[PATCH v2] AArch64 pauth: Indicate unmasked address in backtrace for kernel

[Kuan-Ying Lee]

Armv8.3-a Pointer Authentication causes the function return address to
be changed. GDB needs to use address bit[55] to know which mode is active
and unmask the link register in order to get backtrace.
---
v2:
 - Rewrite the commit message
 - Add a new function aarch64_unmask_address() for PAC
   suggested by Luis in [1]
 - Define VA range select bit
 [1] https://sourceware.org/pipermail/gdb-patches/2021-October/182835.html

 gdb/aarch64-tdep.c | 16 +++++++++++++++-
 gdb/aarch64-tdep.h |  4 ++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 42b8494980..a96aed7567 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -243,6 +243,20 @@ class instruction_reader : public abstract_instruction_reader
 
 } // namespace
 
+/* Return unmasked address by using different unmask method
+   depending on AARCH64_PAC_VA_RANGE_BIT  */
+
+static CORE_ADDR
+aarch64_unmask_address(CORE_ADDR address, CORE_ADDR mask)
+{
+    /* Unmask kernel mode and user mode addresses appropriately based on
+       the VA range bit  */
+    if (address & AARCH64_PAC_VA_RANGE_MASK)
+        return address | mask;
+    else
+        return address & ~mask;
+}
+
 /* If address signing is enabled, mask off the signature bits from the link
    register, which is passed by value in ADDR, using the register values in
    THIS_FRAME.  */
@@ -257,7 +271,7 @@ aarch64_frame_unmask_lr (struct gdbarch_tdep *tdep,
     {
       int cmask_num = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);
       CORE_ADDR cmask = frame_unwind_register_unsigned (this_frame, cmask_num);
-      addr = addr & ~cmask;
+      addr = aarch64_unmask_address(addr, cmask);
 
       /* Record in the frame that the link register required unmasking.  */
       set_frame_previous_pc_masked (this_frame);
diff --git a/gdb/aarch64-tdep.h b/gdb/aarch64-tdep.h
index 7bf612b012..9c50988d6e 100644
--- a/gdb/aarch64-tdep.h
+++ b/gdb/aarch64-tdep.h
@@ -59,6 +59,10 @@ struct regset;
    single-stepped instruction.  */
 #define AARCH64_DISPLACED_MODIFIED_INSNS 1
 
+/* VA range select bit for Pointer Authentication Code (PAC) feature  */
+#define AARCH64_PAC_VA_RANGE_BIT 55
+#define AARCH64_PAC_VA_RANGE_MASK (1ULL << AARCH64_PAC_VA_RANGE_BIT)
+
 /* Target-dependent structure in gdbarch.  */
 struct gdbarch_tdep
 {
-- 
2.25.1