shm_open() fails with EACCESS for root user

[Radek Krejci]

In Ubuntu 20.04 I'm having problem with accessing shared memory segments by different users. The associated files are created using shm_open() with the combination of O_CREAT and O_RDWR flags and placed in /dev/shm. The example of such a code is in the enclosed source file.

When I run it for the first time, it works as expected and /dev/shm/testshm is created:

$ ls -la /dev/shm
total 0
drwxrwxrwt 2 root root 60 Aug 3 16:34 .
drwxr-xr-x 18 root root 4580 Aug 3 12:41 ..
-rw-rw-r-- 1 krejci krejci 0 Aug 3 14:37 testshm

Then running program again under the same user works fine, but when I try to run the program as root (or any other user), I'm getting "Permission denied":

# id
uid=0(root) gid=0(root) groups=0(root)
# ./test
return value -1
result 13: Permission denied

The example program works as expected on previous Ubuntu (18.04, 19.10) releases as well as on other linux distros - other users has access according to the set permissions (root is always able to access it).

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: libc6 2.31-0ubuntu9
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: skip
Date: Mon Aug 3 17:35:07 2020
InstallationDate: Installed on 2020-08-03 (0 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: glibc
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.