From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8D0271624D2; Fri, 2 May 2025 14:26:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746195987; cv=none; b=euGZ2cRgVfh8aZKbSbTbwwURKHkA/KsaBfSY+/zPIRFRkkh5G5jRiXxuqt12cZAF9/jll0bfHcPn8JNQOShJyQPFMrZy8oOy+9vt7JtbT5BmCx4m3pdZgln6FvfPsFNj70qV0c/syVTI6aY0QBdhDjnx02OK3EcN61Y3K5L0aR8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746195987; c=relaxed/simple; bh=WnH+dqe0udCsTb2yb68GJWUxQUnxpidgGIRNl7dIGho=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hl884Ftg1DiCT+zmwfjwuOkmDQH8jup8FUtQx+WQQTto3jZy51maLBBeWAp5ChmphQGN6V9QkExRtn0/3mhfi8OlPFVQvyrArDDeh5iYsTIKamdoAjrpi16m0aISyzMNTmDuXLE9g5F4X17lZUhai07b65a+va+BIEvJanDtf1w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=RRdT92hS; arc=none smtp.client-ip=95.215.58.171 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="RRdT92hS" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1746195982; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=qhhZm+7v2L3Je3++vQOTPSZ1GcPWDVHw6xWa+wHNGkg=; b=RRdT92hSwCCMwy891Mow09cD2Li4if9SkyeAymSds629ZfnqXVXZ5C91HLIfGYIYRi4i8F BjDHJZvDbyJlUuaXcLd2p/XFKxUuZymv0Eq75l8NFFoSn35kGx9FGjoSpOHzeu2G6n1Rsj T965yfd0A4H7jWOz/naFCzI8fFEC0I8= Date: Fri, 2 May 2025 22:25:59 +0800 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg_cnt] helpers in raw tracepoint programs To: Alexei Starovoitov Cc: Kafai Wan , Song Liu , Jiri Olsa , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Matt Bobrowski , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Mykola Lysenko , Shuah Khan , LKML , bpf , linux-trace-kernel , Network Development , "open list:KERNEL SELFTEST FRAMEWORK" References: <20250426160027.177173-1-mannkafai@gmail.com> <20250426160027.177173-2-mannkafai@gmail.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Leon Hwang In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/5/1 00:53, Alexei Starovoitov wrote: > On Wed, Apr 30, 2025 at 8:55 AM Leon Hwang wrote: >> >> >> >> On 2025/4/30 20:43, Kafai Wan wrote: >>> On Wed, Apr 30, 2025 at 10:46 AM Alexei Starovoitov >>> wrote: >>>> >>>> On Sat, Apr 26, 2025 at 9:00 AM KaFai Wan wrote: >>>>> >> [...] >> >> >> bpf_get_func_arg() will be very helpful for bpfsnoop[1] when tracing tp_btf. >> >> In bpfsnoop, it can generate a small snippet of bpf instructions to use >> bpf_get_func_arg() for retrieving and filtering arguments. For example, >> with the netif_receive_skb tracepoint, bpfsnoop can use >> bpf_get_func_arg() to filter the skb argument using pcap-filter(7)[2] or >> a custom attribute-based filter. This will allow bpfsnoop to trace >> multiple tracepoints using a single bpf program code. > > I doubt you thought it through end to end. > When tracepoint prog attaches we have this check: > /* > * check that program doesn't access arguments beyond what's > * available in this tracepoint > */ > if (prog->aux->max_ctx_offset > btp->num_args * sizeof(u64)) > return -EINVAL; > > So you cannot have a single bpf prog attached to many tracepoints > to read many arguments as-is. > You can hack around that limit with probe_read, > but the values won't be trusted and you won't be able to pass > such untrusted pointers into skb and other helpers/kfuncs. I understand that a single bpf program cannot be attached to multiple tracepoints using tp_btf. However, the same bpf code can be reused to create multiple bpf programs, each attached to a different tracepoint. For example: SEC("fentry") int BPF_PROG(fentry_fn) { /* ... */ return