netdev - Re: [PATCH net] bpf: don't zero out the info struct in bpf_obj_get_info_by

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Message-ID: <20170725161547.64d1cba5@cakuba.netronome.com>
Date:   Tue, 25 Jul 2017 16:15:47 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     netdev@...r.kernel.org, oss-drivers@...ronome.com,
        alexei.starovoitov@...il.com, kafai@...com
Subject: Re: [PATCH net] bpf: don't zero out the info struct in
 bpf_obj_get_info_by_fd()

On Wed, 26 Jul 2017 00:59:49 +0200, Daniel Borkmann wrote:
> > @@ -418,6 +420,8 @@ static void test_bpf_obj_id(void)
> >   		nr_id_found++;
> >
> >   		err = bpf_obj_get_info_by_fd(prog_fd, &prog_info, &info_len);
> > +		prog_infos[i].jited_prog_insns = 0;
> > +		prog_infos[i].xlated_prog_insns = 0;  
> 
> Can you elaborate why this one above is needed?

Ah, I removed the comment about it at the last minute.  The check below
compares the info we get here with info we got reading the programs in
the earlier loop - using memcmp().  This call, however, doesn't fill in
the pointers for jited and xlated images, so the memcmp() would fail.

It used to work when bpf_obj_get_info_by_fd() was zeroing info, since
the pointers would be cleared by it, and no dump ever returned, it
didn't matter that the call sites differ.

> >   		CHECK(err || info_len != sizeof(struct bpf_prog_info) ||
> >   		      memcmp(&prog_info, &prog_infos[i], info_len),
> >   		      "get-prog-info(next_id->fd)",