Messages in this thread | | | Date | Tue, 18 Mar 2008 14:32:22 +0000 | From | Al Viro <> | Subject | Re: RFC: /dev/stdin, symlinks & permissions |
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On Tue, Mar 18, 2008 at 08:54:45AM -0400, Theodore Tso wrote:
> The main issue is that at the moment, when you open /proc/self/fd/X, > what you get is a new struct file, since the inode is opened a second > time. That is why you have to go through the access control checks a > second time, and why there are issues when you have /dev/stdin > pointing to a tty which was owned by user 1, and then when you su to > user 2, you get a "permission denied" error. > > On other operating systems, opening /proc/self/fd/X gives you a > duplicate of the file descriptor. That means that the seek pointer is > also duplicated. This has been remarked upon before. Linux 1.2 did > things "right" (as in, the same as Plan 9 and Solaris), but it was > changed in Linux 2.0. Please see: > > http://www.ussg.iu.edu/hypermail/linux/kernel/9609.2/0371.html
The real issue is that it was not Plan 9 semantics to start with.
See 9/port/devproc.c and 9/port/devdup.c; the former is procfs and while it does have <pid>/fd, the sucker is not a directory - it's a text file containing (more or less) the pathnames of opened files of that process. The latter is an entirely different thing - it's a separate filesystem (#d instead of #p, FWIW). There you have per-descriptor files to open and yes, that'll give you dup(). What you do not have there is per-process part.
IOW, you can get pathnames of opened files for other processes via procfs *AND* you can get open-that-does-only-dup for files in your descriptor table - on a separate filesystem.
1.2 tried to mix both. I'm not actually sure that it was a good idea wrt security, while we are at it...
We could implement Plan 9 style dupfs, but to do that without excessive ugliness we'd need to change prototype of ->open() - it must be able to return a reference to struct file different from anything it got from caller; probably the least painful way would be to make it return NULL => success, use struct file passed to ->open() ERR_PTR(-err) => error pointer to struct file => success, caller should drop the reference to struct file it had passed to ->open() and use the return value. Still a mind-boggling amount of churn - probably too much to bother with.
PS: from Plan 9 proc(3) [they use section 3 for kernel filesystems]: The read-only fd file lists the open file descriptors of the process. The first line of the file is its current directory; subsequent lines list, one per line, the open files, giving the decimal file descriptor number; whether the file is open for read (r), write, (w), or both (rw); the type, device number, and qid of the file; its I/O unit (the amount of data that may be transferred on the file as a contiguous piece; see iounit(2)), its I/O offset; and its name at the time it was opened.
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