> On Feb 22, 2019, at 11:34 AM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > >> 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. > If so, the uapi is wrong: a long-sized number does not reliably identify an address if you don’t separately know whether it’s a user or kernel address. s390x and 4G:4G x86_32 are the notable exceptions. I have lobbied for RISC-V and future x86_64 to join the crowd. I don’t know whether I’ll win this fight, but the uapi will probably have to change for at least s390x. What to do about existing scripts is a different question.
