From: Jakub Sitnicki <jakub@cloudflare.com>
To: Xu Kuohai <xukuohai@huawei.com>
Cc: bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
linux-kselftest@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>,
Zi Shen Lim <zlim.lnx@gmail.com>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
Yonghong Song <yhs@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
"David S . Miller" <davem@davemloft.net>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
David Ahern <dsahern@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, hpa@zytor.com, Shuah Khan <shuah@kernel.org>,
Jakub Kicinski <kuba@kernel.org>,
Jesper Dangaard Brouer <hawk@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Pasha Tatashin <pasha.tatashin@soleen.com>,
Ard Biesheuvel <ardb@kernel.org>,
Daniel Kiss <daniel.kiss@arm.com>,
Steven Price <steven.price@arm.com>,
Sudeep Holla <sudeep.holla@arm.com>,
Marc Zyngier <maz@kernel.org>,
Peter Collingbourne <pcc@google.com>,
Mark Brown <broonie@kernel.org>, Delyan Kratunov <delyank@fb.com>,
Kumar Kartikeya Dwivedi <memxor@gmail.com>
Subject: Re: [PATCH bpf-next v3 5/7] bpf, arm64: Support to poke bpf prog
Date: Thu, 12 May 2022 12:54:07 +0200 [thread overview]
Message-ID: <87wneryq8z.fsf@cloudflare.com> (raw)
In-Reply-To: <5fb30cc0-dcf6-75ec-b6fa-38be3e99dca6@huawei.com>
On Wed, May 11, 2022 at 11:12 AM +08, Xu Kuohai wrote:
> On 5/10/2022 5:36 PM, Jakub Sitnicki wrote:
>> On Sun, Apr 24, 2022 at 11:40 AM -04, Xu Kuohai wrote:
[...]
>>> @@ -281,12 +290,15 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>>> *
>>> */
>>>
>>> + if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
>>> + emit(A64_BTI_C, ctx);
>>
>> I'm no arm64 expert, but this looks like a fix for BTI.
>>
>> Currently we never emit BTI because ARM64_BTI_KERNEL depends on
>> ARM64_PTR_AUTH_KERNEL, while BTI must be the first instruction for the
>> jump target [1]. Am I following correctly?
>>
>> [1] https://lwn.net/Articles/804982/
>>
>
> Not quite correct. When the jump target is a PACIASP instruction, no
> Branch Target Exception is generated, so there is no need to insert a
> BTI before PACIASP [2].
>
> In order to attach trampoline to bpf prog, a MOV and NOP are inserted
> before the PACIASP, so BTI instruction is required to avoid Branch
> Target Exception.
>
> The reason for inserting NOP before PACIASP instead of after PACIASP is
> that no call frame is built before entering trampoline, so there is no
> return address on the stack and nothing to be protected by PACIASP.
>
> [2]
> https://developer.arm.com/documentation/ddi0596/2021-12/Base-Instructions/BTI--Branch-Target-Identification-?lang=en
That makes sense. Thanks for the explanation!
next prev parent reply other threads:[~2022-05-12 10:55 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-24 15:40 [PATCH bpf-next v3 0/7] bpf trampoline for arm64 Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 1/7] arm64: ftrace: Add ftrace direct call support Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 2/7] ftrace: Fix deadloop caused by direct call in ftrace selftest Xu Kuohai
2022-04-25 15:05 ` Steven Rostedt
2022-04-26 7:36 ` Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 3/7] bpf: Move is_valid_bpf_tramp_flags() to the public trampoline code Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 4/7] bpf, arm64: Impelment bpf_arch_text_poke() for arm64 Xu Kuohai
2022-05-10 11:45 ` Jakub Sitnicki
2022-05-11 3:18 ` Xu Kuohai
2022-05-13 14:59 ` Mark Rutland
2022-05-16 6:55 ` Xu Kuohai
2022-05-16 7:18 ` Mark Rutland
2022-05-16 7:58 ` Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 5/7] bpf, arm64: Support to poke bpf prog Xu Kuohai
2022-05-10 9:36 ` Jakub Sitnicki
2022-05-11 3:12 ` Xu Kuohai
2022-05-12 10:54 ` Jakub Sitnicki [this message]
2022-04-24 15:40 ` [PATCH bpf-next v3 6/7] bpf, arm64: bpf trampoline for arm64 Xu Kuohai
2022-04-24 15:40 ` [PATCH bpf-next v3 7/7] selftests/bpf: Fix trivial typo in fentry_fexit.c Xu Kuohai
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