This issue is on RHEL9 (AlmaLinux 9.4) and MariaDB v10.5.27.

I have (many) mariadb servers that have the InnoDB engine disabled and use
MyISAM as the default storage engine. As such, these DB's do not contain
"ibdata1" or "ib_logfile0" files.

I attempted to run mariabackup on these DB's with the following command:

mariabackup --backup --target-dir=/somedir --user=root --password=xxx

The program core dumps with the below error message.

I was able to get the backup program to run successfully by installing
ibdata1 and ib_logfile0 files from a freshly initialized empty DB, but
it seems mariabackup could handle this case more gracefully, or better
yet, provide an option to exclude InnoDB from the backup.

Using the "--print-defaults" option of mariabackup does indicate that
it recognizes that the "--skip-innodb" and "default-storage-engine=myisam"
options are set.

mariabackup would have been started with the following arguments:
--datadir=/var/lib/mysql --socket=/var/lib/mysql/mysql.sock --log-error=/var/log/mariadb/mariadb.log --pid-file=/run/mariadb/mariadb.pid --datadir=/scratch/var/mysql
--log-error=/scratch/var/mysql/mariadb.log --pid-file=/run/mariadb/mariadb.pid --skip-innodb --default-storage-engine=myisam --symbolic-links=1

[root@dbhost tmp]# mariabackup --backup --target-dir=/tmp/somedir --user=root --password=xxx
[00] 2025-03-27 14:19:31 Connecting to server host: localhost, user: root, password: set, port: not set, socket: not set
[00] 2025-03-27 14:19:31 Using server version 10.5.27-MariaDB
mariabackup based on MariaDB server 10.5.27-MariaDB Linux (x86_64)
[00] 2025-03-27 14:19:31 uses posix_fadvise().
[00] 2025-03-27 14:19:31 cd to /scratch/var/mysql/
[00] 2025-03-27 14:19:31 open files limit requested 0, set to 1024
[00] 2025-03-27 14:19:31 mariabackup: using the following InnoDB configuration:
[00] 2025-03-27 14:19:31 innodb_data_home_dir = .
[00] 2025-03-27 14:19:31 innodb_data_file_path = ibdata1:10M:autoextend
[00] 2025-03-27 14:19:31 innodb_log_group_home_dir = .
[00] 2025-03-27 14:19:31 InnoDB: Using Linux native AIO
2025-03-27 14:19:31 0 [Note] InnoDB: Number of pools: 1
2025-03-27 14:19:31 0 [Warning] InnoDB: Unable to open "./ib_logfile0" to check native AIO read support.
2025-03-27 14:19:31 0 [Warning] InnoDB: Linux Native AIO disabled.
2025-03-27 14:19:31 0 [ERROR] InnoDB: Operating system error number 2 in a file operation.
2025-03-27 14:19:31 0 [ERROR] InnoDB: The error means the system cannot find the path specified.
2025-03-27 14:19:31 0 [ERROR] InnoDB: If you are installing InnoDB, remember that you must create directories yourself, InnoDB does not create them.
2025-03-27 14:19:31 0 [ERROR] InnoDB: File ./ib_logfile0: 'open' returned OS error 71.
2025-03-27 14:19:31 0 [ERROR] [FATAL] InnoDB: open(./ib_logfile0) returned Generic error
250327 14:19:31 [ERROR] mysqld got signal 6 ;
Sorry, we probably made a mistake, and this is a bug.

Your assistance in bug reporting will enable us to fix this for the next release.
To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed,
something is definitely wrong and this may fail.

Server version: 10.5.27-MariaDB source revision: 6aa47fae304bd1f3efc0ebd2035bc1481ded73a3
key_buffer_size=0
read_buffer_size=131072
max_used_connections=0
max_threads=1
thread_count=0
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 6071 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0x0 thread_stack 0x49000
addr2line: 'mariabackup': No such file
Printing to addr2line failed
mariabackup(my_print_stacktrace+0x3f)[0x55779288b07f]