Re: ISA: BPF_MSH and deprecated packet access instructions

[Yonghong Song <yonghong.song@linux.dev>]

On 1/27/24 4:53 PM, dthaler1968@googlemail.com wrote:
>> -----Original Message-----
>> From: Yonghong Song <yonghong.song@linux.dev>
>> Sent: Saturday, January 27, 2024 4:27 PM
>> To: dthaler1968@googlemail.com
>> Cc: bpf@ietf.org; bpf@vger.kernel.org
>> Subject: Re: ISA: BPF_MSH and deprecated packet access instructions
>>
>>
>> On 1/27/24 10:50 AM, dthaler1968@googlemail.com wrote:
>>> Under "Load and store instructions", various mode modifiers are
>> documented.
>>> I notice that BPF_MSH (0xa0) is not documented, but appears to be in
>>> use in various projects, including Linux, BSD, seccomp, etc. and is
>>> even documented in various books such as
>>>
>> https://www.google.com/books/edition/Programming_Linux_Hacker_Tools_Un
>>> covere
>>>
>> d/yqHVAwAAQBAJ?hl=en&gbpv=1&dq=%22BPF_MSH%22&pg=PA129&printsec
>> =frontco
>>> ver
>>>
>>> Should we document it as deprecated and add it to the set of
>>> deprecated instructions (the legacy conformance group) like BPF_ABS
>>> and BPF_IND already are?
>>>
>>> Also, for purposes of the IANA registry of instructions where we list
>>> which opcodes are "(deprecated, implementation-specific)", I currently
>>> list all possible BPF_ABS and BPF_IND opcodes regardless of whether
>>> they were ever used (I didn't check which were used and which might
>>> not have been), so I could just list all possible BPF_MSH opcodes
>>> similarly.  But if we know that some were never used then I don't need
>>> to do so, so I guess I should
>>> ask:
>>> do we have a list of which combinations were actually used or should
>>> we continue to just deprecate all combinations?
>>>
>>> As an example,
>>> https://github.com/seccomp/libseccomp/blob/main/tools/scmp_bpf_disasm.
>>> c#L68 lists 6 variants of BPF_MSH: LD and LDX, for B, H, and W (but
>>> not DW).
>>> Other sources like the book page referenced above, and the BSD man
>>> page, list only BPF_LDX | BPF_B | BPF_MSH, which is in Linux sources
>>> such as
>>> https://elixir.bootlin.com/linux/v6.8-rc1/source/lib/test_bpf.c#L368
>>   From kernel source code (net/core/filter.c), the only supported format is
>>      BPF_LDX | BPF_MSH | BPF_B
>>
>> The insn (BPF_LDX | BPF_MSH | BPF_B) is only used when cBPF (classic BPF) is
>> converted to BPF insn set. If the current BPF program has this insn, verifier will
>> reject it and bpf kernel interpreter does not support this insn either. So
>> technically, (BPF_LDX | BPF_MSH | BPF_B) is not supported by BPF program.
>>
>>> So, should we list the DW variants as deprecated, or never assigned?
>>> Should we list the H, W, and LD variants as deprecated, or never assigned?
> Thanks for confirming.  I think the doc is ok as is for this part.
>
>>> What about DW and LDX variants of BPF_IND and BPF_ABS?
> What about this question?

I don't know how to do proper wording in the standard. But
DW and LDX variants of BPF_IND/BPF_ABS are not supported
by verifier for now and they are considered illegal insns.

>
> Dave
>