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
| ||
|
Message-ID: <e8c82119-e443-2557-a7e9-bf6a5d5a7ea9@huawei.com> Date: Wed, 10 Aug 2022 16:10:23 +0800 From: Xu Kuohai <xukuohai@...wei.com> To: Florent Revest <revest@...omium.org> CC: Mark Rutland <mark.rutland@....com>, <bpf@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>, <linux-kselftest@...r.kernel.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, Zi Shen Lim <zlim.lnx@...il.com>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, "David S . Miller" <davem@...emloft.net>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, David Ahern <dsahern@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, Pasha Tatashin <pasha.tatashin@...een.com>, Ard Biesheuvel <ardb@...nel.org>, Daniel Kiss <daniel.kiss@....com>, Steven Price <steven.price@....com>, Sudeep Holla <sudeep.holla@....com>, Marc Zyngier <maz@...nel.org>, Peter Collingbourne <pcc@...gle.com>, Mark Brown <broonie@...nel.org>, Delyan Kratunov <delyank@...com>, Kumar Kartikeya Dwivedi <memxor@...il.com>, Wang ShaoBo <bobo.shaobowang@...wei.com>, <cj.chengjian@...wei.com>, <huawei.libin@...wei.com>, <xiexiuqi@...wei.com>, <liwei391@...wei.com> Subject: Re: [PATCH bpf-next v5 1/6] arm64: ftrace: Add ftrace direct call support On 8/10/2022 1:03 AM, Florent Revest wrote: > On Thu, Jun 9, 2022 at 6:27 AM Xu Kuohai <xukuohai@...wei.com> wrote: >> On 6/7/2022 12:35 AM, Mark Rutland wrote: >>> On Thu, May 26, 2022 at 10:48:05PM +0800, Xu Kuohai wrote: >>>> On 5/26/2022 6:06 PM, Mark Rutland wrote: >>>>> On Thu, May 26, 2022 at 05:45:03PM +0800, Xu Kuohai wrote: >>>>>> On 5/25/2022 9:38 PM, Mark Rutland wrote: >>>>>>> On Wed, May 18, 2022 at 09:16:33AM -0400, Xu Kuohai wrote: >>>>>>>> As noted in that thread, I have a few concerns which equally apply here: >>>>>>> >>>>>>> * Due to the limited range of BL instructions, it's not always possible to >>>>>>> patch an ftrace call-site to branch to an arbitrary trampoline. The way this >>>>>>> works for ftrace today relies upon knowingthe set of trampolines at >>>>>>> compile-time, and allocating module PLTs for those, and that approach cannot >>>>>>> work reliably for dynanically allocated trampolines. >>>>>> >>>>>> Currently patch 5 returns -ENOTSUPP when long jump is detected, so no >>>>>> bpf trampoline is constructed for out of range patch-site: >>>>>> >>>>>> if (is_long_jump(orig_call, image)) >>>>>> return -ENOTSUPP; >>>>> >>>>> Sure, my point is that in practice that means that (from the user's PoV) this >>>>> may randomly fail to work, and I'd like something that we can ensure works >>>>> consistently. >>>>> >>>> >>>> OK, should I suspend this work until you finish refactoring ftrace? >>> >>> Yes; I'd appreciate if we could hold on this for a bit. >>> >>> I think with some ground work we can avoid most of the painful edge cases and >>> might be able to avoid the need for custom trampolines. >>> >> >> I'v read your WIP code, but unfortunately I didn't find any mechanism to >> replace bpf trampoline in your code, sorry. >> >> It looks like bpf trampoline and ftrace works can be done at the same >> time. I think for now we can just attach bpf trampoline to bpf prog. >> Once your ftrace work is done, we can add support for attaching bpf >> trampoline to regular kernel function. Is this OK? > > Hey Mark and Xu! :) > > I'm interested in this feature too and would be happy to help. > > I've been trying to understand what you both have in mind to figure out a way > forward, please correct me if I got anything wrong! :) > > > It looks like, currently, there are three places where an indirection to BPF is > technically possible. Chronologically these are: > > - the function's patchsite (currently there are 2 nops, this could become 4 > nops with Mark's series on per call-site ops) > > - the ftrace ops (currently called by iterating over a global list but could be > called more directly with Mark's series on per-call-site ops or by > dynamically generated branches with Wang's series on dynamic trampolines) > > - a ftrace