Re: [PATCH bpf-next v3 13/21] bpf: add bpf_seq_printf and bpf_seq_write helpers

[Alexei Starovoitov <ast@fb.com>]

On 5/8/20 9:18 PM, Yonghong Song wrote:
> 
> 
> On 5/8/20 12:44 PM, Andrii Nakryiko wrote:
>> On Wed, May 6, 2020 at 10:40 PM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> Two helpers bpf_seq_printf and bpf_seq_write, are added for
>>> writing data to the seq_file buffer.
>>>
>>> bpf_seq_printf supports common format string flag/width/type
>>> fields so at least I can get identical results for
>>> netlink and ipv6_route targets.
>>>
>>> For bpf_seq_printf and bpf_seq_write, return value -EOVERFLOW
>>> specifically indicates a write failure due to overflow, which
>>> means the object will be repeated in the next bpf invocation
>>> if object collection stays the same. Note that if the object
>>> collection is changed, depending how collection traversal is
>>> done, even if the object still in the collection, it may not
>>> be visited.
>>>
>>> bpf_seq_printf may return -EBUSY meaning that internal percpu
>>> buffer for memory copy of strings or other pointees is
>>> not available. Bpf program can return 1 to indicate it
>>> wants the same object to be repeated. Right now, this should not
>>> happen on no-RT kernels since migrate_disable(), which guards
>>> bpf prog call, calls preempt_disable().
>>>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>> ---
>>>   include/uapi/linux/bpf.h       |  32 +++++-
>>>   kernel/trace/bpf_trace.c       | 200 +++++++++++++++++++++++++++++++++
>>>   scripts/bpf_helpers_doc.py     |   2 +
>>>   tools/include/uapi/linux/bpf.h |  32 +++++-
>>>   4 files changed, 264 insertions(+), 2 deletions(-)
>>>
>>
>> Was a bit surprised by behavior on failed memory read, I think it's
>> important to emphasize and document this. But otherwise:
>>
>> Acked-by: Andrii Nakryiko <andriin@fb.com>
>>
>> [...]
>>
>>> +               if (fmt[i] == 's') {
>>> +                       /* try our best to copy */
>>> +                       if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
>>> +                               err = -E2BIG;
>>> +                               goto out;
>>> +                       }
>>> +
>>> +                       bufs->buf[memcpy_cnt][0] = 0;
>>> +                       strncpy_from_unsafe(bufs->buf[memcpy_cnt],
>>> +                                           (void *) (long) 
>>> args[fmt_cnt],
>>> +                                           MAX_SEQ_PRINTF_STR_LEN);
>>
>> So the behavior is that we try to read string, but if it fails, we
>> treat it as empty string? That needs to be documented, IMHO. My
>> expectation was that entire printf would fail.
> 
> Let me return proper error. Currently, two possible errors may happen:
>    - user provide an invalid address, yes, an error should be returned
>      and we should not do anything
>    - user provide a valid address, but it needs page fault happening
>      to read the content. With current implementation,
>      strncpy_from_unsafe will return fail. Future sleepable
>      bpf program will help for this case, so an error means a
>      real address error.

It matches what bpf_trace_printk() is doing.
I suggest to defer any improvements to later patches.
Both should be consistent.