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
| ||
|
Date: Wed, 11 Nov 2015 13:58:07 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Will Deacon <will.deacon@....com> Cc: Daniel Borkmann <daniel@...earbox.net>, Arnd Bergmann <arnd@...db.de>, "Shi, Yang" <yang.shi@...aro.org>, linaro-kernel@...ts.linaro.org, Eric Dumazet <eric.dumazet@...il.com>, Z Lim <zlim.lnx@...il.com>, Alexei Starovoitov <ast@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Xi Wang <xi.wang@...il.com>, Catalin Marinas <catalin.marinas@....com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org> Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction On Wed, Nov 11, 2015 at 12:38:31PM +0000, Will Deacon wrote: > > Hmm, gcc doesn't have an eBPF compiler backend, so this won't work on > > gcc at all. The eBPF backend in LLVM recognizes the __sync_fetch_and_add() > > keyword and maps that to a BPF_XADD version (BPF_W or BPF_DW). In the > > interpreter (__bpf_prog_run()), as Eric mentioned, this maps to atomic_add() > > and atomic64_add(), respectively. So the struct bpf_insn prog[] you saw > > from sock_example.c can be regarded as one possible equivalent program > > section output from the compiler. > > Ok, so if I understand you correctly, then __sync_fetch_and_add() has > different semantics depending on the backend target. That seems counter > to the LLVM atomics Documentation: > > http://llvm.org/docs/Atomics.html > > which specifically calls out the __sync_* primitives as being > sequentially-consistent and requiring barriers on ARM (which isn't the > case for atomic[64]_add in the kernel). > > If we re-use the __sync_* naming scheme in the source language, I don't > think we can overlay our own semantics in the backend. The > __sync_fetch_and_add primitive is also expected to return the old value, > which doesn't appear to be the case for BPF_XADD. Yikes. That's double fail. Please don't do this. If you use the __sync stuff (and I agree with Will, you should not) it really _SHOULD_ be sequentially consistent, which means full barriers all over the place. And if you name something XADD (exchange and add, or fetch-add) then it had better return the previous value. atomic*_add() does neither. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists