[SOLVED] C : How to block opening files "above" a specified directory?

Question

SOLVED: Used
man 3 realpath
Hello Archers,
I have this one problem I've been struggling with. I kind of solved it but the solution's hackish.
My question is how do you prevent the opening of files "above" a specified directory? The reason behind this is that I'm writing a simple sharing server, something like Xyne's quickserve, but in C and with a GTK GUI. I'm using the libmicrohttpd library.
The program works this way: the user selects a port and a directory to be "served", and then he can access the files under the directory he chose. However, there obviously needs to be a way to prevent the user from opening files above the directory by using ".."'s for example.
For now, I simply plan on rejecting all queries containing ".." (http://github.com/houbysoft/quickshare/ … hare.c#L59), but that's obviously very imperfect -- and you could get around it if there is a symlink in the directory for example.
Any pointers will be very appreciated, thanks.
Last edited by y27 (2010-03-17 02:59:14)

Answer 1

tavianator wrote:
man realpath wrote:BUGS
       The  POSIX.1-2001  standard  version of this function is broken by design, since it is impossible to
       determine a suitable size for the output buffer, resolved_path.  According to POSIX.1-2001 a  buffer
       of  size PATH_MAX suffices, but PATH_MAX need not be a defined constant, and may have to be obtained
       using pathconf(3).  And asking pathconf(3) does not really help, since, on the one hand POSIX  warns
       that  the  result  of  pathconf(3) may be huge and unsuitable for mallocing memory, and on the other
       hand pathconf(3) may return -1 to signify that PATH_MAX is not bounded.   The  resolved_path == NULL
       feature,  not  standardized  in  POSIX.1-2001,  but standardized in POSIX.1-2008, allows this design
       problem to be avoided.
So be careful to either pass resolved_path as NULL and bite the potential unportablilty bullet, or copy canonicalize_file_name or canonicalize_filename_mode from gnulib.
Yeah, I am passing NULL.