I made a system call and recompiled the kernel, however upon running the system call it returned Killed. So, to trace it, I used strace, which shows the following message:
syscall_0x224(0x7ffda7199738, 0x7ffda7199748, 0x55743750a6d0, 0x7f9f20df7d80, 0x7f9f20df7d80, 0x7ffda7199730) = ?
What does this mean (not the hex, the question mark)?
It means that the system call was terminated, and didn't (couldn't) return a value. An example is given in the strace manual:
Interruption of a (restartable) system call by a signal delivery is
processed differently as kernel terminates the system call and also
arranges its immediate reexecution after the signal handler
completes.
read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted)
It seems, like your system call, read here was terminated, and has no return value. (Unlike your system call, read here was scheduled for re-execution.)
Other system calls, which don't return (e.g., exit_group), also show ?:
~ strace -e exit_group /bin/true
exit_group(0) = ?
+++ exited with 0 +++
This means that your strace doesn't know about syscall_0x224 (neither do I [1]), and doesn't know what kind of value returns and how to decode it.
You can have a look at the source here:
if (sys_res & RVAL_NONE)
tprints("= ?");
If you have compiled some module that adds another system call, do not expect strace to magically know about it.
[1] I wasn't able to find any syscall with nr = 0x224 / 548 (the x32 pwritev2 is the last = 547).