BPF_OK; } The above fentry code can be compiled into multiple bpf programs to trace different kernel functions. Each program can then use the bpf_get_func_arg() helper to access the arguments of the traced function. With this patch, tp_btf will gain similar flexibility. For example: SEC("tp_btf") int BPF_PROG(tp_btf_fn) { /* ... */ return BPF_OK; } Here, bpf_get_func_arg() can be used to access tracepoint arguments. Currently, due to the lack of bpf_get_func_arg() support in tp_btf, bpfsnoop[1] uses bpf_probe_read_kernel() to read tracepoint arguments. This is also used when filtering specific argument attributes. For instance, to filter the skb argument of the netif_receive_skb tracepoint by 'skb->dev->ifindex == 2', the translated bpf instructions with bpf_probe_read_kernel() would look like this: bool filter_arg(__u64 * args): ; filter_arg(__u64 *args) 209: (79) r1 = *(u64 *)(r1 +0) /* all tracepoint's argument has been read into args using bpf_probe_read_kernel() */ 210: (bf) r3 = r1 211: (07) r3 += 16 212: (b7) r2 = 8 213: (bf) r1 = r10 214: (07) r1 += -8 215: (85) call bpf_probe_read_kernel#-125280 216: (79) r3 = *(u64 *)(r10 -8) 217: (15) if r3 == 0x0 goto pc+10 218: (07) r3 += 224 219: (b7) r2 = 8 220: (bf) r1 = r10 221: (07) r1 += -8 222: (85) call bpf_probe_read_kernel#-125280 223: (79) r3 = *(u64 *)(r10 -8) 224: (67) r3 <<= 32 225: (77) r3 >>= 32 226: (b7) r0 = 1 227: (15) if r3 == 0x2 goto pc+1 228: (af) r0 ^= r0 229: (95) exit If bpf_get_func_arg() is supported in tp_btf, the bpf program will instead look like: static __noinline bool filter_skb(void *ctx) { struct sk_buff *skb; (void) bpf_get_func_arg(ctx, 0, (__u64 *) &skb); return skb->dev->ifindex == 2; } This will simplify the generated code and eliminate the need for bpf_probe_read_kernel() calls. However, in my tests (on kernel 6.8.0-35-generic, Ubuntu 24.04 LTS), the pointer returned by bpf_get_func_arg() is marked as a scalar rather than a trusted pointer: 0: R1=ctx() R10=fp0 ; if (!filter_skb(ctx)) 0: (85) call pc+3 caller: R10=fp0 callee: frame1: R1=ctx() R10=fp0 4: frame1: R1=ctx() R10=fp0 ; filter_skb(void *ctx) 4: (bf) r3 = r10 ; frame1: R3_w=fp0 R10=fp0 ; 5: (07) r3 += -8 ; frame1: R3_w=fp-8 ; (void) bpf_get_func_arg(ctx, 0, (__u64 *) &skb); 6: (b7) r2 = 0 ; frame1: R2_w=0 7: (85) call bpf_get_func_arg#183 ; frame1: R0_w=scalar() ; return skb->dev->ifindex == 2; 8: (79) r1 = *(u64 *)(r10 -8) ; frame1: R1_w=scalar() R10=fp0 fp-8=mmmmmmmm ; return skb->dev->ifindex == 2; 9: (79) r1 = *(u64 *)(r1 +16) R1 invalid mem access 'scalar' processed 7 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0 If the returned skb is a trusted pointer, the verifier will accept something like: static __noinline bool filter_skb(struct sk_buff *skb) { return skb->dev->ifindex == 2; } Which will compile into much simpler and more efficient instructions: bool filter_skb(struct sk_buff * skb): ; return skb->dev->ifindex == 2; 92: (79) r1 = *(u64 *)(r1 +16) ; return skb->dev->ifindex == 2; 93: (61) r1 = *(u32 *)(r1 +224) 94: (b7) r0 = 1 ; return skb->dev->ifindex == 2; 95: (15) if r1 == 0x2 goto pc+1 96: (b7) r0 = 0 ; return skb->dev->ifindex == 2; 97: (95) exit In conclusion: 1. It will be better if the pointer returned by bpf_get_func_arg() is trusted, only when the argument index is a known constant. 2. Adding bpf_get_func_arg() support to tp_btf will significantly simplify and improve tools like bpfsnoop. [1] https://github.com/bpfsnoop/bpfsnoop Thanks, Leon