[binutils-gdb] gdb/dwarf: fix internal error in dwarf2_fetch_cfa_info for FDEs without a CFA rule

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=38e1f10568dc62a9c7fb1fb2f508595d787d0171

commit 38e1f10568dc62a9c7fb1fb2f508595d787d0171
Author: Simon Marchi <[email protected]>
Date:   Thu Apr 23 13:37:06 2026 -0400

    gdb/dwarf: fix internal error in dwarf2_fetch_cfa_info for FDEs without a CFA rule
    
    Commit a910478f65a5 ("gdb/dwarf: fix internal error when FDEs do not
    describe the CFA") fixed the switch on cfa_how in dwarf2_frame_cache,
    but there is a very similar switch in dwarf2_fetch_cfa_info that has
    the same issue.  If the unwind info does not set a rule for the CFA,
    fs.regs.cfa_how is left as CFA_UNSET and we hit the default case,
    triggering an internal error.
    
    dwarf2_fetch_cfa_info is called from two places, both while compiling
    a DWARF expression that contains DW_OP_call_frame_cfa:
    
     - dwarf2_compile_expr_to_ax, when compiling to agent-expression
       bytecode
    
     - do_compile_dwarf_expr_to_c, when compiling to C source (used by
       the "compile" commands).
    
    Unlike dwarf2_frame_cache, where we can degrade gracefully by setting
    undefined_retaddr, the callers of dwarf2_fetch_cfa_info need a concrete
    CFA to generate code, so we can't continue.  Throw an error with the
    same message used a few lines above, when the FDE itself cannot be
    found.
    
    Extend the debug-frame-no-cfa.exp test to reproduce this case: add a
    compile unit describing the "main" function and with a DW_AT_frame_base
    that uses DW_OP_call_frame_cfa and a local variable "x" that uses
    DW_OP_fbreg (the evaluation of which requires the CFA).  Then, try to
    translate the location of "x" to agent-expression bytecode with "maint"
    agent.  This hits the internal error without the corresponding fix.
    
    Change-Id: I82349e3d9259c8f943eabee5c2fce360876feee8
    Approved-by: Kevin Buettner <[email protected]>

Diff:
---
 gdb/dwarf2/frame.c                              |  3 +++
 gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp | 36 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
index 2301d914637..70895e90696 100644
--- a/gdb/dwarf2/frame.c
+++ b/gdb/dwarf2/frame.c
@@ -793,6 +793,9 @@ dwarf2_fetch_cfa_info (struct gdbarch *gdbarch, CORE_ADDR pc,
   /* Calculate the CFA.  */
   switch (fs.regs.cfa_how)
     {
+    case CFA_UNSET:
+      error (_("Could not compute CFA; needed to translate this expression"));
+
     case CFA_REG_OFFSET:
       {
 	int regnum = dwarf_reg_to_regnum_or_error (gdbarch, fs.regs.cfa_reg);
diff --git a/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp b/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp
index 8442b5c3ddb..3f7d77cf074 100644
--- a/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp
+++ b/gdb/testsuite/gdb.dwarf2/debug-frame-no-cfa.exp
@@ -26,6 +26,36 @@ set asm_file [standard_output_file $srcfile2]
 Dwarf::assemble $asm_file {
     get_func_info main
 
+    cu { version 5 } {
+	DW_TAG_compile_unit {
+	    DW_AT_name debug-frame-no-cfa.c
+	    DW_AT_comp_dir /tmp
+	} {
+	    declare_labels int_type_label
+
+	    int_type_label: DW_TAG_base_type {
+		DW_AT_name "int"
+		DW_AT_encoding @DW_ATE_signed
+		DW_AT_byte_size 4 DW_FORM_sdata
+	    }
+
+	    DW_TAG_subprogram {
+		MACRO_AT_func { main }
+		DW_AT_frame_base {
+		    DW_OP_call_frame_cfa
+		} SPECIAL_expr
+	    } {
+		DW_TAG_variable {
+		    DW_AT_name x
+		    DW_AT_type :$int_type_label
+		    DW_AT_location {
+			DW_OP_fbreg 0
+		    } SPECIAL_expr
+		}
+	    }
+	}
+    }
+
     frame {
 	declare_labels cie_label
 
@@ -52,3 +82,9 @@ if { ![runto_main] } {
 # Make sure to match a single line, so that the test fails if an error
 # about stack unwind is printed after frame 0.
 gdb_test "backtrace" "^#0 \[^\r\n\]* main \[^\r\n\]*"
+
+# Translating the location of "x" to an agent expression requires
+# evaluating its frame base, which uses DW_OP_call_frame_cfa.  This would
+# cause an internal error in dwarf2_fetch_cfa_info.
+gdb_test "maint agent x" \
+    "Could not compute CFA; needed to translate this expression"