确保文件为（X级别）的有效方法？ 在某个目录下

Question

I need to make sure a certain file is inside (doesn't matter how many levels) a certain directory.

To visualize:

A/ + file

or

A/ + x/x/x/..../file

It has to be under A, doesn't matter how many levels.

The file path (after the plus sign) is given to the program by the user.

The thing is that I can't use chroot, because the program also needs to access some folders outside A (lets say B) but users (that are connected via socket interface) should not be able to access them via A/ + ../B/file

This will be done in an inner inner loop so it has to perform well. I have thought of counting '/', extracting number of '..', extracting number of '/./' and comparing it >= 1 but it both does not perform well and feels open to bugs since the input is given by the user.

Is there a function that checks this that I don't know of or some other way?

EDIT:

If there is a way that will grant me access to folder B even after I chroot (file handles remain after chroot if I remember right, I need folder version of that), that is also OK. All I need is to access B , create/read/write files in it after the chroot.

EDIT2:

By the way, I know a way to do this using UID and such but I need an alternative.

Answer 1

You can call ftw() or nftw() in a recursive function.

While doing this, you need to make sure that you do not follow symlinks or limit the traversal to them.

Initial start may scan the frequently visited folders for memoization until a request is received and while the system is idle. All requests are added to the memoization (hashtable etc.)

You can find the details here .

Answer 2

I think the easiest way to do this would be to append the user-supplied path string to end of the A directory's path, and then use the POSIX realpath() function to simplify the resulting path string. (realpath() will iron out any ".." or symlink shenanigans and give you a simple, absolute file-path string)

Then compare that path to the A directory's simplified/absolute path. If the file-path string starts with A's path-string (eg strncmp(file_path_str, a_dir_path_str, strlen(a_dir_path_str))==0), then allow access to the file; otherwise deny access.

Answer 3

Assuming the "user" doesn't have free access to the system, but has sufficient privileges in itself, I'd expect that chroot is the solution you are looking for.

So, instead of validating the directory structure of the input, simply change the "root" so that it points at "A/" - and then (if need be) remove "A/" from the start of the path, and accept the rest.