How to fix "dmesg: klogctl: Permission denied" for normal user on Android?

dmesg calls C library's klogctl wrapper function for kernel's syslog syscall which reads kernel's ring buffer by accessing /dev/kmsg character device. adb shell dmesg returns permissions denied error because adbd which runs with context u:r:shell:s0 is not allowed by SELinux to read kernel log. You need to explicitly allow it (requires root):

~# supolicy --live 'allow shell kernel system syslog_read'

If property ro.logd.kernel is set to true (default on debuggable builds), Android's logging daemon (logd) writes kernel messages to a separate buffer named kernel, which can be read using logcat (doesn't require root):

~$ logcat -b kernel