From: Yonghong Song <yhs@fb.com>
To: Florent Revest <revest@chromium.org>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
KP Singh <kpsingh@chromium.org>, bpf <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Florent Revest <revest@google.com>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper
Date: Sun, 13 Dec 2020 22:47:01 -0800 [thread overview]
Message-ID: <221fb873-80fc-5407-965e-b075c964fa13@fb.com> (raw)
In-Reply-To: <CABRcYm+zjC-WH2gxtfEX5S6mZj-5_ByAzVd5zi3aRmQv-asYqg@mail.gmail.com>
On 12/11/20 6:40 AM, Florent Revest wrote:
> On Wed, Dec 2, 2020 at 10:18 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>> I still think that adopting printk/vsnprintf for this instead of
>> reinventing the wheel
>> is more flexible and easier to maintain long term.
>> Almost the same layout can be done with vsnprintf
>> with exception of \0 char.
>> More meaningful names, etc.
>> See Documentation/core-api/printk-formats.rst
>
> I agree this would be nice. I finally got a bit of time to experiment
> with this and I noticed a few things:
>
> First of all, because helpers only have 5 arguments, if we use two for
> the output buffer and its size and two for the format string and its
> size, we are only left with one argument for a modifier. This is still
> enough for our usecase (where we'd only use "%ps" for example) but it
> does not strictly-speaking allow for the same layout that Andrii
> proposed.
See helper bpf_seq_printf. It packs all arguments for format string and
puts them into an array. bpf_seq_printf will unpack them as it parsed
through the format string. So it should be doable to have more than
"%ps" in format string.
>
>> If we force fmt to come from readonly map then bpf_trace_printk()-like
>> run-time check of fmt string can be moved into load time check
>> and performance won't suffer.
>
> Regarding this bit, I have the impression that this would not be
> possible, but maybe I'm missing something ? :)
>
> The iteration that bpf_trace_printk does over the format string
> argument is not only used for validation. It is also used to remember
> what extra operations need to be done based on the modifier types. For
> example, it remembers whether an arg should be interpreted as 32bits or
> 64bits. In the case of string printing, it also remembers whether it is
> a kernel-space or user-space pointer so that bpf_trace_copy_string can
> be called with the right arg. If we were to run the iteration over the format
> string in the verifier, how would you recommend that we
> "remember" the modifier type until the helper gets called ?
>
next prev parent reply other threads:[~2020-12-14 6:48 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-26 16:57 [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper Florent Revest
2020-11-26 16:57 ` [PATCH bpf-next 2/2] selftests/bpf: Add bpf_kallsyms_lookup test Florent Revest
2020-12-02 0:57 ` Andrii Nakryiko
2020-11-27 2:32 ` [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper KP Singh
2020-11-27 9:25 ` Florent Revest
2020-11-27 9:27 ` Florent Revest
2020-11-27 7:35 ` Yonghong Song
2020-11-27 9:20 ` Florent Revest
2020-11-27 11:20 ` KP Singh
2020-11-27 16:09 ` Yonghong Song
2020-12-02 0:55 ` Andrii Nakryiko
2020-12-02 20:32 ` Florent Revest
2020-12-02 21:18 ` Alexei Starovoitov
2020-12-11 14:40 ` Florent Revest
2020-12-14 6:47 ` Yonghong Song [this message]
2020-12-17 15:31 ` Florent Revest
2020-12-17 17:26 ` Yonghong Song
2020-12-18 3:20 ` Alexei Starovoitov
2020-12-18 4:39 ` Yonghong Song
2020-12-18 18:53 ` Andrii Nakryiko
2020-12-18 20:36 ` Alexei Starovoitov
2020-12-18 20:47 ` Andrii Nakryiko
2020-12-22 20:38 ` Florent Revest
2020-12-22 20:52 ` Florent Revest
2020-12-22 14:18 ` Christoph Hellwig
2020-12-22 20:17 ` Florent Revest
2020-12-23 7:50 ` Christoph Hellwig
2020-12-02 0:47 ` Andrii Nakryiko
2020-11-27 17:20 ` kernel test robot
2020-11-27 17:20 ` [RFC PATCH] bpf: bpf_kallsyms_lookup_proto can be static kernel test robot
2020-11-29 1:07 ` [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper Alexei Starovoitov
2020-11-30 16:23 ` Florent Revest
2020-12-01 2:41 ` Alexei Starovoitov
2020-12-01 20:25 ` Florent Revest
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