On Fri, Feb 22, 2019 at 11:27:05AM -0800, Alexei Starovoitov 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. Slightly related and FWIW, BCC's eBPF-based opensnoop tool [1] installs a kprobe on do_sys_open to monitor calls to the open syscall globally. do_sys_open() has prototype: long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode); This causes a "blank" filename to be displayed by opensnoop when I run it on my Pixel 3 (arm64), possibly because this is a user pointer. However, it works fine on x86-64. So it seems to me that on arm64, reading user pointers directly still doesn't work even if there is a distinction between user/kernel addresses. In that case reading the user pointer using user accessors (possibly using bpf_probe_user_read helper) should be needed to fix this issue (as Yonghong also privately discussed with me). [1] https://github.com/iovisor/bcc/blob/master/tools/opensnoop.py#L140 thanks! - Joel > > 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. >
