netdev - Re: [PATCH bpf-next] bpf: libbpf: retry program creation without the name

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Date:   Tue, 20 Nov 2018 15:20:55 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Stanislav Fomichev <sdf@...ichev.me>,
        Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        ast@...nel.org, vladum@...gle.com
Subject: Re: [PATCH bpf-next] bpf: libbpf: retry program creation without the
 name

On Wed, Nov 21, 2018 at 12:18:57AM +0100, Daniel Borkmann wrote:
> On 11/21/2018 12:04 AM, Alexei Starovoitov wrote:
> > On Tue, Nov 20, 2018 at 01:19:05PM -0800, Stanislav Fomichev wrote:
> >> On 11/20, Alexei Starovoitov wrote:
> >>> On Mon, Nov 19, 2018 at 04:46:25PM -0800, Stanislav Fomichev wrote:
> >>>> [Recent commit 23499442c319 ("bpf: libbpf: retry map creation without
> >>>> the name") fixed this issue for maps, let's do the same for programs.]
> >>>>
> >>>> Since commit 88cda1c9da02 ("bpf: libbpf: Provide basic API support
> >>>> to specify BPF obj name"), libbpf unconditionally sets bpf_attr->name
> >>>> for programs. Pre v4.14 kernels don't know about programs names and
> >>>> return an error about unexpected non-zero data. Retry sys_bpf without
> >>>> a program name to cover older kernels.
> >>>>
> >>>> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> >>>> ---
> >>>>  tools/lib/bpf/bpf.c | 10 ++++++++++
> >>>>  1 file changed, 10 insertions(+)
> >>>>
> >>>> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> >>>> index 961e1b9fc592..cbe9d757c646 100644
> >>>> --- a/tools/lib/bpf/bpf.c
> >>>> +++ b/tools/lib/bpf/bpf.c
> >>>> @@ -212,6 +212,16 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
> >>>>  	if (fd >= 0 || !log_buf || !log_buf_sz)
> >>>>  		return fd;
> >>>>  
> >>>> +	if (fd < 0 && errno == E2BIG && load_attr->name) {
> >>>> +		/* Retry the same syscall, but without the name.
> >>>> +		 * Pre v4.14 kernels don't support prog names.
> >>>> +		 */
> >>>
> >>> I'm afraid that will put unnecessary stress on the kernel.
> >>> This check needs to be tighter.
> >>> Like E2BIG and anything in the log_buf probably means that
> >>> E2BIG came from the verifier and nothing to do with prog_name.
> >>> Asking kernel to repeat is an unnecessary work.
> >>>
> >>> In general we need to think beyond this single prog_name field.
> >>> There are bunch of other fields in bpf_load_program_xattr() and older kernels
> >>> won't support them. Are we going to zero them out one by one
> >>> and retry? I don't think that would be practical.
> >> I general, we don't want to zero anything out. However,
> >> for this particular problem the rationale is the following:
> >> In commit 88cda1c9da02 we started unconditionally setting {prog,map}->name
> >> from the 'higher' libbpfc layer which breaks users on the older kernels.
> >>
> >>> Also libbpf silently ignoring prog_name is not great for debugging.
> >>> A warning is needed.
> >>> But it cannot be done out of lib/bpf/bpf.c, since it's a set of syscall
> >>> wrappers.
> >>> Imo such "old kernel -> lets retry" feature should probably be done
> >>> at lib/bpf/libbpf.c level. inside load_program().
> >> For maps bpftools calls bpf_create_map_xattr directly, that's why
> >> for maps I did the retry on the lower level (and why for programs I initially
> >> thought about doing the same). However, in this case maybe asking
> >> user to omit 'name' argument might be a better option.
> >>
> >> For program names, I agree, we might think about doing it on the higher
> >> level (although I'm not sure whether we want to have different API
> >> expectations, i.e. bpf_create_map_xattr ignoring the name and
> >> bpf_load_program_xattr not ignoring the name).
> >>
> >> So given that rationale above, what do you think is the best way to
> >> move forward?
> >> 1. Same patch, but tighten the retry check inside bpf_load_program_xattr ?
> >> 2. Move this retry logic into load_program and have different handling
> >>    for bpf_create_map_xattr vs bpf_load_program_xattr ?
> >> 3. Do 2 and move the retry check for maps from bpf_create_map_xattr
> >>    into bpf_object__create_maps ?
> >>
> >> (I'm slightly leaning towards #3)
> > 
> > me too. I think it's cleaner for maps to do it in
> > bpf_object__create_maps().
> > Originally bpf.c was envisioned to be a thin layer on top of bpf syscall.
> > Whereas 'smart bits' would go into libbpf.c
> 
> Can't we create in bpf_object__load() a small helper bpf_object__probe_caps()
> which would figure this out _once_ upon start with a few things to probe for
> availability in the underlying kernel for maps and programs? E.g. programs
> it could try to inject a tiny 'r0 = 0; exit' snippet where we figure out
> things like prog name support etc. Given underlying kernel doesn't change, we
> would only try this once and it doesn't require fallback every time.

+1. great idea!