On Mon, 5 Apr 2021 09:56:48 -0500 "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx> wrote: > > > On 4/5/21 8:24 AM, Masami Hiramatsu wrote: > > Hi Madhaven, > > > > On Sat, 3 Apr 2021 22:29:12 -0500 > > "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > >>>> Check for kretprobe > >>>> =================== > >>>> > >>>> For functions with a kretprobe set up, probe code executes on entry > >>>> to the function and replaces the return address in the stack frame with a > >>>> kretprobe trampoline. Whenever the function returns, control is > >>>> transferred to the trampoline. The trampoline eventually returns to the > >>>> original return address. > >>>> > >>>> A stack trace taken while executing in the function (or in functions that > >>>> get called from the function) will not show the original return address. > >>>> Similarly, a stack trace taken while executing in the trampoline itself > >>>> (and functions that get called from the trampoline) will not show the > >>>> original return address. This means that the caller of the probed function > >>>> will not show. This makes the stack trace unreliable. > >>>> > >>>> Add the kretprobe trampoline to special_functions[]. > >>>> > >>>> FYI, each task contains a task->kretprobe_instances list that can > >>>> theoretically be consulted to find the orginal return address. But I am > >>>> not entirely sure how to safely traverse that list for stack traces > >>>> not on the current process. So, I have taken the easy way out. > >>> > >>> For kretprobes, unwinding from the trampoline or kretprobe handler > >>> shouldn't be a reliability concern for live patching, for similar > >>> reasons as above. > >>> > >> > >> Please see previous answer. > >> > >>> Otherwise, when unwinding from a blocked task which has > >>> 'kretprobe_trampoline' on the stack, the unwinder needs a way to get the > >>> original return address. Masami has been working on an interface to > >>> make that possible for x86. I assume something similar could be done > >>> for arm64. > >>> > >> > >> OK. Until that is available, this case needs to be addressed. > > > > Actually, I've done that on arm64 :) See below patch. > > (and I also have a similar code for arm32, what I'm considering is how > > to unify x86/arm/arm64 kretprobe_find_ret_addr(), since those are very > > similar.) > > > > This is applicable on my x86 series v5 > > > > https://lore.kernel.org/bpf/161676170650.330141.6214727134265514123.stgit@devnote2/ > > > > Thank you, > > > > > > I took a brief look at your changes. Looks reasonable. > > However, for now, I am going to include the kretprobe_trampoline in the special_functions[] > array until your changes are merged. At that point, it is just a matter of deleting > kretprobe_trampoline from the special_functions[] array. That is all. > > I hope that is fine with everyone. Agreed, that is reasonable unless my series is merged. Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
