There are real, and subtle differences between su and sudo

Most of the time, sudo just works. Every now and then, it doesn’t. Most recently was with a build I am working on, where I got a “permission denied” error for creating a directory.

The reason for this was non-obvious at first. You “are” superuser after all when you sudo, right? Aren’t you?

Sort of.

Your effective user ID has been set to the superuser. Your real user ID still is yours. This means things like your temp directory are not necessarily yours … er … the real user ID of the temp directory owner might be different from the effective user ID you are building as. And if you have a root_squash on an NFS mount, or your system uses one or the other security mechanisms to prevent privilege escalation … here be dragons.

So it seems, during a build of rust 0.14.0, I ran head first into this. I will freely admit that my mouth was agape for a bit. I will not admit to drool falling out, and have rapidly deleted any such webcam video.

Ok, more seriously, it was a WTF moment. Took me a second to understand, as my prompt says # when I sudo -s. The make was run as sudo. The make command failed under sudo with a permission (!!!@@!@@!) error. Then a fast ‘su’ and off to the races we went.


While I want to dig into this more, my goal here was building rust in a reliable and repeatable manner. I don’t have that going quite yet. Very close, but I’ve now run into LLVM/clang oddities, and have switched back to gcc for it. Build completes now, but install is still problematic because of this issue.

I could just build as root user, and other build environments I’ve built do that. I’ve been trying to get away from that, as it is a bad habit, and an errant make file could wreak havoc. But the converse is also true, in that during installation, often you need to be root to install into specific paths.

I can change that assumption, and create a specific path owned by a specific user, and off to the races I go. I prefer that model, and then let the admins set up sudo access to the tree.

Viewed 39605 times by 1915 viewers