[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.

7 files changed, 150 insertions, 16 deletions

diff --git a/gas/ChangeLog b/gas/ChangeLog
index dbb8a37..73cadc7 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,22 @@
+2018-12-05  Sam Tebbs  <sam.tebbs@arm.com>
+
+	* 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.
+
 2018-12-04  wu.heng  <wu.heng@zte.com.cn>
 
 	PR 23939
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},
diff --git a/gas/doc/c-aarch64.texi b/gas/doc/c-aarch64.texi
index f38fdf5..ca84c9d 100644
--- a/gas/doc/c-aarch64.texi
+++ b/gas/doc/c-aarch64.texi
@@ -435,6 +435,14 @@ as the @code{.dword} directive.
 @c YYYYYYYYYYYYYYYYYYYYYYYYYY
 @c ZZZZZZZZZZZZZZZZZZZZZZZZZZ
 
+@cindex @code{.cfi_b_key_frame} directive, AArch64
+@item	@code{.cfi_b_key_frame}
+The @code{.cfi_b_key_frame} directive inserts a 'B' character into the CIE
+corresponding to the current frame's FDE, meaning that its return address has
+been signed with the B-key.  If two frames are signed with differing keys then
+they will not share the same CIE.  This information is intended to be used by
+the stack unwinder in order to properly authenticate return addresses.
+
 @end table
 
 @node AArch64 Opcodes
diff --git a/gas/dw2gencfi.c b/gas/dw2gencfi.c
index 4add173..ff5c0df 100644
--- a/gas/dw2gencfi.c
+++ b/gas/dw2gencfi.c
@@ -403,6 +403,7 @@ struct cie_entry
   unsigned char per_encoding;
   unsigned char lsda_encoding;
   expressionS personality;
+  enum pointer_auth_key pauth_key;
   struct cfi_insn_data *first, *last;
 };
 
@@ -414,22 +415,6 @@ static struct fde_entry **last_fde_data = &all_fde_data;
 /* List of CIEs so that they could be reused.  */
 static struct cie_entry *cie_root;
 
-/* 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;
-};
-
 /* Construct a new FDE structure and add it to the end of the fde list.  */
 
 static struct fde_entry *
@@ -448,6 +433,7 @@ alloc_fde_entry (void)
   fde->per_encoding = DW_EH_PE_omit;
   fde->lsda_encoding = DW_EH_PE_omit;
   fde->eh_header_type = EH_COMPACT_UNKNOWN;
+  fde->pauth_key = AARCH64_PAUTH_KEY_A;
 
   return fde;
 }
@@ -1872,6 +1858,8 @@ output_cie (struct cie_entry *cie, bfd_boolean eh_frame, int align)
       if (cie->lsda_encoding != DW_EH_PE_omit)
 	out_one ('L');
       out_one ('R');
+      if (cie->pauth_key == AARCH64_PAUTH_KEY_B)
+	out_one ('B');
     }
   if (cie->signal_frame)
     out_one ('S');
@@ -2052,6 +2040,7 @@ select_cie_for_fde (struct fde_entry *fde, bfd_boolean eh_frame,
       if (CUR_SEG (cie) != CUR_SEG (fde))
 	continue;
       if (cie->return_column != fde->return_column
+	  || cie->pauth_key != fde->pauth_key
 	  || cie->signal_frame != fde->signal_frame
 	  || cie->per_encoding != fde->per_encoding
 	  || cie->lsda_encoding != fde->lsda_encoding)
@@ -2158,6 +2147,7 @@ select_cie_for_fde (struct fde_entry *fde, bfd_boolean eh_frame,
   cie->lsda_encoding = fde->lsda_encoding;
   cie->personality = fde->personality;
   cie->first = fde->data;
+  cie->pauth_key = fde->pauth_key;
 
   for (i = cie->first; i ; i = i->next)
     if (i->insn == DW_CFA_advance_loc
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.  */
diff --git a/gas/testsuite/gas/aarch64/pac_ab_key.d b/gas/testsuite/gas/aarch64/pac_ab_key.d
new file mode 100644
index 0000000..a428633
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/pac_ab_key.d
@@ -0,0 +1,54 @@
+#objdump: --dwarf=frames
+# Test assembling a file with functions signed by two different pointer
+# authentication keys. It must interpret .cfi_b_key_frame properly and emit a
+# 'B' character into the correct CIE's augmentation string.
+
+.+:     file .+
+
+Contents of the .eh_frame section:
+
+00000000 0000000000000010 00000000 CIE
+  Version:               1
+  Augmentation:          "zR"
+  Code alignment factor: 4
+  Data alignment factor: -8
+  Return address column: 30
+  Augmentation data:     1b
+  DW_CFA_def_cfa: r31 \(sp\) ofs 0
+
+00000014 0000000000000018 00000018 FDE cie=00000000 pc=0000000000000000..0000000000000008
+  DW_CFA_advance_loc: 4 to 0000000000000004
+  DW_CFA_GNU_window_save
+  DW_CFA_advance_loc: 4 to 0000000000000008
+  DW_CFA_def_cfa_offset: 16
+  DW_CFA_offset: r29 \(x29\) at cfa-16
+  DW_CFA_offset: r30 \(x30\) at cfa-8
+  DW_CFA_nop
+  DW_CFA_nop
+
+00000030 0000000000000014 00000000 CIE
+  Version:               1
+  Augmentation:          "zRB"
+  Code alignment factor: 4
+  Data alignment factor: -8
+  Return address column: 30
+  Augmentation data:     1b
+  DW_CFA_def_cfa: r31 \(sp\) ofs 0
+  DW_CFA_nop
+  DW_CFA_nop
+  DW_CFA_nop
+
+00000048 000000000000001c 0000001c FDE cie=00000030 pc=0000000000000008..0000000000000010
+  DW_CFA_advance_loc: 4 to 000000000000000c
+  DW_CFA_GNU_window_save
+  DW_CFA_advance_loc: 4 to 0000000000000010
+  DW_CFA_def_cfa_offset: 16
+  DW_CFA_offset: r29 \(x29\) at cfa-16
+  DW_CFA_offset: r30 \(x30\) at cfa-8
+  DW_CFA_nop
+  DW_CFA_nop
+  DW_CFA_nop
+  DW_CFA_nop
+  DW_CFA_nop
+  DW_CFA_nop
+
diff --git a/gas/testsuite/gas/aarch64/pac_ab_key.s b/gas/testsuite/gas/aarch64/pac_ab_key.s
new file mode 100644
index 0000000..4b328e7
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/pac_ab_key.s
@@ -0,0 +1,31 @@
+	.arch armv8-a
+	.text
+	.align	2
+	.global	_Z5foo_av
+	.type	_Z5foo_av, %function
+_Z5foo_av:
+.LFB0:
+	.cfi_startproc
+	hint	25 // paciasp
+	.cfi_window_save
+	stp	x29, x30, [sp, -16]!
+	.cfi_def_cfa_offset 16
+	.cfi_offset 29, -16
+	.cfi_offset 30, -8
+	.cfi_endproc
+.LFE0:
+	.size	_Z5foo_av, .-_Z5foo_av
+	.align	2
+	.global	_Z5foo_bv
+	.type	_Z5foo_bv, %function
+_Z5foo_bv:
+.LFB1:
+	.cfi_startproc
+	.cfi_b_key_frame
+	hint	27 // pacibsp
+	.cfi_window_save
+	stp	x29, x30, [sp, -16]!
+	.cfi_def_cfa_offset 16
+	.cfi_offset 29, -16
+	.cfi_offset 30, -8
+	.cfi_endproc