/etc/profile, ~/.bashrc and no other profile files are sourced for users, only for root

Question

Since I've upgraded my server from Debian 8 to 9 I have a strange behavior with bash. When I log in via ssh, none of the profile files are loaded, I get the following prompt:

Linux myserver 4.9.0-7-amd64 #1 SMP Debian 4.9.110-1 (2018-07-05) x86_64
Last login: Mon Jul 16 22:37:12 2018 from x.x.x.x
-bash-4.4$

Running echo $0 returns -bash. I've put an echo "sourcing ..." line into /etc/profile, ~/.bashrc to see when they are invoked, but none of these lines are displayed. However, if I source these files manually, the text is displayed and the file is sourced as it should.

Using strace (found here) to see which files were open I see that all of the profiles files were open. echo exit | strace bash -li |& less | grep '^open'

I get the following output:

...
open("/etc/profile", O_RDONLY)          = 3
open("/etc/bash.bashrc", O_RDONLY)      = 3
open("/etc/profile.d/", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
open("/etc/profile.d/bash_completion.sh", O_RDONLY) = 3
...
open("/home/myuser/.bash_profile", O_RDONLY) = 3
open("/home/myuser/.bash_history", O_RDONLY) = 3
open("/home/myuser/.bash_history", O_RDONLY) = 3
...
open("/home/myuser/.bash_logout", O_RDONLY) = 3
open("/etc/bash.bash_logout", O_RDONLY) = -1 ENOENT (No such file or directory)
...

The problem only occurs when I log in as a normal user, once I su to root, the profile files are loaded as they should. I suspect that maybe only root has now permission to invoke the files, but I am going crazy as everything I checked is as it should be.

Any hint or idea is very much appreciated.

Answer 1

Found the solution, which was around permissions as I suspected. Somehow the setgid bit was set on /bin/bash.

The left system is the one with the problem, the right is a working one:

To solve the problem I run the following command:

   sudo chmod g-s /bin/bash