netdev - Re: [PATCH net] samples/bpf: bpf_load.c order of prog

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Message-ID: <20170530163410.mhag3k5nssohcofd@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 30 May 2017 09:34:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] samples/bpf: bpf_load.c order of prog_fd[] should
 correspond with ELF order

On Tue, May 30, 2017 at 02:37:51PM +0200, Jesper Dangaard Brouer wrote:
> An eBPF ELF file generated with LLVM can contain several program
> section, which can be used for bpf tail calls.  The bpf prog file
> descriptors are accessible via array prog_fd[].
> 
> At-least XDP samples assume ordering, and uses prog_fd[0] is the main
> XDP program to attach.  The actual order of array prog_fd[] depend on
> whether or not a bpf program section is referencing any maps or not.
> Not using a map result in being loaded/processed after all other
> prog section.  Thus, this can lead to some very strange and hard to
> debug situation, as the user can only see a FD and cannot correlated
> that with the ELF section name.
> 
> The fix is rather simple, and even removes duplicate memcmp code.
> Simply load program sections as the last step, instead of
> load_and_attach while processing the relocation section.
> 
> When working with tail calls, it become even more essential that the
> order of prog_fd[] is consistant, like the current dependency of the
> map_fd[] order.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Looks fine, but imo net-next is better, since it's not a bugfix
to anything in the net tree, but a general improvement.

Acked-by: Alexei Starovoitov <ast@...nel.org>