Bug 192837

Created attachment 146044 [details]
patch for the modification suggested above

Problem:

When calling /usr/bin/su, there are several unconditional calls to tcsetpgrp, which changes control of the terminal. This causes problems when the su call is a part of a pipeline and other processes in that pipeline require terminal access. For example, if I run the following:

root@host$ su -m nobody -c 'find /' | less

...less will get stuck in the background. This is a problem when the call to su is embedded in a script (e.g., root scripts that need to occasionally do something as a normal user), and that script is a part of a pipeline, because one can't simply move the rest of the pipeline into the su command.

The calls to tcsetpgrp are only necessary because su forks and creates a new process group for the child. Because the child potentially needs the terminal for authentication or executing the command, it needs terminal control, which takes it away from whatever process group su is a part of, e.g., a pipeline.


Solution:

I don't see a good reason for the fork+setpgid+waitpid code (https://svnweb.freebsd.org/base/stable/10/usr.bin/su/su.c?revision=256281&view=markup#l445). Really, the only thing it accomplishes is having the original suid process hang around until the command finishes, and it prevents the command from being a part of the pipeline it's embedded in. (e.g., in "su -m nobody -c 'find /' | less", "find" and "less" will not be in the same process group.) The fork code causes problems under these limited circumstances, without any apparent benefit. I therefore suggest that the fork code be removed, providing expected behavior to su. (Just for comparison, GNU su doesn't fork, and it exhibits the expected behavior.)