On Fri, Nov 02, 2018 at 09:16:58AM -0400, Steven Rostedt wrote: > On Fri, 2 Nov 2018 17:59:32 +1100 > Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > > As an aside, I just tested with the frame unwinder and it isn't thrown > > off-course by kretprobe_trampoline (though obviously the stack is still > > wrong). So I think we just need to hook into the ORC unwinder to get it > > to continue skipping up the stack, as well as add the rewriting code for > > the stack traces (for all unwinders I guess -- though ideally we should > > I agree that this is the right solution. Sounds good to me. However, it would be *really* nice if function graph and kretprobes shared the same infrastructure, like they do for function entry. There's a lot of duplicated effort there. > > do this without having to add the same code to every architecture). > > True, and there's an art to consolidating the code between > architectures. > > I'm currently looking at function graph and seeing if I can consolidate > it too. And I'm also trying to get multiple uses to hook into its > infrastructure. I think I finally figured out a way to do so. > > The reason it is difficult, is that you need to maintain state between > the entry of a function and the exit for each task and callback that is > registered. Hence, it's a 3x tuple (function stack, task, callbacks). > And this must be maintained with preemption. A task may sleep for > minutes, and the state needs to be retained. > > The only state that must be retained is the function stack with the > task, because if that gets out of sync, the system crashes. But the > callback state can be removed. > > Here's what is there now: > > When something is registered with the function graph tracer, every > task gets a shadowed stack. A hook is added to fork to add shadow > stacks to new tasks. Once a shadow stack is added to a task, that > shadow stack is never removed until the task exits. > > When the function is entered, the real return code is stored in the > shadow stack and the trampoline address is put in its place. > > On return, the trampoline is called, and it will pop off the return > code from the shadow stack and return to that. > > The issue with multiple users, is that different users may want to > trace different functions. On entry, the user could say it doesn't want > to trace the current function, and the return part must not be called > on exit. Keeping track of which user needs the return called is the > tricky part. > > Here's what I plan on implementing: > > Along with a shadow stack, I was going to add a 4096 byte (one page) > array that holds 64 8 byte masks to every task as well. This will allow > 64 simultaneous users (which is rather extreme). If we need to support > more, we could allocate another page for all tasks. The 8 byte mask > will represent each depth (allowing to do this for 64 function call > stack depth, which should also be enough). > > Each user will be assigned one of the masks. Each bit in the mask > represents the depth of the shadow stack. When a function is called, > each user registered with the function graph tracer will get called > (if they asked to be called for this function, via the ftrace_ops > hashes) and if they want to trace the function, then the bit is set in > the mask for that stack depth. > > When the function exits the function and we pop off the return code > from the shadow stack, we then look at all the bits set for the > corresponding users, and call their return callbacks, and ignore > anything that is not set. > > > When a user is unregistered, it the corresponding bits that represent > it are cleared, and it the return callback will not be called. But the > tasks being traced will still have their shadow stack to allow it to > get back to normal. > > I'll hopefully have a prototype ready by plumbers. Why do we need multiple users? It would be a lot simpler if we could just enforce a single user per fgraphed/kretprobed function (and return -EBUSY if it's already being traced/probed). > And this too will require each architecture to probably change. As a > side project to this, I'm going to try to consolidate the function > graph code among all the architectures as well. Not an easy task. Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the arches? If so, I think have an old crusty patch which attempted to that. I could try to dig it up if you're interested. -- Josh
