[Miscellaneous Android Development] rlrevell:
Edit: SUCCESS! I tried it with Magisk 17.1 and Magisk Manager 5.9.1 and all is working.

Hey! Followed these steps on my LG LMX420 (Android 9) and everything seemed to work, I got the correct output, but su gives:

mmh4x:/ $ su
read (4 != 0) failed with 0: Success
1|mmh4x:/ $ ps -eZ | grep magisk
u:r:untrusted_app:s0:c245,c256,c512,c768 u0_a245 5197 2356 1000848 119632 0 0 S com.topjohnwu.magisk
mmh4x:/ $

The magisk daemon has apparently crashed. I can restart it but the same happens again, here's strace excerpt from su:

socket(PF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC, 0) = 3
clock_gettime(CLOCK_MONOTONIC, {538, 725052847}) = 0
connect(3, {sa_family=AF_LOCAL, sun_path=@"d30138f2310a9fb9c54a3e0c21f58591"}, 35) = 0
clock_gettime(CLOCK_MONOTONIC, {538, 725647078}) = 0
write(3, "\1\0\0\0", 4) = 4
read(3, "", 4) = 0
write(2, "read (4 != 0) failed with 0: Suc"..., 37read (4 != 0) failed with 0: Success
) = 37

Edit: The su incovation is causing magiskd to crash with a null pointer dereference:

--------- beginning of crash
12-27 14:43:39.055 6953 8121 F libc : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7d0 in tid 8121 (magisk.bin), pid 6953 (magisk.bin)
12-27 14:43:39.139 8130 8130 F DEBUG : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
12-27 14:43:39.141 8130 8130 F DEBUG : Build fingerprint: 'lge/mmh4x_fg/mmh4x:9/PKQ1.190414.001/191822103aa3c:user/release-keys'
12-27 14:43:39.141 8130 8130 F DEBUG : Revision: '0'
12-27 14:43:39.142 8130 8130 F DEBUG : ABI: 'arm'
12-27 14:43:39.142 8130 8130 F DEBUG : pid: 6953, tid: 8121, name: magisk.bin >>> magiskd <<<
12-27 14:43:39.142 8130 8130 F DEBUG : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7d0
12-27 14:43:39.142 8130 8130 F DEBUG : Cause: null pointer dereference
12-27 14:43:39.142 8130 8130 F DEBUG : r0 000007d0 r1 000007d0 r2 00000000 r3 00000000
12-27 14:43:39.142 8130 8130 F DEBUG : r4 00000000 r5 b39b82bc r6 8cf2a9f7 r7 b39b7ebc
12-27 14:43:39.142 8130 8130 F DEBUG : r8 b39b8368 r9 00000000 r10 00000001 r11 8000002f
12-27 14:43:39.142 8130 8130 F DEBUG : ip b3f275a0 sp b39b7db8 lr b3eccce9 pc b3e9cc9c
12-27 14:43:39.163 8130 8130 F DEBUG :
12-27 14:43:39.163 8130 8130 F DEBUG : backtrace:
12-27 14:43:39.163 8130 8130 F DEBUG : #00 pc 0001ac9c /system/lib/libc.so (strlen+72)
12-27 14:43:39.163 8130 8130 F DEBUG : #01 pc 0004ace5 /system/lib/libc.so (__vfprintf+3224)
12-27 14:43:39.164 8130 8130 F DEBUG : #02 pc 0005f935 /system/lib/libc.so (vsnprintf+108)
12-27 14:43:39.164 8130 8130 F DEBUG : #03 pc 00006b25 /system/lib/liblog.so (__android_log_vprint+40)
12-27 14:43:39.164 8130 8130 F DEBUG : #04 pc 00010e5d /sbin/magisk.bin
12-27 14:43:39.164 8130 8130 F DEBUG : #05 pc 00007a89 /sbin/magisk.bin
12-27 14:43:39.164 8130 8130 F DEBUG : #06 pc 0000c0d1 /sbin/magisk.bin
12-27 14:43:39.164 8130 8130 F DEBUG : #07 pc 00003e03 /sbin/magisk.bin
12-27 14:43:39.164 8130 8130 F DEBUG : #08 pc 00063c69 /system/lib/libc.so (__pthread_start(void*)+22)
12-27 14:43:39.164 8130 8130 F DEBUG : #09 pc 0001dec9 /system/lib/libc.so (__start_thread+24)
12-27 14:43:39.189 2718 2933 I chatty : uid=1000(system) Thread-9 expire 1 line

It's not recognizing arguments correctly either:

1|mmh4x:/data/local/tmp $ magisk --list
magisk
su
resetprop
magiskhide
imgtool
mmh4x:/data/local/tmp $ magisk su
Magisk v18.1(18100) (by topjohnwu) multi-call binary

Usage: magisk [applet [arguments]...]
or: magisk [options]...

Any ideas?