Re: [PATCH bpf-next 2/3] bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW_TP

[Song Liu <songliubraving@fb.com>]

> On Nov 6, 2020, at 2:08 PM, Martin KaFai Lau <kafai@fb.com> wrote:
> 
> This patch enables the FENTRY/FEXIT/RAW_TP tracing program to use
> the bpf_sk_storage_(get|delete) helper, so those tracing programs
> can access the sk's bpf_local_storage and the later selftest
> will show some examples.
> 
> The bpf_sk_storage is currently used in bpf-tcp-cc, tc,
> cg sockops...etc which is running either in softirq or
> task context.
> 
> This patch adds bpf_sk_storage_get_tracing_proto and
> bpf_sk_storage_delete_tracing_proto.  They will check
> in runtime that the helpers can only be called when serving
> softirq or running in a task context.  That should enable
> most common tracing use cases on sk.
> 
> During the load time, the new tracing_allowed() function
> will ensure the tracing prog using the bpf_sk_storage_(get|delete)
> helper is not tracing any *sk_storage*() function itself.
> The sk is passed as "void *" when calling into bpf_local_storage.
> 
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> ---
> include/net/bpf_sk_storage.h |  2 +
> kernel/trace/bpf_trace.c     |  5 +++
> net/core/bpf_sk_storage.c    | 73 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 80 insertions(+)
> 
> diff --git a/include/net/bpf_sk_storage.h b/include/net/bpf_sk_storage.h
> index 3c516dd07caf..0e85713f56df 100644
> --- a/include/net/bpf_sk_storage.h
> +++ b/include/net/bpf_sk_storage.h
> @@ -20,6 +20,8 @@ void bpf_sk_storage_free(struct sock *sk);
> 
> extern const struct bpf_func_proto bpf_sk_storage_get_proto;
> extern const struct bpf_func_proto bpf_sk_storage_delete_proto;
> +extern const struct bpf_func_proto bpf_sk_storage_get_tracing_proto;
> +extern const struct bpf_func_proto bpf_sk_storage_delete_tracing_proto;
> 
> struct bpf_local_storage_elem;
> struct bpf_sk_storage_diag;
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index e4515b0f62a8..cfce60ad1cb5 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -16,6 +16,7 @@
> #include <linux/syscalls.h>
> #include <linux/error-injection.h>
> #include <linux/btf_ids.h>
> +#include <net/bpf_sk_storage.h>
> 
> #include <uapi/linux/bpf.h>
> #include <uapi/linux/btf.h>
> @@ -1735,6 +1736,10 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> 		return &bpf_skc_to_tcp_request_sock_proto;
> 	case BPF_FUNC_skc_to_udp6_sock:
> 		return &bpf_skc_to_udp6_sock_proto;
> +	case BPF_FUNC_sk_storage_get:
> +		return &bpf_sk_storage_get_tracing_proto;
> +	case BPF_FUNC_sk_storage_delete:
> +		return &bpf_sk_storage_delete_tracing_proto;
> #endif
> 	case BPF_FUNC_seq_printf:
> 		return prog->expected_attach_type == BPF_TRACE_ITER ?
> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> index 001eac65e40f..1a41c917e08d 100644
> --- a/net/core/bpf_sk_storage.c
> +++ b/net/core/bpf_sk_storage.c
> @@ -6,6 +6,7 @@
> #include <linux/types.h>
> #include <linux/spinlock.h>
> #include <linux/bpf.h>
> +#include <linux/btf.h>
> #include <linux/btf_ids.h>
> #include <linux/bpf_local_storage.h>
> #include <net/bpf_sk_storage.h>
> @@ -378,6 +379,78 @@ const struct bpf_func_proto bpf_sk_storage_delete_proto = {
> 	.arg2_type	= ARG_PTR_TO_BTF_ID_SOCK_COMMON,
> };
> 
> +static bool bpf_sk_storage_tracing_allowed(const struct bpf_prog *prog)
> +{
> +	const struct btf *btf_vmlinux;
> +	const struct btf_type *t;
> +	const char *tname;
> +	u32 btf_id;
> +
> +	if (prog->aux->dst_prog)
> +		return false;
> +
> +	/* Ensure the tracing program is not tracing
> +	 * any *sk_storage*() function and also
> +	 * use the bpf_sk_storage_(get|delete) helper.
> +	 */
> +	switch (prog->expected_attach_type) {
> +	case BPF_TRACE_RAW_TP:
> +		/* bpf_sk_storage has no trace point */
> +		return true;
> +	case BPF_TRACE_FENTRY:
> +	case BPF_TRACE_FEXIT:
> +		btf_vmlinux = bpf_get_btf_vmlinux();
> +		btf_id = prog->aux->attach_btf_id;
> +		t = btf_type_by_id(btf_vmlinux, btf_id);

What happens to fentry/fexit attach to other BPF programs? I guess
we should check for t == NULL?

Thanks,
Song

> +		tname = btf_name_by_offset(btf_vmlinux, t->name_off);
> +		return !strstr(tname, "sk_storage");
> +	default:
> +		return false;
> +	}
> +
> +	return false;
> +}

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