perf trace: Handle failure when trace point folder is missed

On Arm64 a case is perf tools fails to find the corresponding trace
point folder for system calls listed in the table 'syscalltbl_arm64',
e.g. the generated system call table contains "lookup_dcookie" but we
cannot find out the matched trace point folder for it.

We need to figure out if there have any issue for the generated system
call table, on the other hand, we need to handle the case when trace
point folder is missed under sysfs, this patch sets the flag
syscall::nonexistent as true and returns the error from
trace__read_syscall_info().

Another problem is for trace__syscall_info(), it returns two different
values if a system call doesn't exist: at the first time calling
trace__syscall_info() it returns NULL when the system call doesn't exist,
later if call trace__syscall_info() again for the same missed system
call, it returns pointer of syscall. trace__syscall_info() checks the
condition 'syscalls.table[id].name == NULL', but the name will be
assigned in the first invoking even the system call is not found.

So checking system call's name in trace__syscall_info() is not the right
thing to do, this patch simply checks flag syscall::nonexistent to make
decision if a system call exists or not, finally trace__syscall_info()
returns the consistent result (NULL) if a system call doesn't existed.

Fixes: b8b1033fcaa091d8 ("perf trace: Mark syscall ids that are not allocated to avoid unnecessary error messages")
Signed-off-by: Leo Yan <[e-mail]>
Acked-by: Ian Rogers <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: [e-mail]
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-trace.c

···
 		sc->tp_format = trace_event__tp_format("syscalls", tp_name);
 	}
 
+	/*
+	 * Fails to read trace point format via sysfs node, so the trace point
+	 * doesn't exist.  Set the 'nonexistent' flag as true.
+	 */
+	if (IS_ERR(sc->tp_format)) {
+		sc->nonexistent = true;
+		return PTR_ERR(sc->tp_format);
+	}
+
 	if (syscall__alloc_arg_fmts(sc, IS_ERR(sc->tp_format) ?
 					RAW_SYSCALL_ARGS_NUM : sc->tp_format->format.nr_fields))
 		return -ENOMEM;
-
-	if (IS_ERR(sc->tp_format))
-		return PTR_ERR(sc->tp_format);
 
 	sc->args = sc->tp_format->format.fields;
 	/*
···
 	    (err = trace__read_syscall_info(trace, id)) != 0)
 		goto out_cant_read;
 
-	if (trace->syscalls.table[id].name == NULL) {
-		if (trace->syscalls.table[id].nonexistent)
-			return NULL;
+	if (trace->syscalls.table && trace->syscalls.table[id].nonexistent)
 		goto out_cant_read;
-	}
 
 	return &trace->syscalls.table[id];