Re: [PATCH net] bpf: x86: fix epilogue generation for eBPF programs

[Daniel Borkmann]

On 11/28/2014 06:55 AM, Alexei Starovoitov wrote:
> On Thu, Nov 27, 2014 at 1:52 AM, Daniel Borkmann <dborkman@xxxxxxxxxx> wrote:
> > On 11/27/2014 06:02 AM, Alexei Starovoitov wrote:
> > > classic BPF has a restriction that last insn is always BPF_RET.
> > > eBPF doesn't have BPF_RET instruction and this restriction.
> > > It has BPF_EXIT insn which can appear anywhere in the program
> > > one or more times and it doesn't have to be last insn.
> > > Fix eBPF JIT to emit epilogue when first BPF_EXIT is seen
> > > and all other BPF_EXIT instructions will be emitted as jump.
> > >
> > > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> > > ---
> > > Note, this bug is applicable only to native eBPF programs
> > > which first were introduced in 3.18, so no need to send it
> > > to stable and therefore no 'Fixes' tag.
> >
> > Btw, even if it's not sent to -stable, a 'Fixes:' tag is useful
> > information for backporting and regression tracking, preferably
> > always mentioned where it can clearly be identified.
>
> Well I didn't mention it, as I said, because I don't think it
> needs backporting. Otherwise with the tag the tools might
> pick it up automatically? Just a guess.

No, Dave selects -stable material on a case-by-case basis and bundles
it up eventually; after -net was merged, it's then pushed to -stable
by himself (see Documentation/networking/netdev-FAQ.txt +114). So the
comment below "---" is absolutely okay.

It can well be, that some people/companies cannot switch for various
reasons immediately to the next kernels, but nevertheless would like
to have a certain features included, so generally regression tracking
via 'Fixes:' tag is extremely useful/valuable. ;)

> Fixes: 622582786c9e ("net: filter: x86: internal BPF JIT")

...
> > Why this type change here? This seems a bit out of context (I would
> > have expected a mention of this in the commit message, otherwise).
>
> The reason for signed is the following:
> jmp offset to epilogue is computed as:
> jmp_offset = ctx->cleanup_addr - addrs[i]
> when cleanup_addr was always last insn it wasn't a problem,
> since result of subtraction was positive.
> Now, since epilogue will be in the middle of JITed
> code the jmps to epilogue may be negative

Ok, thanks for the clarification, Alexei.
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