From: Florent Revest <revest@chromium.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
KP Singh <kpsingh@kernel.org>,
Brendan Jackman <jackmanb@google.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Steven Rostedt <rostedt@goodmis.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/2] bpf: Implement formatted output helpers with bstr_printf
Date: Wed, 28 Apr 2021 02:20:39 +0200 [thread overview]
Message-ID: <CABRcYm+eWh5=eM9mgOsCU6-TACi-y5kviCf9Kbqxfzvgq9u5BA@mail.gmail.com> (raw)
In-Reply-To: <CAADnVQLQmt0-D_e=boXoK=FLRoXv9xzkCwM24zpbZERrEexLCw@mail.gmail.com>
On Wed, Apr 28, 2021 at 1:46 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Apr 27, 2021 at 10:43 AM Florent Revest <revest@chromium.org> wrote:
> > + if (fmt[i + 1] == 'B') {
> > + if (tmp_buf) {
> > + err = snprintf(tmp_buf,
> > + (tmp_buf_end - tmp_buf),
> > + "%pB",
> ...
> > + if ((tmp_buf_end - tmp_buf) < sizeof_cur_ip) {
>
> I removed a few redundant () like above
Oh, sorry about that.
> and applied.
Nice! :)
> > if (fmt[i] == 'l') {
> > - cur_mod = BPF_PRINTF_LONG;
> > + sizeof_cur_arg = sizeof(long);
> > i++;
> > }
> > if (fmt[i] == 'l') {
> > - cur_mod = BPF_PRINTF_LONG_LONG;
> > + sizeof_cur_arg = sizeof(long long);
> > i++;
> > }
>
> This bit got me thinking.
> I understand that this is how bpf_trace_printk behaved
> and the sprintf continued the tradition, but I think it will
> surprise bpf users.
> The bpf progs are always 64-bit. The sizeof(long) == 8
> inside any bpf program. So printf("%ld") matches that long.
Yes, this also surprised me.
> The clang could even do type checking to make sure the prog
> is passing the right type into printf() if we add
> __attribute__ ((format (printf))) to bpf_helper_defs.h
> But this sprintf() implementation will trim the value to 32-bit
> to satisfy 'fmt' string on 32-bit archs.
> So bpf program behavior would be different on 32 and 64-bit archs.
> I think that would be confusing, since the rest of bpf prog is
> portable. The progs work the same way on all archs
> (except endianess, of course).
> I'm not sure how to fix it though.
> The sprintf cannot just pass 64-bit unconditionally, since
> bstr_printf on 32-bit archs will process %ld incorrectly.
> The verifier could replace %ld with %Ld.
> The fmt string is a read only string for bpf_snprintf,
> but for bpf_trace_printk it's not and messing with it at run-time
> is not good. Copying the fmt string is not great either.
> Messing with internals of bstr_printf is ugly too.
Indeed, none of these solutions are satisfying.
> Maybe we just have to live with this quirk ?
If we were starting from scratch, maybe just banning %ld could have
been an option, but now that bpf_trace_printk has been behaving like
this for a while, I think it might be best to just keep the behavior
as it is.
> Just add a doc to uapi/bpf.h to discourage %ld and be done?
More doc is always good. Something like "Note: %ld behaves differently
depending on the host architecture, it is recommended to avoid it and
use %d or %lld instead" in the helper description of the three
helpers? If you don't have the time to do it today, I can send a patch
tomorrow.
next prev parent reply other threads:[~2021-04-28 0:20 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 17:43 [PATCH bpf-next v2 0/2] Implement formatted output helpers with bstr_printf Florent Revest
2021-04-27 17:43 ` [PATCH bpf-next v2 1/2] seq_file: Add a seq_bprintf function Florent Revest
2021-04-27 17:43 ` [PATCH bpf-next v2 2/2] bpf: Implement formatted output helpers with bstr_printf Florent Revest
2021-04-27 23:46 ` Alexei Starovoitov
2021-04-28 0:20 ` Florent Revest [this message]
2021-04-28 0:51 ` Alexei Starovoitov
2021-04-28 14:52 ` Florent Revest
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CABRcYm+eWh5=eM9mgOsCU6-TACi-y5kviCf9Kbqxfzvgq9u5BA@mail.gmail.com' \
--to=revest@chromium.org \
--cc=alexei.starovoitov@gmail.com \
--cc=andrii@kernel.org \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=jackmanb@google.com \
--cc=kpsingh@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@rasmusvillemoes.dk \
--cc=rostedt@goodmis.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).