On 11/28/2014 06:55 AM, Alexei Starovoitov wrote:

On Thu, Nov 27, 2014 at 1:52 AM, Daniel Borkmann [off-list ref] wrote:

quoted

On 11/27/2014 06:02 AM, Alexei Starovoitov wrote:

quoted

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 <redacted>
---
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")

...

quoted

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.