Date:   Wed, 21 Nov 2018 00:18:57 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Stanislav Fomichev <sdf@...ichev.me>
Cc:     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 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.

> 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.
> 

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