> -----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? 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? Dave
