[Lldb-commits] [PATCH] D124198: [LLDB][Unwind] Add stack scanning as fallback unwind plan if no symbol file is available.

[Pavel Labath via Phabricator via lldb-commits]

labath added a comment.

The patch description could definitely use more details about the motivation for the change, and a description of how it works. Apart from the fact that it uses the raSearch feature I introduced a while back, I don't know much about how it works (and given the planned changes, I am not going to try to understand that), but I think I can provide some details/thoughts about the motivation.

(IIUC), the motivating case is some code in kernel32.dll, which ends up setting rbp to zero. I don't know why it does that, but I suspect it may have something to do with the windows syscall convention (I know that ebp is used in i386 linux, for instance).  While it would be definitely interesting to understand why it does that, I don't think it really matters -- one can never really exclude the possibility that some code (inline asm perhaps) will be messing with the frame pointer. So, scanning the stack for return addresses as a *last* last resort (the architectural default plan is already a kind of a last resort option) seems like it could be useful, regardless of the architecture (although the actual algorithm may differ somewhat for x86, which stores the return address on the stack directly, and say arm, which uses a link register). And given that the choice is between "showing nothing" and "showing something potentially incorrect", I don't think we need to be extra careful about vetting this address. All the checks Greg mentioned seem fine, but I don't see a compelling reason to implement them (all of them), especially given that they would not be useful in this case -- all we have is a minidump core file with some stack memory and a list of loaded modules (not necessarily the modules themselves).  In this case, all we can do is to check whether the pointer points to some known code region (which is already done IIRC).


Repository:
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  https://reviews.llvm.org/D124198/new/

https://reviews.llvm.org/D124198