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Date: Thu, 17 Oct 2019 00:08:23 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: Alexei Starovoitov <alexei.starovoitov@...il.com> Cc: Alexei Starovoitov <ast@...nel.org>, "David S. Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>, Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com> Subject: Re: [PATCH v3 bpf-next 06/11] bpf: implement accurate raw_tp context access via BTF On 10/16/19 11:28 PM, Alexei Starovoitov wrote: > On Wed, Oct 16, 2019 at 2:22 PM Daniel Borkmann <daniel@...earbox.net> wrote: >> On 10/16/19 5:25 AM, Alexei Starovoitov wrote: >>> libbpf analyzes bpf C program, searches in-kernel BTF for given type name >>> and stores it into expected_attach_type. >>> The kernel verifier expects this btf_id to point to something like: >>> typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, void *loc); >>> which represents signature of raw_tracepoint "kfree_skb". >>> >>> Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb' >>> and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint. >>> In first case it passes btf_id of 'struct sk_buff *' back to the verifier core >>> and 'void *' in second case. >>> >>> Then the verifier tracks PTR_TO_BTF_ID as any other pointer type. >>> Like PTR_TO_SOCKET points to 'struct bpf_sock', >>> PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on. >>> PTR_TO_BTF_ID points to in-kernel structs. >>> If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF >>> then PTR_TO_BTF_ID#1234 points to one of in kernel skbs. >>> >>> When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32) >>> the btf_struct_access() checks which field of 'struct sk_buff' is >>> at offset 32. Checks that size of access matches type definition >>> of the field and continues to track the dereferenced type. >>> If that field was a pointer to 'struct net_device' the r2's type >>> will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device' >>> in vmlinux's BTF. >>> >>> Such verifier analysis prevents "cheating" in BPF C program. >>> The program cannot cast arbitrary pointer to 'struct sk_buff *' >>> and access it. C compiler would allow type cast, of course, >>> but the verifier will notice type mismatch based on BPF assembly >>> and in-kernel BTF. >>> >>> Signed-off-by: Alexei Starovoitov <ast@...nel.org> >> >> Overall set looks great! >> >> [...] >>> +int btf_struct_access(struct bpf_verifier_log *log, >>> + const struct btf_type *t, int off, int size, >>> + enum bpf_access_type atype, >>> + u32 *next_btf_id) >>> +{ >>> + const struct btf_member *member; >>> + const struct btf_type *mtype; >>> + const char *tname, *mname; >>> + int i, moff = 0, msize; >>> + >>> +again: >>> + tname = __btf_name_by_offset(btf_vmlinux, t->name_off); >> >> More of a high-level question wrt btf_ctx_access(), is there a reason the ctx >> access is only done for raw_tp? I presume kprobes is still on todo (?), what >> about uprobes which also have pt_regs and could benefit from this work, but is >> not fixed to btf_vmlinux to search its ctx type. > > Optimized kprobes via ftrace entry point are on immediate todo list > to follow up. I'm still debating on the best way to handle it. > uprobes - I haven't though about. Likely necessary as well. > Not sure what types to give to pt_regs yet. > >> I presume BPF_LDX | BPF_PROBE_MEM | BPF_* would need no additional encoding, >> but JIT emission would have to differ depending on the prog type. > > you mean for kprobes/uprobes? Why would it need to be different? > The idea was to keep LDX|PROBE_MEM as normal LDX|MEM load as much as possible. Agree, makes sense. > The only difference vs normal load is to populate extable which is > arch dependent. Wouldn't you also need to switch to USER_DS similarly to what probe_kernel_read() vs probe_user_read() differentiates? Thanks, Daniel
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