[PATCH] Add BPF Kernel Function bpf_ptrace_vprintk

[Eric Yan <eric.yan@xxxxxxxx>]

add a kfunc 'bpf_ptrace_vprintk' printing bpf msg with trace_marker
format requirement so that these msgs can be retrieved by android
perfetto by default and well represented in perfetto UI.

[testing prog]
const volatile bool ptrace_enabled = true;
extern int bpf_ptrace_vprintk(char *fmt, u32 fmt_size, const void *args, u32 args__sz) __ksym;

({                                    \
    if (!ptrace_enabled) { \
        bpf_printk(fmt, __VA_ARGS__);     \
    } else {                              \
        char __fmt[] = fmt;               \
        _Pragma("GCC diagnostic push")    \
        _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")  \
        u64 __params[] = { __VA_ARGS__ }; \
        _Pragma("GCC diagnostic pop")     \
        bpf_ptrace_vprintk(__fmt, sizeof(__fmt), __params, sizeof(__params)); \
    }                                  \
})

SEC("perf_event")
int do_sample(struct bpf_perf_event_data *ctx)
{
        u64 ip = PT_REGS_IP(&ctx->regs);
        u64 id = bpf_get_current_pid_tgid();
        s32 pid = id >> 32;
        s32 tid = id;
        debug_printk("N|%d|BPRF-%d|BPRF:%llx", pid, tid, ip);
        return 0;
}

[output]:
       app-3151    [000] d.h1.  6059.904239: tracing_mark_write: N|2491|BPRF-3151|BPRF:58750d0eec

Signed-off-by: Eric Yan <eric.yan@xxxxxxxx>
---
 kernel/bpf/helpers.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1a43d06eab28..e3d17d3b17c5 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2521,6 +2521,39 @@ __bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)
 	return p;
 }
 
+static noinline void tracing_mark_write(char *buf)
+{
+	trace_printk(buf);
+}
+
+/**
+ * same as bpf_trace_vprintk, except for a trace_marker format requirement
+ */
+__bpf_kfunc int bpf_ptrace_vprintk(char *fmt, u32 fmt_size, const void *args, u32 args__sz)
+{
+	struct bpf_bprintf_data data = {
+		.get_bin_args	= true,
+		.get_buf	= true,
+	};
+	int ret, num_args;
+
+	if (args__sz & 7 || args__sz > MAX_BPRINTF_VARARGS * 8 || (args__sz && !args))
+		return -EINVAL;
+	num_args = args__sz / 8;
+
+	ret = bpf_bprintf_prepare(fmt, fmt_size, args, num_args, &data);
+	if (ret < 0)
+		return ret;
+
+	ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt, data.bin_args);
+
+	tracing_mark_write(data.buf);
+
+	bpf_bprintf_cleanup(&data);
+
+	return ret;
+}
+
 /**
  * bpf_dynptr_slice() - Obtain a read-only pointer to the dynptr data.
  * @p: The dynptr whose data slice to retrieve
@@ -3090,6 +3123,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_ptrace_vprintk)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
-- 
2.34.1