Table '/tmp/#sql_1_0.MAI' is read only

Question

When I try to use SHOW FULL FIELDS FROM or some other SHOW COLUMNS syntax query I get this strange error message:

MariaDB [mysql]> show full fields from user;
ERROR 1036 (HY000): Table '/tmp/#sql_126_0.MAI' is read only

Tmp dir is present and has 777 permissions. Changing tmp dir in my.cnf doesn't help.

Chowning that dir to mysql user doesn't help either.

Steps to reproduce:

1. Install CentOS 7 into VM (or any sufficiently old kernel)
2. Install Docker on CentOS 7 (so that we can use alpine)
3. Create alpine-based image with MariaDB 10.1.28 (apk add mysql mysql-client) or use readily available image
4. Run mysql container docker run --name mariadb_container mariadb_image
5. Connect to mysql using docker exec -ti mariadb_container mysql -A
6. Run SHOW FULL FIELDS FROM some_table query

Any suggestions on how to trace that problem?

Answer 1

Warning: this is a working theory, not confirmed by any independent sources.

There are 3 points of failure that may explain why the bug is reproducible under the very specific conditions (CentOS 7, docker, alpine, mariadb 10.1.22+):

• some linux kernel bug specific to /proc/self/fd/%d permissions for container threads and setuid
• /proc/-based realpath implementation in musl library
• changes in mariadb to fix mi_open race condition security issue

Links to discussions:

• docker4drupal
• Alpine mariadb package
• docker issue describing wrong readlink behavior
• discussion of musl implementation of realpath

Links to code:

• Kernel fix for fd permissions security issue (2013, included in kernel 3.12+)
• Kernel fix to fix regression introduced by previous fix (2015, included in kernel 4.4+)
• realpath implementation in musl
• mi_open fix in mariadb

Approaches to fix:

• upgrade to the latest kernel version (no new versions available in CentOS/RHEL, so switch to another OS or make a petition to RedHat to backport the fix)
• investigate issues in musl realpath implementation
• investigate issues in mariadb mi_open implementation (specifically changes to this file in 10.1.22)

Workarounds:

• Use glibc-based mariadb published on DockerHub: https://hub.docker.com/r/library/mariadb/tags/
• Switch to mariadb 10.1.20 (latest stable version before the bug): https://hub.docker.com/r/sizmek/mariadb/tags/

Answer 2

You should try to downgrade your Mariadb to version 10.1.17 (worked for me)