[PATCH 2/2] gdb/dwarf: fix internal error when FDEs do not describe the CFA

[Simon Marchi <simon.marchi@efficios.com>]

From: Simon Marchi <simon.marchi@polymtl.ca>

This patch fixes an internal error problem that happens when a frame
description entry does not defined the Canonical Frame Address (CFA).
This problem was initially reported downstream as a ROCgdb issue (see
Bug trailer below), but I wrote a reproducer that uses the .debug_frame
functionality added to the DWARF assembler in the previous patch.

The error is:

    /home/smarchi/src/binutils-gdb/gdb/dwarf2/frame.c:1046: internal-error: Unknown CFA rule.

The original bug was encountered while debugging a GPU kernel written
with Triton [1].  From what I understand, the generated kernel does not
really use a stack, so the .debug_frame contents generated is quite
bare:

    $ readelf --debug-dump=frames k
    Contents of the .debug_frame section:

    00000000 000000000000000c ffffffff CIE
      Version:               4
      Augmentation:          ""
      Pointer Size:          8
      Segment Size:          0
      Code alignment factor: 4
      Data alignment factor: 4
      Return address column: 16

      DW_CFA_nop

    00000010 0000000000000014 00000000 FDE cie=00000000 pc=0000000000001600..0000000000001704

For those who don't speak fluent .debug_frame, what we see here is a
Frame Description Entry (FDE) that doesn't define any register rule,
referring to a Common Information Entry (CIE) that also doesn't define
any initial register rule.  This is equivalent to having no unwind
information at all.  One question is: why generate these at all?  I
suppose that this is an edge case, that the compiler is written in a way
that that presumes there will always be some unwind info.  That there is
not "if unwind info is empty, skip emitting the FDE" check.  Anyway, the
important thing for us is that these can be found in the wild, so GDB
shouldn't crash.

The first part of the fix is to handle CFA_UNSET in dwarf2_frame_cache
(and do nothing).  CFA_UNSET is the initial state when we start
interpreting a CFA program, meaning that we don't know yet how the CFA
is defined.  In our case, it remains unset after interpreting the CFA
program.

Then, we would ideally want to get into this `if` below that sets
undefined_retaddr:

  if (fs.retaddr_column < fs.regs.reg.size ()
      && fs.regs.reg[fs.retaddr_column].how == DWARF2_FRAME_REG_UNDEFINED)
    cache->undefined_retaddr = true;

Setting undefined_retaddr has two effects:

 - dwarf2_frame_this_id won't try to build a frame id from the CFA
 - dwarf2_frame_unwind_stop_reason will return UNWIND_OUTERMOST, which
   is the most accurate thing we can return here (there is not outer
   frame)

However, the way it is written currently, we don't get info the if.
`fs.regs.reg.size ()` is 0, so the condition always evaluates to false.
The `fs.regs.reg` is a vector that is expanded as needed: if an
operation sets a rule for register N, then we'll resize the vector so it
holds at least `N + 1` elements.  But conceptually, all register columns
initially contain "undefined".  If we arrive to this condition and the
vector hasn't been expanded to include a given column, then it means
that the rule for this column is "undefined".  Therefore, rewrite the
condition to consider the return address as undefined if the vector is
too small to include retaddr_column.

Here are some relevant references to DWARF 5:

 - Section 6.4.1. ("Structure of Call Frame Information")

     The default rule for all columns before interpretation of the initial
     instructions is the undefined rule.

 - Section 6.4.4 ("Call Frame Calling Address")

     If a Return Address register is defined in the virtual unwind
     table, and its rule is undefined (for example, by
     DW_CFA_undefined), then there is no return address and no call
     address, and the virtual unwind of stack activations is complete.

Add a test case written using the DWARF assembler that reproduces the
issue.  The user experience in this case is that the frame appears as
the outer most frame:

    (gdb) bt
    #0  0x000055555555511d in main ()
    (gdb) up
    ❌️ Initial frame selected; you cannot go up.
    (gdb) frame 1
    ❌️ No frame at level 1.

[1] https://triton-lang.org/

Change-Id: I67c717ff03a41c0630a73ce9549d88ff363e8cea
Bug: https://github.com/ROCm/ROCgdb/issues/47
---
 gdb/dwarf2/frame.c                            |  7 ++-
 .../gdb.dwarf2/debug-frame-no-cfa.exp         | 51 +++++++++++++++++++
 2 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp

diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
index 152bebef0e30..fe74a4f65223 100644
--- a/gdb/dwarf2/frame.c
+++ b/gdb/dwarf2/frame.c
@@ -962,6 +962,9 @@ dwarf2_frame_cache (const frame_info_ptr &this_frame, void **this_cache)
       /* Calculate the CFA.  */
       switch (fs.regs.cfa_how)
 	{
+	case CFA_UNSET:
+	  break;
+
 	case CFA_REG_OFFSET:
 	  cache->cfa = read_addr_from_reg (this_frame, fs.regs.cfa_reg);
 	  if (fs.armcc_cfa_offsets_reversed)
@@ -1074,8 +1077,8 @@ incomplete CFI data; unspecified registers (e.g., %s) at %s"),
 	}
     }
 
-  if (fs.retaddr_column < fs.regs.reg.size ()
-      && fs.regs.reg[fs.retaddr_column].how == DWARF2_FRAME_REG_UNDEFINED)
+  if (fs.retaddr_column >= fs.regs.reg.size ()
+      || fs.regs.reg[fs.retaddr_column].how == DWARF2_FRAME_REG_UNDEFINED)
     cache->undefined_retaddr = true;
 
   dwarf2_tailcall_sniffer_first (this_frame, &cache->tailcall_cache,
diff --git a/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp b/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp
new file mode 100644
index 000000000000..e8397b253e41
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp
@@ -0,0 +1,51 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test GDB against an FDE in the .debug_frame section that doesn't set a rule
+# for the CFA.
+
+load_lib dwarf.exp
+
+require dwarf2_support
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    get_func_info main
+
+    frame {
+	declare_labels cie_label
+
+	cie_label: CIE {
+	    return_address_register 16
+	} {}
+
+	FDE $cie_label $main_start $main_len {} {
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	[list $srcfile $asm_file] {nodebug}] } {
+    return
+}
+
+if { ![runto_main] } {
+    return
+}
+
+# This would cause an internal error in dwarf2_frame_cache.
+gdb_test "backtrace" "#0 .* main .*"
-- 
2.53.0