[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, 23 insertions, 0 deletions

diff --git a/gas/dw2gencfi.h b/gas/dw2gencfi.h
index cf534b4..2b1362a 100644
--- a/gas/dw2gencfi.h
+++ b/gas/dw2gencfi.h
@@ -135,6 +135,27 @@ enum {
   EH_COMPACT_HAS_LSDA
 };
 
+enum pointer_auth_key {
+  AARCH64_PAUTH_KEY_A,
+  AARCH64_PAUTH_KEY_B
+};
+
+/* Stack of old CFI data, for save/restore.  */
+struct cfa_save_data
+{
+  struct cfa_save_data *next;
+  offsetT cfa_offset;
+};
+
+/* Current open FDE entry.  */
+struct frch_cfi_data
+{
+  struct fde_entry *cur_fde_data;
+  symbolS *last_address;
+  offsetT cur_cfa_offset;
+  struct cfa_save_data *cfa_save_stack;
+};
+
 struct fde_entry
 {
   struct fde_entry *next;
@@ -162,6 +183,8 @@ struct fde_entry
   /* For out of line tables and FDEs.  */
   symbolS *eh_loc;
   int sections;
+  /* The pointer authentication key used.  Only used for AArch64.  */
+  enum pointer_auth_key pauth_key;
 };
 
 /* The list of all FDEs that have been collected.  */