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

[Simon Marchi <simark@simark.ca>]

On 2026-04-23 22:18, Kevin Buettner wrote:
> 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>
> 

Thanks, pushed.

Simon