On Wed, Feb 20, 2019 at 9:10 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > On Tue, 19 Feb 2019 14:03:30 -0500 > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > > Basically, a kprobe is mostly used for debugging what's happening in a > > > > live kernel, to read any address. > > > > > > My point is that "any address" is not sufficient to begin with. You > > > need "kernel or user". > > > > > > Having a flag for what _kind_ of kernel address is ok might then be > > > required for other cases if they might not be ok with following page > > > tables to IO space.. > > > > > > > Good point. Looks like we should add a new flag for kprobe > > trace parameters, that tell kprobes if the address is expected to be > > user or kernel. That would be good regardless of the duplicate > > meanings, as we could use copy_from_user without touching KERNEL_DS, if > > the probe argument specifically states "this is user space". For > > example, when probing do_sys_open, and you want to read what path string > > was passed into the kernel. > > > > Masami, thoughts? > > Let me ensure what you want. So you want to access a "string" in user-space, > not a data structure? In that case, it is very easy to me. It is enough to > add a "ustring" type to kprobe events. For example, do_sys_opsn's path > variable is one example. That will be +0(+0(%si)):ustring, and fetcher > finally copy the string using strncpy_from_user() instead of > strncpy_from_unsafe(). (*) [...] > (*) BTW, there is another concern to use _from_user APIs in kprobe. Are those > APIs might sleep?? If you want to access userspace without sleeping, and ignore data in non-present pages, you can do `pagefault_disable(); err = __copy_from_user_inatomic(...); pagefault_enable();`. (Actually, maybe the kernel should have a helper for that...)
