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

[Kevin Buettner <kevinb@redhat.com>]

On Thu, 23 Apr 2026 13:37:06 -0400
Simon Marchi <simon.marchi@efficios.com> wrote:

> 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.

LGTM.

Approved-by: Kevin Buettner <kevinb@redhat.com>