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

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

Hi Szabolcs,

> i would keep the assert: how[reg] must be either UNSAVED or UNDEFINED
> here, other how[reg] means the toggle cfi instruction is mixed with
> incompatible instructions for the pseudo reg.
>
> and i would add a comment about this e.g. saying that UNSAVED/UNDEFINED
> how[reg] is used for tracking the return address signing status and
> other how[reg] is not allowed here.

I've added the assert back and updated the comment.

Cheers,
Wilco

v3: Improve comments, add assert.

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 encode the return
address signing state in REG_UNSAVED and REG_UNDEFINED.

Passes bootstrap & regress, OK for commit?

libgcc/
        PR target/107678
        * unwind-dw2.c (execute_cfa_program): Use REG_UNSAVED/UNDEFINED
        to encode return address signing state.
        * config/aarch64/aarch64-unwind.h (aarch64_demangle_return_addr)
        Check current return address signing state.
        (aarch64_frob_update_contex): Remove.

---

diff --git a/libgcc/config/aarch64/aarch64-unwind.h b/libgcc/config/aarch64/aarch64-unwind.h
index 26db9cbd9e5c526e0c410a4fc6be2bedb7d261cf..1afc3f9d308b95bc787398263e629bab226ff1ba 100644
--- a/libgcc/config/aarch64/aarch64-unwind.h
+++ b/libgcc/config/aarch64/aarch64-unwind.h
@@ -29,8 +29,6 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 
 #define MD_DEMANGLE_RETURN_ADDR(context, fs, addr) \
   aarch64_demangle_return_addr (context, fs, addr)
-#define MD_FROB_UPDATE_CONTEXT(context, fs) \
-  aarch64_frob_update_context (context, fs)
 
 static inline int
 aarch64_cie_signed_with_b_key (struct _Unwind_Context *context)
@@ -55,42 +53,27 @@ aarch64_cie_signed_with_b_key (struct _Unwind_Context *context)
 
 static inline void *
 aarch64_demangle_return_addr (struct _Unwind_Context *context,
-			      _Unwind_FrameState *fs ATTRIBUTE_UNUSED,
+			      _Unwind_FrameState *fs,
 			      _Unwind_Word addr_word)
 {
   void *addr = (void *)addr_word;
-  if (context->flags & RA_SIGNED_BIT)
+  const int reg = DWARF_REGNUM_AARCH64_RA_STATE;
+
+  if (fs->regs.how[reg] == REG_UNSAVED)
+    return addr;
+
+  /* Return-address signing state is toggled by DW_CFA_GNU_window_save (where
+     REG_UNDEFINED means enabled), or set by a DW_CFA_expression.  */
+  if (fs->regs.how[reg] == REG_UNDEFINED
+      || (_Unwind_GetGR (context, reg) & 0x1) != 0)
     {
       _Unwind_Word salt = (_Unwind_Word) context->cfa;
       if (aarch64_cie_signed_with_b_key (context) != 0)
 	return __builtin_aarch64_autib1716 (addr, salt);
       return __builtin_aarch64_autia1716 (addr, salt);
     }
-  else
-    return addr;
-}
-
-/* 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.  */
-
-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.  */
-    context->flags |= RA_SIGNED_BIT;
-  else
-    context->flags &= ~RA_SIGNED_BIT;
 
-  return;
+  return addr;
 }
 
 #endif /* defined AARCH64_UNWIND_H && defined __ILP32__ */
diff --git a/libgcc/unwind-dw2.c b/libgcc/unwind-dw2.c
index eaceace20298b9b13344aff9d1fe9ee5f9c7bd73..55fe35520106e848c5d4aea4e7104bf4a0c14891 100644
--- a/libgcc/unwind-dw2.c
+++ b/libgcc/unwind-dw2.c
@@ -139,7 +139,6 @@ struct _Unwind_Context
 #define EXTENDED_CONTEXT_BIT ((~(_Unwind_Word) 0 >> 2) + 1)
   /* Bit reserved on AArch64, return address has been signed with A or B
      key.  */
-#define RA_SIGNED_BIT ((~(_Unwind_Word) 0 >> 3) + 1)
   _Unwind_Word flags;
   /* 0 for now, can be increased when further fields are added to
      struct _Unwind_Context.  */
@@ -1204,10 +1203,15 @@ 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.  */
+	     return address signing status.  The REG_UNDEFINED/UNSAVED states
+	     mean RA signing is enabled/disabled.  */
 	  reg = DWARF_REGNUM_AARCH64_RA_STATE;
-	  gcc_assert (fs->regs.how[reg] == REG_UNSAVED);
-	  fs->regs.reg[reg].loc.offset ^= 1;
+	  gcc_assert (fs->regs.how[reg] == REG_UNSAVED
+		      || fs->regs.how[reg] == REG_UNDEFINED);
+	  if (fs->regs.how[reg] == REG_UNSAVED)
+	    fs->regs.how[reg] = REG_UNDEFINED;
+	  else
+	    fs->regs.how[reg] = REG_UNSAVED;
 #else
 	  /* ??? Hardcoded for SPARC register window configuration.  */
 	  if (__LIBGCC_DWARF_FRAME_REGISTERS__ >= 32)