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. Madhavan
