[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <f78b0a02-9469-32c5-d8af-78335010660b@fb.com>
Date: Fri, 8 May 2020 22:30:41 -0700
From: Alexei Starovoitov <ast@...com>
To: Yonghong Song <yhs@...com>,
Andrii Nakryiko <andrii.nakryiko@...il.com>
CC: Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
Martin KaFai Lau <kafai@...com>,
Networking <netdev@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v3 13/21] bpf: add bpf_seq_printf and
bpf_seq_write helpers
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@...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@...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@...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.
Powered by blists - more mailing lists