> On Feb 22, 2019, at 11:34 AM, Alexei Starovoitov
> <alexei.starovoi...@gmail.com> 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.starovoi...@gmail.com> 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.
