bpf/perf: Call perf_prepare_sample() before bpf_prog_run()

When the BPF program calls bpf_cast_to_kern_ctx(), it assumes the program will
access perf sample data directly and call perf_prepare_sample() to make sure
the sample data is populated.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: namhyung

include/linux/bpf.h

@@ -1341,6 +1341,7 @@ struct bpf_prog {
 				enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
 				call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */
 				call_get_func_ip:1, /* Do we call get_func_ip() */
+				call_cast_kctx:1, /* Do we call bpf_cast_to_kern_ctx() */
 				tstamp_type_access:1; /* Accessed __sk_buff->tstamp_type */
 	enum bpf_prog_type	type;		/* Type of BPF program */
 	enum bpf_attach_type	expected_attach_type; /* For some prog types */

kernel/bpf/verifier.c

@@ -9236,6 +9236,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
 				regs[BPF_REG_0].btf = desc_btf;
 				regs[BPF_REG_0].btf_id = meta.ret_btf_id;
+				env->prog->call_cast_kctx = 1;
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
 				ret_t = btf_type_by_id(desc_btf, meta.arg_constant.value);
 				if (!ret_t || !btf_type_is_struct(ret_t)) {

kernel/events/core.c

@@ -10332,6 +10332,7 @@ static void bpf_overflow_handler(struct perf_event *event,
 		.event = event,
 	};
 	struct bpf_prog *prog;
+	struct perf_event_header dummy;
 	int ret = 0;
 
 	ctx.regs = perf_arch_bpf_user_pt_regs(regs);
@@ -10346,6 +10347,8 @@ static void bpf_overflow_handler(struct perf_event *event,
 			data->callchain = perf_callchain(event, regs);
 			data->sample_flags |= PERF_SAMPLE_CALLCHAIN;
 		}
+		if (prog->call_cast_kctx)
+			perf_prepare_sample(&dummy, data, event, regs);
 
 		ret = bpf_prog_run(prog, &ctx);
 	}