[lldb-dev] LLDB might not be handling DW_CFA_restore or DW_CFA_restore_extended correctly in all cases

Fangrui Song via lldb-dev lldb-dev at lists.llvm.org
Thu Oct 8 16:48:21 PDT 2020

On 2020-10-08, Greg Clayton via lldb-dev wrote:
>Hello LLDB devs,
>This is a deep dive into an issue I found in the LLDB handling of DWARF call frame information, so no need to read further if this doesn't interest you!
>I am in the process of adding some support to LLVM for parsing the opcode state machines for CIE and FDE objects that produces unwind rows. While making unit tests to test DW_CFA_restore and DW_CFA_restore_extended opcodes, I read the DWARF spec that states:
>"The DW_CFA_restore instruction takes a single operand (encoded with the opcode) that represents a register number. The required action is to change the rule for the indicated register to the rule assigned it by the initial_instructions in the CIE."
>Looking at the LLDB code in DWARFCallFrameInfo.cpp I see code that is simplified to:
>case DW_CFA_restore:
>  if (unwind_plan.IsValidRowIndex(0) &&
>      unwind_plan.GetRowAtIndex(0)->GetRegisterInfo(reg_num, reg_location))
>          row->SetRegisterInfo(reg_num, reg_location);
>  break;
>The issue is, the CIE contains initial instructions, but it doesn't push a row after doing these instructions, the FDE will push a row when it emits a DW_CFA_advance_loc, DW_CFA_advance_loc1, DW_CFA_advance_loc2, DW_CFA_advance_loc4 or DW_CFA_set_loc opcode. So the DWARF spec says we should restore the register rule to be what it was in the CIE's initial instructions, but we are restoring it to the first row that was parsed. This will mostly not get us into trouble because .debug_frame and .eh_frame usually have a DW_CFA_advance_locXXX or DW_CFA_set_loc opcode as the first opcode, but it isn't a requirement and a FDE could modify a register value prior to pushing the first row at index zero. So we might be restoring the register incorrectly in some cases according to the spec. Also, what if there was no value specified in the CIE's initial instructions for a register? Should we remove the register value to match the state of the CIE's initial instructions if there is no rule for the register? We are currently leaving this register as having the same value if there is no value for the register in the first row.
>Let me know what you think.
>Greg Clayton

Thanks for noticing this. This is indeed a bug.

nongnu.org libunwind is correct:

llvm libunwind (DwarfParser.hpp) makes an approximation by only tracking
the first instruction setting up a register (it works in practice
because initial instructions don't use multiple setup instructions on one register)

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