[PATCH] libgcc: Fix uninitialized RA signing on AArch64 [PR107678]

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

A recent change only initializes the regs.how[] during Dwarf unwinding
which resulted in an uninitialized offset used in return address signing
and random failures during unwinding.  The fix is to use REG_SAVED_OFFSET
as the state where the return address signing bit is valid, and if the
state is REG_UNSAVED, initialize it to 0.

Passes bootstrap & regress, OK for commit?

libgcc/
        PR target/107678
        * unwind-dw2.c (execute_cfa_program): Initialize offset of
        DWARF_REGNUM_AARCH64_RA_STATE if in REG_UNSAVED state.
        * config/aarch64/aarch64-unwind.h (aarch64_frob_update_contex):
        Check state is REG_SAVED_OFFSET before using offset for RA state.

---

diff --git a/libgcc/config/aarch64/aarch64-unwind.h b/libgcc/config/aarch64/aarch64-unwind.h
index 26db9cbd9e5c526e0c410a4fc6be2bedb7d261cf..597133b3d708a50a366c8bfeff57475f5522b3f6 100644
--- a/libgcc/config/aarch64/aarch64-unwind.h
+++ b/libgcc/config/aarch64/aarch64-unwind.h
@@ -71,21 +71,15 @@ aarch64_demangle_return_addr (struct _Unwind_Context *context,
 }
 
 /* Do AArch64 private initialization on CONTEXT based on frame info FS.  Mark
-   CONTEXT as return address signed if bit 0 of DWARF_REGNUM_AARCH64_RA_STATE is
-   set.  */
+   CONTEXT as having a signed return address if DWARF_REGNUM_AARCH64_RA_STATE
+   is initialized (REG_SAVED_OFFSET state) and the offset has bit 0 set.  */
 
 static inline void
 aarch64_frob_update_context (struct _Unwind_Context *context,
 			     _Unwind_FrameState *fs)
 {
-  const int reg = DWARF_REGNUM_AARCH64_RA_STATE;
-  int ra_signed;
-  if (fs->regs.how[reg] == REG_UNSAVED)
-    ra_signed = fs->regs.reg[reg].loc.offset & 0x1;
-  else
-    ra_signed = _Unwind_GetGR (context, reg) & 0x1;
-  if (ra_signed)
-    /* The flag is used for re-authenticating EH handler's address.  */
+  if (fs->regs.how[DWARF_REGNUM_AARCH64_RA_STATE] == REG_SAVED_OFFSET
+      && (fs->regs.reg[DWARF_REGNUM_AARCH64_RA_STATE].loc.offset & 1) != 0)
     context->flags |= RA_SIGNED_BIT;
   else
     context->flags &= ~RA_SIGNED_BIT;
diff --git a/libgcc/unwind-dw2.c b/libgcc/unwind-dw2.c
index eaceace20298b9b13344aff9d1fe9ee5f9c7bd73..87f2ae065b67982ce48f74e45523d9c754a7661c 100644
--- a/libgcc/unwind-dw2.c
+++ b/libgcc/unwind-dw2.c
@@ -1203,11 +1203,16 @@ execute_cfa_program (const unsigned char *insn_ptr,
 
 	case DW_CFA_GNU_window_save:
 #if defined (__aarch64__) && !defined (__ILP32__)
-	  /* This CFA is multiplexed with Sparc.  On AArch64 it's used to toggle
-	     return address signing status.  */
-	  reg = DWARF_REGNUM_AARCH64_RA_STATE;
-	  gcc_assert (fs->regs.how[reg] == REG_UNSAVED);
-	  fs->regs.reg[reg].loc.offset ^= 1;
+	 /* This CFA is multiplexed with Sparc.  On AArch64 it's used to toggle
+	    the return address signing status.  It is initialized at the first
+	    use and the state is stored in bit 0 of the offset.  */
+	 reg = DWARF_REGNUM_AARCH64_RA_STATE;
+	 if (fs->regs.how[reg] == REG_UNSAVED)
+	   {
+	     fs->regs.how[reg] = REG_SAVED_OFFSET;
+	     fs->regs.reg[reg].loc.offset = 0;
+	   }
+	 fs->regs.reg[reg].loc.offset ^= 1;
 #else
 	  /* ??? Hardcoded for SPARC register window configuration.  */
 	  if (__LIBGCC_DWARF_FRAME_REGISTERS__ >= 32)