[lldb-dev] [llvm-dev] Continuing from dbgtrap on different targets
Jim Ingham via lldb-dev
lldb-dev at lists.llvm.org
Thu Mar 5 10:39:25 PST 2020
That shouldn't be too hard to do. We already have machinery to inject a "step over breakpoint trap" ThreadPlan when we continue, so we could use the same detection point to just move the PC before continuing in the case of an unrecognized trap.
> On Mar 5, 2020, at 10:10 AM, Jim Ingham via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> BTW, I think the better way to handle this in lldb is not to move the PC past the trap when you stop, but that when CONTINUING past a trap that we don't recognize, we always make sure we start past the trap. That way it won't look weird when people compare crash logs coming from hitting a trap with backtraces in lldb, but we would still get the desired behavior.
>> On Mar 4, 2020, at 11:29 PM, Pavel Labath <pavel at labath.sk> wrote:
>> On 04/03/2020 21:45, Jim Ingham via llvm-dev wrote:
>>> As you have seen, different machine architectures do different things after hitting a trap. On x86_64, the trap instruction is executed, and then you stop, so the PC is left after the stop. On arm64, when execution halts the pc is still pointing at the trap instruction.
>>> I don't think lldb should be in the business of telling systems how they should report stops, especially since that is certainly something we can handle in lldb.
>>> For traps that lldb recognizes as ones it is using for breakpoints, it already has to handle this difference for you. But for traps we know nothing about we don't do anything special.
>>> I think it would be entirely reasonable that whenever lldb encounters a trap instruction that isn't one of ours it should always move the PC after the trap before returning control to the user. I can't see why you would want to keep hitting the trap over and over. I've received several bugs (on the Apple bug reporter side) for this feature. This might be something we teach lldb-server & debugserver to do, rather than lldb but that's an implementation detail...
>>> For now, on architectures where the trap doesn't execute, you just need to move the pc past the trap by hand (with the "thread jump" command) before continuing. That has always been safe on arm64 so far as I can tell.
>> Yes, this is something that has bugged me too.
>> While I think it would be nice if the OSes hid these architecture quirks
>> (hell, I think it would be nice if the CPU manufacturers made this
>> consistent so that the OS doesn't need to hide it), I think that
>> changing that at this point is very unlikely, and so working around it
>> in lldb is probably the best we can do.
>> I am not sure what is the official position on continuing from a debug
>> trap, but I think that without that ability, the concept would be pretty
>> useless. A quick example <https://godbolt.org/z/-8voBz> shows that clang
>> produces the "expected" output even at -O3. In fact, on aarch64,
>> __builtin_debugtrap() and __builtin_trap() produce the same instruction,
>> and the only difference between them is that the latter also triggers
>> DCE of everything coming after it.
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