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
_______________________________________________
lldb-dev mailing list
lldb-dev@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
