Hi Jann, On Wed, 20 Feb 2019 14:57:31 +0100 Jann Horn <jannh@xxxxxxxxxx> wrote: > 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...) Ok, we are going back to the start point of this thread :) http://lkml.kernel.org/r/20190215174712.372898450@xxxxxxxxxxx So, if user tells kprobe it is user-pointer, we check it with access_ok(), and will do something similar to the strnlen_user() and strncpy_from_user(), but using __copy_from_user_inatomic() and pagefault_disable() for kprobes. Thank you! -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
