-----Original Message-----
From: Yonghong Song <yonghong.song@xxxxxxxxx>
Sent: Saturday, January 27, 2024 4:27 PM
To: dthaler1968@xxxxxxxxxxxxxx
Cc: bpf@xxxxxxxx; bpf@xxxxxxxxxxxxxxx
Subject: Re: ISA: BPF_MSH and deprecated packet access instructions
On 1/27/24 10:50 AM, dthaler1968@xxxxxxxxxxxxxx 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?
