On Fri, Feb 22, 2019 at 02:30:26PM -0500, Steven Rostedt wrote: > On Fri, 22 Feb 2019 11:27:05 -0800 > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > > On Fri, Feb 22, 2019 at 09:43:14AM -0800, Linus Torvalds wrote: > > > > > > Then we should still probably fix up "__probe_kernel_read()" to not > > > allow user accesses. The easiest way to do that is actually likely to > > > use the "unsafe_get_user()" functions *without* doing a > > > uaccess_begin(), which will mean that modern CPU's will simply fault > > > on a kernel access to user space. > > > > On bpf side the bpf_probe_read() helper just calls probe_kernel_read() > > and users pass both user and kernel addresses into it and expect > > that the helper will actually try to read from that address. > > > > If __probe_kernel_read will suddenly start failing on all user addresses > > it will break the expectations. > > How do we solve it in bpf_probe_read? > > Call probe_kernel_read and if that fails call unsafe_get_user byte-by-byte > > in the loop? > > That's doable, but people already complain that bpf_probe_read() is slow > > and shows up in their perf report. > > We're changing kprobes to add a specific flag to say that we want to > differentiate between kernel or user reads. Can this be done with > bpf_probe_read()? If it's showing up in perf report, I doubt a single so you're saying you will break existing kprobe scripts? I don't think it's a good idea. It's not acceptable to break bpf_probe_read uapi.