trampoline tail call (currently, this is after restoring a full > pt_regs but this could become an args only restoration with Mark's series on > DYNAMIC_FTRACE_WITH_ARGS) > > > If we first consider the situation when only a BPF program is attached to a > kernel function: > - Using the patchsite for indirection (proposed by Xu, same as on x86) > Pros: > - We have BPF trampolines anyway because they are required for orthogonal > features such as calling BPF programs as functions, so jumping into that > existing JITed code is straightforward > - This has the minimum overhead (eg: these trampolines only save the actual > number of args used by the function in ctx and avoid indirect calls) > Cons: > - If the BPF trampoline is JITed outside BL's limits, attachment can > randomly fail > > - Using a ftrace op for indirection (proposed by Mark) > Pros: > - BPF doesn't need to care about BL's range, ftrace_caller will be in range > Cons: > - The ftrace trampoline would first save all args in an ftrace_regs only for > the BPF op to then re-save them in a BPF ctx array (as per BPF calling > convention) so we'd effectively have to do the work of saving args twice > - BPF currently uses DYNAMIC_FTRACE_WITH_DIRECT_CALLS APIs. Either arm64 > should implement DIRECT_CALLS with... an indirect call :) (that is, the > arch_ftrace_set_direct_caller op would turn back its ftrace_regs into > arguments for the BPF trampoline) or BPF would need to use a different > ftrace API just on arm64 (to define new ops, which, unless if they would be > dynamically JITed, wouldn't be as performant as the existing BPF > trampolines) > > - Using a ftrace trampoline tail call for indirection (not discussed yet iiuc) > Pros: > - BPF also doesn't need to care about BL's range > - This also leverages the existing BPF trampolines > Cons: > - This also does the work of saving/restoring arguments twice > - DYNAMIC_FTRACE_WITH_DIRECT_CALLS depends on DYNAMIC_FTRACE_WITH_REGS now > although in practice the registers kept by DYNAMIC_FTRACE_WITH_ARGS > should be enough to call BPF trampolines > > If we consider the situation when both ftrace ops and BPF programs are attached > to a kernel function: > - Using the patchsite for indirection can't solve this > > - Using a ftrace op for indirection (proposed by Mark) or using a ftrace > trampoline tail call as an indirection (proposed by Xu, same as on x86) have > the same pros & cons as in the BPF only situation except that this time we > pay the cost of registers saving twice for good reasons (we need args in both > ftrace_regs and the BPF ctx array formats anyway) > > > Unless I'm missing something, it sounds like the following approach would work: > - Always patch patchsites with calls to ftrace trampolines (within BL ranges) > - Always go through ops and have arch_ftrace_set_direct_caller set > ftrace_regs->direct_call (instead of pt_regs->orig_x0 in this patch) > - If ftrace_regs->direct_call != 0 at the end of the ftrace trampoline, tail > call it > > Once Mark's series on DYNAMIC_FTRACE_WITH_ARGS is merged, we would need to have > DYNAMIC_FTRACE_WITH_DIRECT_CALLS > depend on DYNAMIC_FTRACE_WITH_REGS || DYNAMIC_FTRACE_WITH_ARGS > BPF trampolines (the only users of this API now) only care about args to the > attachment point anyway so I think this would work transparently ? > > Once Mark's series on per-callsite ops is merged, the second step (going > through ops) would be significantly faster in the situation where only one > program is used, therefore one arch_ftrace_set_direct_caller op. > > Once Wang's series on dynamic trampolines is merged, the second step (going > through ops) would also be significantly faster in the case when multiple ops > are attached. > > > What are your thoughts? If this sounds somewhat sane, I'm happy to help out > with the implementation as well :) > Hi Florent, I'm struggling with how to attach bpf trampoline to regular kernel functions. I think your suggestion is fine. Thanks for the help! > Thanks! > Florent > .
Powered by blists - more mailing lists