Date:   Tue, 29 Nov 2016 16:28:40 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jakub Kicinski <kubakici@...pl>, netdev@...r.kernel.org,
        Brenden Blanco <bblanco@...mgrid.com>,
        Thomas Graf <tgraf@...g.ch>, Wangnan <wangnan0@...wei.com>,
        He Kuang <hekuang@...wei.com>, kernel-team@...com
Subject: Re: bpf debug info

On Tue, Nov 29, 2016 at 09:23:10PM +0100, Daniel Borkmann wrote:
> On 11/29/2016 07:51 PM, Alexei Starovoitov wrote:
> >On Tue, Nov 29, 2016 at 03:38:18PM +0000, Jakub Kicinski wrote:
> [...]
> >>>>So next step is to improve verifier messages to be more human friendly.
> >>>>The step after is to introduce BPF_COMMENT pseudo instruction
> >>>>that will be ignored by the interpreter yet it will contain the text
> >>>>of original source code. Then llvm-objdump step won't be necessary.
> >>>>The bpf loader will load both instructions and pieces of C sources.
> >>>>Then verifier errors should be even easier to read and humans
> >>>>can easily understand the purpose of the program.
> >>>
> >>>So the BPF_COMMENT pseudo insn will get stripped away from the insn array
> >>>after verification step, so we don't need to hold/account for this mem? I
> >>>assume in it's ->imm member it will just hold offset into text blob?
> >>
> >>Associating any form of opaque data with programs always makes me
> >>worried about opening a side channel of communication with a specialized
> >>user space implementations/compilers.  But I guess if the BPF_COMMENTs
> >>are stripped in the verifier as Daniel assumes drivers and JITs will
> >>never see it.
> >
> >yes. the idea that it's a comment. It can contain any text,
> >not only C code, but any other language.
> >It's definitely going to be stripped before JITs and kernel will
> >not make any safety or translation decisions based on such comment.
> >
> >>Just to clarify, however - is there any reason why pushing the source
> >>code into the kernel is necessary?  Or is it just for convenience?
> >>Provided the user space loader has access to the debug info it should
> >>have no problems matching the verifier output to code lines?
> >
> >correct. just for convenience. The user space has to keep .o around,
> >since it can crash, would have to reload and so on.
> >Only for some script that ssh-es into servers and wants to see
> >what is being loaded, it might help to dump full asm and these comments
> >along with prog_digest that Daniel is working on in parallel.
> 
> Which would mean we'd need to keep it around somewhere (prog aux data?)
> in post-verification time (so potentially drivers/JITs could see it, too,
> just not inside insn stream). Some API glue code could probably blind
> this information for the JITing time to stop incentive of playing side
> channel games (e.g. core code could encrypt the pointer value and only
> core kernel knows how to access that data, no modules, no out-of-tree
> code). The other thing I'm wondering is, when we strip this info anyway
> from the insn stream to keep it in aux data (so it can later be reconstructed
> on a dump), then perhaps that is best done before prog loading time? It
> would then allow to keep complexity with stripping that insns out of the
> verifier. If semantics are that these comments are acting as a hole/gap
> (in a similar sense of what we have with cBPF today), then it can never
> become a jmp target and loaders could strip it out already (instead of
> teaching DFS, etc about it), and prepare a meta data structure in bpf_attr
> for bpf(2), and verifier works based on that one. What makes this problematic
> however is when you have rewrites in the kernel (ctx access, constant
> blinding, etc), but perhaps they could just adjust the offsets from that
> meta data thing as well?

yes. all correct.
if we keep comment==nop instructions as part of the program, it's not great,
since we'll be wasting performance for no strong reason.
If we remove them from instruction stream after the verifier then they
can only be useful in verifier messages and that's not much better
than existing 'llvm-objdump -S file.o' approach.
Hmm.

> >Alternatively instead of doing BPF_COMMENT we can load the whole .o
> >as-is into bpffs as a blob. Later (based on digest) the kernel can
> >dump such .o back for user space to run objdump on. It all can be
> >done without kernel involvement. Like tc command can copy .o and so on.
> >But not everything is using tc.
> 
> That means kernel must ensure/verify that loaded insns also come from
> that claimed object file; not sure if easily possible w/o parsing elf.
> It could work if the kernel loads everything based on the content of
> the object file itself,

it will only check that program section of file contain valid insns.
the user may still cheat with junk in dwarf section of such elf.
Sounds more and more that this should really be solved by user space
and correlation to be done via prog_digest.

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