Re: [PATCH v11 net-next 00/12] eBPF syscall, verifier, testsuite

[Alexei Starovoitov]

On Thu, Sep 11, 2014 at 6:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Sep 11, 2014 at 3:29 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> On Thu, Sep 11, 2014 at 2:54 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>>
>>>> the verifier log contains full trace. Last unsafe instruction + error
>>>> in many cases is useless. What we found empirically from using
>>>> it over last 2 years is that developers have different learning curve
>>>> to adjust to 'safe' style of C. Pretty much everyone couldn't
>>>> figure out why program is rejected based on last error. Therefore
>>>> verifier emits full log. From the 1st insn all the way till the last
>>>> 'unsafe' instruction. So the log is multiline output.
>>>> 'Understanding eBPF verifier messages' section of
>>>> Documentation/networking/filter.txt provides few trivial
>>>> examples of these multiline messages.
>>>> Like for the program:
>>>>   BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
>>>>   BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
>>>>   BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
>>>>   BPF_LD_MAP_FD(BPF_REG_1, 0),
>>>>   BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
>>>>   BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
>>>>   BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
>>>>   BPF_EXIT_INSN(),
>>>> the verifier log_buf is:
>>>>   0: (7a) *(u64 *)(r10 -8) = 0
>>>>   1: (bf) r2 = r10
>>>>   2: (07) r2 += -8
>>>>   3: (b7) r1 = 0
>>>>   4: (85) call 1
>>>>   5: (15) if r0 == 0x0 goto pc+1
>>>>    R0=map_ptr R10=fp
>>>>   6: (7a) *(u64 *)(r0 +4) = 0
>>>>   misaligned access off 4 size 8
>>>>
>>>> It will surely change over time as verifier becomes smarter,
>>>> supports new types, optimizations and so on.
>>>> So this log is not an ABI. It's for humans to read.
>>>> The log explains _how_ verifier came to conclusion
>>>> that the program is unsafe.
>>>
>>> Given that you've already arranged (I think) for the verifier to be
>>> compilable in the kernel and in userspace, would it make more sense to
>>> have the kernel version just say yes or no and to make it easy for
>>> user code to retry verification in userspace if they want a full
>>> explanation?
>>
>> Good memory :) Long ago I had a hack where I compiled
>> verifier.o for kernel and linked it with userspace wrappers to
>> have the same verifier for userspace. It was very fragile.
>> and maps were not separate objects and there were no fds.
>> It's not feasible anymore, since different subsystems
>> will configure different bpf_context and helper functions and
>> verifier output is dynamic based on maps that were created.
>> For example, if user's samples/bpf/sock_example.c does
>> bpf_create_map(HASH, sizeof(key) * 2, ...);
>> instead of
>> bpf_create_map(HASH, sizeof(key), ...);
>> the same program will be rejected in first case and will be
>> accepted in the second, because map sizes and ebpf
>> program expectations are mismatching.
>
> Hmm.
>
> This actually furthers my thought that the relocations should be a
> real relocation table.  Then you could encode the types of the
> referenced objects in the table, and a program could be verified
> without looking up the fds.  The only extra step would be to confirm
> that the actual types referenced match those in the table.

It's not the type is being checked, but one particular map instance
with user specified key/value sizes. type is not helpful. type is not
even used during verification. Only key_size and value_size of
elements are meaningful and they're looked up dynamically by fd.
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