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

[Jason Molenda via lldb-dev]

> On Oct 8, 2020, at 9:17 PM, Greg Clayton <clayborg at gmail.com> wrote:
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>> On Oct 8, 2020, at 8:55 PM, Jason Molenda <jmolenda at apple.com> wrote:
>> 
>> Good bug find!
>> 
>> It seems to me that DWARFCallFrameInfo should push the initial CIE register setup instructions as being the state at offset 0 in the function (in fact I'd say it's a bug if it's not). If that were done, then getting RowAtIndex(0) should be synonymous with get-the-CIE-register-unwind-rules, and this code would be correct.
>> 
>> Looking at DWARFCallFrameInfo::FDEToUnwindPlan, we do
>> 
>> unwind_plan.SetPlanValidAddressRange(range);
>> UnwindPlan::Row *cie_initial_row = new UnwindPlan::Row;
>> *cie_initial_row = cie->initial_row;
>> UnwindPlan::RowSP row(cie_initial_row);
>> 
>> unwind_plan.SetRegisterKind(GetRegisterKind());
>> unwind_plan.SetReturnAddressRegister(cie->return_addr_reg_num);
>> 
>> cie->initial_row is set by DWARFCallFrameInfo::HandleCommonDwarfOpcode.
>> 
>> I think the bug here is DWARFCallFrameInfo::FDEToUnwindPlan not pushing that initial row at offset 0, isn't it? We don't really use DWARF CFI on darwin any more so I don't have a lot of real world experience here.
> 
> The only opcodes that push a row are DW_CFA_advance_locXXX and DW_CFA_set_loc, so I don't think that is the right fix. I think we need to pass a copy of just the registers from the "cie->initial_row" object around to the opcode parsing code for restoration purposes.


I think everything I'm saying here is besides the point, though.  Unless we ALWAYS push the initial unwind state (from the CIE) to an UnwindPlan, the DW_CFA_restore is not going to work.  If an unwind rule for r12 says "DW_CFA_restore" and at offset 0 in the function, the unwind rule for r12 was "same" (i.e. no rule), but we return the RowAtIndex(0) and the first instruction, I don't know, spills it or something, then the DW_CFA_restore would set the r12 rule to "r12 was spilled" instead of "r12 is same".

So the only way DW_CFA_restore would behave correctly, with this, is if we always push a Row at offset 0 with the rules from the CIE, or with no rules at all, just the initial unwind state showing how the CFA is set and no register rules.


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>>> On Oct 8, 2020, at 4:01 PM, Greg Clayton <clayborg at gmail.com> 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
>>> 
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