/proc/kmsg provides a root-only, read-only, consuming view of the kernel log buffer. It’s equivalent to calling
syslog(2) with the
SYSLOG_ACTION_READ action. As mentioned in the
A process must have superuser privileges to read this file, and only one process should read this
file. This file should not be read if a syslog process is
running which uses the
syslog(2) system call facility to log
/dev/kmsg provides access to the same kernel log buffer, but in an easier-to-use fashion. Reads are tracked per open, so multiple processes can read in parallel, and entries aren’t removed from the buffer as they are read.
/dev/kmsg also provides write access to the log buffer, so it can be used to add entries to the log buffer. See the
/dev/kmsg documentation for details.
As for why both are present, and why one is in
/proc (albeit not process-related) and one in
/proc/kmsg is an old convenience “export” of kernel internals, and
/dev/kmsg is a more recent addition, designed as a usable interface to the log buffer.