On Wed, 9 Aug 2023 18:17:47 +0200 Florent Revest <revest@xxxxxxxxxxxx> wrote: > On Wed, Aug 9, 2023 at 6:09 PM Florent Revest <revest@xxxxxxxxxxxx> wrote: > > > > On Wed, Aug 9, 2023 at 4:10 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > > > Hi Florent, > > > > > > On Wed, 9 Aug 2023 12:28:38 +0200 > > > Florent Revest <revest@xxxxxxxxxxxx> wrote: > > > > > > > On Mon, Aug 7, 2023 at 8:48 AM Masami Hiramatsu (Google) > > > > <mhiramat@xxxxxxxxxx> wrote: > > > > > > > > > > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> > > > > > > > > > > This allows fprobes to be available with CONFIG_DYNAMIC_FTRACE_WITH_ARGS > > > > > instead of CONFIG_DYNAMIC_FTRACE_WITH_REGS, then we can enable fprobe > > > > > on arm64. > > > > > > > > This patch lets fprobe code build on configs WITH_ARGS and !WITH_REGS > > > > but fprobe wouldn't run on these builds because fprobe still registers > > > > to ftrace with FTRACE_OPS_FL_SAVE_REGS, which would fail on > > > > !WITH_REGS. Shouldn't we also let the fprobe_init callers decide > > > > whether they want REGS or not ? > > > > > > Ah, I think you meant FPROBE_EVENTS? Yes I forgot to add the dependency > > > on it. But fprobe itself can work because fprobe just pass the ftrace_regs > > > to the handlers. (Note that exit callback may not work until next patch) > > > > No, I mean that fprobe still registers its ftrace ops with the > > FTRACE_OPS_FL_SAVE_REGS flag: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/fprobe.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n185 > > > > Which means that __register_ftrace_function will return -EINVAL on > > builds !WITH_REGS: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/ftrace.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n338 > > > > As documented here: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/include/linux/ftrace.h?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n188 > > > > There are two parts to using sparse pt_regs. One is "static": having > > WITH_ARGS in the config, the second one is "dynamic": a ftrace ops > > needs to specify that it doesn't want to go through the ftrace > > trampoline that saves a full pt_regs, by not giving > > FTRACE_OPS_FL_SAVE_REGS. If we want fprobe to work on builds > > !WITH_REGS then we should both remove Kconfig dependencies to > > WITH_REGS (like you've done) but also stop passing this ftrace ops > > flag. > > Said in a different way: there are arches that support both WITH_ARGS > and WITH_REGS (like x86 actually). They have two ftrace trampolines > compiled in: ftrace_caller and ftrace_regs_caller, one for each > usecase. If you register to ftrace with the FTRACE_OPS_FL_SAVE_REGS > flag you are telling it that what you want is a pt_regs. If you are > trying to move away from pt_regs and support ftrace_regs in the more > general case (meaning, in the case where it can contain a sparse > pt_regs) then you should stop passing that flag so you go through the > lighter, faster trampoline and test your code in the circumstances > where ftrace_regs isn't just a regular pt_regs but an actually sparse > or light data structure. > > I hope that makes my thoughts clearer? It's a hairy topic ahah Ah, I see your point. static void fprobe_init(struct fprobe *fp) { fp->nmissed = 0; if (fprobe_shared_with_kprobes(fp)) fp->ops.func = fprobe_kprobe_handler; else fp->ops.func = fprobe_handler; fp->ops.flags |= FTRACE_OPS_FL_SAVE_REGS; <---- This flag! } So it should be FTRACE_OPS_FL_ARGS. Let me fix that. Thank you! -- Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
