netdev - Re: [PATCH v2 bpf-next 1/2] bpf: include sub program tags in bpf_prog

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Date:   Wed, 12 Dec 2018 04:38:10 +0000
From:   Martin Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     Song Liu <songliubraving@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/2] bpf: include sub program tags in
 bpf_prog_info

On Wed, Dec 12, 2018 at 03:22:38AM +0100, Daniel Borkmann wrote:
> On 12/11/2018 09:18 AM, Song Liu wrote:
> > Changes from v1:
> > 1. Fix error path as Martin suggested.
> > 
> > This patch adds nr_prog_tags and prog_tags to bpf_prog_info. This is a
> > reliable way for user space to get tags of all sub programs. Before this
> > patch, user space need to find sub program tags via kallsyms.
> > 
> > This feature will be used in BPF introspection, where user space queries
> > information about BPF programs via sys_bpf.
> > 
> > Signed-off-by: Song Liu <songliubraving@...com>
> > ---
> >  include/uapi/linux/bpf.h |  2 ++
> >  kernel/bpf/syscall.c     | 27 +++++++++++++++++++++++++++
> >  2 files changed, 29 insertions(+)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 1bee1135866a..368d185aa32f 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2703,6 +2703,8 @@ struct bpf_prog_info {
> >  	__u32 jited_line_info_cnt;
> >  	__u32 line_info_rec_size;
> >  	__u32 jited_line_info_rec_size;
> > +	__u32 nr_prog_tags;
> > +	__aligned_u64 prog_tags;
> >  } __attribute__((aligned(8)));
> > 
> >  struct bpf_map_info {
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 19c88cff7880..49cb59177db9 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -2322,6 +2322,33 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
> >  		}
> >  	}
> > 
> > +	ulen = info.nr_prog_tags;
> > +	info.nr_prog_tags = prog->aux->func_cnt ? : 1;
> > +	if (ulen) {
> > +		if (bpf_dump_raw_ok()) {
> 
> Hm, why is bpf_dump_raw_ok() needed here? tag is not exposing a kernel
> address. prog tag is in general also visible from fdinfo from unpriv.
> 
> Just looking at the recently merged func_info / line_info I'm not sure
> this is needed there either ... Martin, is there a specific reason this
> was added there as well?
It is mostly to follow info.jited_func_lens which also tests
bpf_dump_raw_ok().  We thought the check iss there even it is
not exposing the kernel address because jited_func_lens is not
useful in general without jited_ksyms.

Same go for func_info when dumping jited insn.  func_info used to only
support jited insn dump.

Considering func_info was later made to support xlated insn also,
I think it makes sense to remove the bpf_dump_raw_ok() check
for func_info, line_info and prog_tags and ask the
userspace to decide if it has all needed details before dumping
info for the xlated insn and jited insn?

The bpf_dump_raw_ok() check for jited_line_info will stay
because jited_line_info contains kernel address.

> > +			__u8 __user (*user_prog_tags)[BPF_TAG_SIZE];
> > +			u32 i;
> > +
> > +			user_prog_tags = u64_to_user_ptr(info.prog_tags);
> > +			ulen = min_t(u32, info.nr_prog_tags, ulen);
> > +			if (prog->aux->func_cnt) {
> > +				for (i = 0; i < ulen; i++) {
> > +					if (copy_to_user(
> > +						    user_prog_tags[i],
> > +						    prog->aux->func[i]->tag,
> > +						    BPF_TAG_SIZE))
> > +						return -EFAULT;
> > +				}
> > +			} else {
> > +				if (copy_to_user(user_prog_tags[0],
> > +						 prog->tag, BPF_TAG_SIZE))
> > +					return -EFAULT;
> > +			}
> > +		} else {
> > +			info.prog_tags = 0;
> > +		}
> > +	}
> > +
> >  done:
> >  	if (copy_to_user(uinfo, &info, info_len) ||
> >  	    put_user(info_len, &uattr->info.info_len))
> > --
> > 2.17.1
> > 
>