-frame.c (_bfd_elf_parse_eh_frame): Add check for 'B'.

gas/
	* dw2gencfi.c (struct cie_entry): Add tc_cie_entry_extras invocation.
	(alloc_fde_entry): Add tc_fde_entry_init_extra invocation.
	(output_cie): Add tc_output_cie_extra invocation.
	(select_cie_for_fde): Add tc_cie_fde_equivalent_extra and
	tc_cie_entry_init_extra invocation.
	(frch_cfi_data, cfa_save_data): Move to dwgencfi.h.
	* config/tc-aarch64.c (s_aarch64_cfi_b_key_frame): Declare.
	(md_pseudo_table): Add "cfi_b_key_frame".
	* config/tc-aarch64.h (tc_fde_entry_extras, tc_cie_entry_extras,
	tc_fde_entry_init_extra, tc_output_cie_extra,
	tc_cie_fde_equivalent_extra, tc_cie_entry_init_extra): Define.
	* dw2gencfi.h (struct fde_entry): Add tc_fde_entry_extras invocation.
	(pointer_auth_key): Define.
	(frch_cfi_data, cfa_save_data): Move from dwgencfi.c.
	* doc/c-aarch64.texi (.cfi_b_key_frame): Add documentation.
	* testsuite/gas/aarch64/(pac_ab_key.d, pac_ab_key.s): New file.

1 files changed, 9 insertions, 0 deletions

diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index 3fed11c..4c97703 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -1993,6 +1993,14 @@ s_aarch64_inst (int ignored ATTRIBUTE_UNUSED)
   demand_empty_rest_of_line ();
 }
 
+static void
+s_aarch64_cfi_b_key_frame (int ignored ATTRIBUTE_UNUSED)
+{
+  demand_empty_rest_of_line ();
+  struct fde_entry *fde = frchain_now->frch_cfi_data->cur_fde_data;
+  fde->pauth_key = AARCH64_PAUTH_KEY_B;
+}
+
 #ifdef OBJ_ELF
 /* Emit BFD_RELOC_AARCH64_TLSDESC_ADD on the next ADD instruction.  */
 
@@ -2067,6 +2075,7 @@ const pseudo_typeS md_pseudo_table[] = {
   {"arch", s_aarch64_arch, 0},
   {"arch_extension", s_aarch64_arch_extension, 0},
   {"inst", s_aarch64_inst, 0},
+  {"cfi_b_key_frame", s_aarch64_cfi_b_key_frame, 0},
 #ifdef OBJ_ELF
   {"tlsdescadd", s_tlsdescadd, 0},
   {"tlsdesccall", s_tlsdesccall, 0},