perf trace: If a syscall arg is marked as 'const', assume it is coming _from_ userspace

We need to decide where to copy syscall arg contents, if at the
syscalls:sys_entry hook, meaning is something that is coming from
user to kernel space, or if it is a response, i.e. if it is something
the _kernel_ is filling in and thus going to userspace.

Since we have 'const' used in those syscalls, and unsure about this
being consistent, doing:

  root@number:~# echo $(grep const /sys/kernel/tracing/events/syscalls/sys_enter_*/format  | grep struct | cut -c47- | cut -d'/' -f1)
  clock_nanosleep clock_settime epoll_pwait2 futex io_pgetevents landlock_create_ruleset listmount mq_getsetattr mq_notify mq_timedreceive mq_timedsend preadv2 preadv prlimit64 process_madvise process_vm_readv process_vm_readv process_vm_writev process_vm_writev pwritev2 pwritev readv rt_sigaction rt_sigtimedwait semtimedop statmount timerfd_settime timer_settime vmsplice writev
  root@number:~#

Seems to indicate that we can use that for the ones that have the
'const' to mark it as coming from user space, do it.

Most notable/frequent syscall that now gets BTF pretty printed in a
system wide 'perf trace' session is:

  root@number:~# perf trace
     21.160 (         ): MediaSu~isor #/1028597 futex(uaddr: 0x7f49e1dfe964, op: WAIT_BITSET|PRIVATE_FLAG, utime: (struct __kernel_timespec){.tv_sec = (__kernel_time64_t)50290,.tv_nsec = (long long int)810362837,}, val3: MATCH_ANY) ...
      21.166 ( 0.000 ms): RemVidChild/6995 futex(uaddr: 0x7f49fcc7fa00, op: WAKE|PRIVATE_FLAG, val: 1)           = 0
      21.169 ( 0.001 ms): RemVidChild/6995 sendmsg(fd: 25<socket:[78915]>, msg: 0x7f49e9af9da0, flags: DONTWAIT) = 280
      21.172 ( 0.289 ms): RemVidChild/6995 futex(uaddr: 0x7f49fcc7fa58, op: WAIT_BITSET|PRIVATE_FLAG|CLOCK_REALTIME, val3: MATCH_ANY) = 0
      21.463 ( 0.000 ms): RemVidChild/6995 futex(uaddr: 0x7f49fcc7fa00, op: WAKE|PRIVATE_FLAG, val: 1)           = 0
      21.467 ( 0.001 ms): RemVidChild/6995 futex(uaddr: 0x7f49e28bb964, op: WAKE|PRIVATE_FLAG, val: 1)           = 1
      21.160 ( 0.314 ms): MediaSu~isor #/1028597  ... [continued]: futex())                                            = 0
      21.469 (         ): RemVidChild/6995 futex(uaddr: 0x7f49fcc7fa5c, op: WAIT_BITSET|PRIVATE_FLAG|CLOCK_REALTIME, val3: MATCH_ANY) ...
      21.475 ( 0.000 ms): MediaSu~isor #/1028597 futex(uaddr: 0x7f49d0223040, op: WAKE|PRIVATE_FLAG, val: 1)           = 0
      21.478 ( 0.001 ms): MediaSu~isor #/1028597 futex(uaddr: 0x7f49e26ac964, op: WAKE|PRIVATE_FLAG, val: 1)           = 1
  ^Croot@number:~#
  root@number:~# cat /sys/kernel/tracing/events/syscalls/sys_enter_futex/format
  name: sys_enter_futex
  ID: 454
  format:
  	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
  	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
  	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
  	field:int common_pid;	offset:4;	size:4;	signed:1;

  	field:int __syscall_nr;	offset:8;	size:4;	signed:1;
  	field:u32 * uaddr;	offset:16;	size:8;	signed:0;
  	field:int op;	offset:24;	size:8;	signed:0;
  	field:u32 val;	offset:32;	size:8;	signed:0;
  	field:const struct __kernel_timespec * utime;	offset:40;	size:8;	signed:0;
  	field:u32 * uaddr2;	offset:48;	size:8;	signed:0;
  	field:u32 val3;	offset:56;	size:8;	signed:0;

  print fmt: "uaddr: 0x%08lx, op: 0x%08lx, val: 0x%08lx, utime: 0x%08lx, uaddr2: 0x%08lx, val3: 0x%08lx", ((unsigned long)(REC->uaddr)), ((unsigned long)(REC->op)), ((unsigned long)(REC->val)), ((unsigned long)(REC->utime)), ((unsigned long)(REC->uaddr2)), ((unsigned long)(REC->val3))
  root@number:~#

Suggested-by: Ian Rogers <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alan Maguire <[email protected]>
Cc: Howard Chu <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/lkml/CAP-5=fWnuQrrBoTn6Rrn6vM_xQ2fCoc9i-AitD7abTcNi-4o1Q@mail.gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: acmel

tools/perf/builtin-trace.c

@@ -2001,11 +2001,14 @@ syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field
 
 		len = strlen(field->name);
 
+		// As far as heuristics (or intention) goes this seems to hold true, and makes sense!
+		if ((field->flags & TEP_FIELD_IS_POINTER) && strstarts(field->type, "const "))
+			arg->from_user = true;
+
 		if (strcmp(field->type, "const char *") == 0 &&
 		    ((len >= 4 && strcmp(field->name + len - 4, "name") == 0) ||
 		     strstr(field->name, "path") != NULL)) {
 			arg->scnprintf = SCA_FILENAME;
-			arg->from_user = true;
 		} else if ((field->flags & TEP_FIELD_IS_POINTER) || strstr(field->name, "addr"))
 			arg->scnprintf = SCA_PTR;
 		else if (strcmp(field->type, "pid_t") == 0)