Dumping a struct to a buffer in an strace like style using BTF

[Arnaldo Carvalho de Melo]

Hi Alan,

	Something I planned to do since forever is to get the contents
of syscall args and print in 'perf trace' using BTF, right now we have
things like:

[root@quaco ~]# perf trace -e connect* ssh localhost
     0.000 ( 0.342 ms): ssh/438068 connect(fd: 3, uservaddr: { .family: INET, port: 22, addr: 127.0.0.1 }, addrlen: 16) = 0
root@localhost's password:

in perf-tools-next when building with BUILD_BPF_SKEL=1 that will hook
into that specific syscall and collect the uservaddr sockaddr struct and
then pretty print it.

That is done manually (the last leg) in
tools/perf/trace/beauty/sockaddr.c:

  syscall_arg__scnprintf_augmented_sockaddr
     af_scnprintfs[family](syscall pointer contents collected via BPF)

which leads to struct sockaddr_in or sockaddr_in6 specific pretty
printers, I wanted to do what these two struct specific pretty printers
do but using BTF.

I guess this is already available, but from a _really_ quick look at
libbpf I couldn't find it, ideas?

I want to try the code at the end of this message for another
multiplexer syscall, bpf(), with this on top of what is at:

git://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git perf-tools-next

Best regards,

- Arnaldo

diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
index 9c1d0b271b20f693..79767422efe9479c 100644
--- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
+++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
@@ -319,6 +319,27 @@ int sys_enter_perf_event_open(struct syscall_enter_args *args)
 	return 1; /* Failure: don't filter */
 }
 
+SEC("tp/syscalls/sys_enter_bpf")
+int sys_enter_bpf(struct syscall_enter_args *args)
+{
+	struct augmented_args_payload *augmented_args = augmented_args_payload();
+	const void *attr = (void *)args->args[1];
+	unsigned int size = args->args[2];
+	unsigned int len = sizeof(augmented_args->args);
+
+        if (augmented_args == NULL)
+		goto failure;
+
+	size &= sizeof(augmented_args->__data) - 1;
+
+	if (bpf_probe_read(&augmented_args->__data, size, attr) < 0)
+		goto failure;
+
+	return augmented__output(args, augmented_args, len + size);
+failure:
+	return 1; /* Failure: don't filter */
+}
+
 SEC("tp/syscalls/sys_enter_clock_nanosleep")
 int sys_enter_clock_nanosleep(struct syscall_enter_args *args)
 {