linux-kernel - Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 1 Dec 2020 12:38:00 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for
 atomic[64]_[fetch_]sub

On Mon, Nov 30, 2020 at 09:18:09AM -0800, Yonghong Song wrote:
> On 11/28/20 5:34 PM, Alexei Starovoitov wrote:
> > On Fri, Nov 27, 2020 at 09:35:07PM -0800, Yonghong Song wrote:
> > > On 11/27/20 9:57 AM, Brendan Jackman wrote:
[...]
> > > > +#define BPF_ATOMIC_SUB(SIZE, DST, SRC, OFF)			\
> > > > +	((struct bpf_insn) {					\
> > > > +		.code  = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC,	\
> > > > +		.dst_reg = DST,					\
> > > > +		.src_reg = SRC,					\
> > > > +		.off   = OFF,					\
> > > > +		.imm   = BPF_SUB })
> > > 
> > > Currently, llvm does not support XSUB, should we support it in llvm?
> > > At source code, as implemented in JIT, user can just do a negate
> > > followed by xadd.
> > 
> > I forgot we have BPF_NEG insn :)
> > Indeed it's probably easier to handle atomic_fetch_sub() builtin
> > completely on llvm side. It can generate bpf_neg followed by atomic_fetch_add.
> 
> Just tried. llvm selectiondag won't be able to automatically
> convert atomic_fetch_sub to neg + atomic_fetch_add. So there
> will be a need in BPFInstrInfo.td to match atomic_fetch_sub IR
> pattern. I will experiment this together with xsub.
> 
> > No need to burden verifier, interpreter and JITs with it.
> > 

I guess it's also worth remembering other archs might have an atomic
subtract.