permissions question 7487

On Monday 13 November 2006 23:26, hug stood up and addressed the mbuttes incomp.os.linux.miscas follows...:

Yes, if he supplies the full path to it or "traverses" a symbolic link elsewhere pointing to it.

The simplest description would be this: the permissions of a file or directory depend upon the permission mask of said file or directory *and* on the permission mask of directory containing said file or directory.

For instance: if you have read and write permission on a file located in a directory where you have read permission but no write permission, then you can alter the file's contents but you cannot delete the file - which would be a write operation on the directory containing it.

A practical example, more along the lines of what you're asking: in most GNU-Linux distributions, *-usr-tmp* is a symbolic link to *-var-tmp.* On my system, *-usris mounted read-only. Yet anything requiring write access to *-usr-tmp* will indeed have write access to it (if the permissions allow it), because *-var-tmp* is mounted writeable.

Similarly, it is possible to have a system running in normal "production" mode with its root filesystem mounted read-only. After all, if the necessary filesystems are separated from the root filesystem and mounted read-write - this would of course also include usingudev(ordevfsfor 2.4 kernels) - then there is no problem. Well, aside from a few other tweaks that need to be taken care of, but it's quite feasible.

The only difference with the situation you are referring to is the writability determined via permissions or via mount options. Mount options are at a lower-level than permissions, but the logical mechanisms with regard to write permissions are the same.

-- With kind regards,

*Aragorn* (registered GNU-Linux user #223157)