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([2620:10d:c090:500::7:25f]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29034dea083sm64462435ad.24.2025.10.10.11.55.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Oct 2025 11:55:27 -0700 (PDT) Message-ID: Subject: Re: bpf_errno. Was: [PATCH RFC bpf-next 1/3] bpf: report probe fault to BPF stderr From: Eduard Zingerman To: Menglong Dong , Kumar Kartikeya Dwivedi , Alexei Starovoitov , Leon Hwang Cc: Andrii Nakryiko , Menglong Dong , Alexei Starovoitov , bpf , LKML , linux-trace-kernel , jiang.biao@linux.dev Date: Fri, 10 Oct 2025 11:55:25 -0700 In-Reply-To: <3349652.5fSG56mABF@7950hx> References: <20250927061210.194502-1-menglong.dong@linux.dev> <0adc5d8a299483004f4796a418420fe1c69f24bc.camel@gmail.com> <3349652.5fSG56mABF@7950hx> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-1.fc42) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2025-10-10 at 20:05 +0800, Menglong Dong wrote: [...] > save errno to r0(Eduard) > ----------------------------------- > Save the errno to r0 in the exception handler of BPF_PROBE_MEM, > and read r0 with a __kfun in BPF program. (Not sure if I understand > it correctly). >=20 > This sounds effective, but won't this break the usage of r0? I mean, > the r0 can be used by the BPF program somewhere. What I meant is that for cases when someone wants to check for memory access error, there is already bpf_probe_read_kernel(). It's return value in r0 and is defined for both success and failure cases. The problem with it, is that it has a function call overhead. But we can workaround that for 1,2,4,8 byte accesses, by replacing helper call by some `BPF_LDX | BPF_PROBE_MEM1 | `, where BPF_PROBE_MEM1 is different from BPF_PROBE_MEM and tells jit that exception handler for this memory access needs to set r0 to -EFAULT if it is executed. The inconvenient part here is that one can't do chaining, like a->b->c, using bpf_probe_read_kernel(). One needs to insert bpf_probe_read_kernel() call at each step of a chain, which is a bit of a pain. Maybe it can be alleviated using some vararg macro. [...]