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

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Date:   Tue, 20 Nov 2018 15:04:46 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        daniel@...earbox.net, ast@...nel.org, vladum@...gle.com
Subject: Re: [PATCH bpf-next] bpf: libbpf: retry program creation without the
 name

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
Right now this boundary is unfortunately blurry.
May be as #4 long term option we'll introduce another 'smart' layer
between bpf.c that will assume the latest kernel and libbpf.c that deals
with elf. May be will call this new layer a 'compat' layer?
For now I think doing #3 as you suggested is probably the best short term.