Date: Fri, 21 Nov 2014 09:11:26 -0500 From: Rich Felker <dalias@...ifal.cx> To: Christoph Hellwig <hch@...radead.org> Cc: David Drysdale <drysdale@...gle.com>, libc-alpha@...rceware.org, Andrew Morton <akpm@...ux-foundation.org>, Linux API <linux-api@...r.kernel.org>, Andy Lutomirski <luto@...capital.net>, musl@...ts.openwall.com Subject: Re: Re: [RFC] Possible new execveat(2) Linux syscall On Fri, Nov 21, 2014 at 02:13:18AM -0800, Christoph Hellwig wrote: > On Sun, Nov 16, 2014 at 02:52:46PM -0500, Rich Felker wrote: > > I've been following the discussions so far and everything looks mostly > > okay. There are still issues to be resolved with the different > > semantics between Linux O_PATH and what POSIX requires for O_EXEC (and > > O_SEARCH) but as long as the intent is that, once O_EXEC is defined to > > save the permissions at the time of open and cause them to be used in > > place of the current file permissions at the time of execveat > > As far as I can tell we only need the little patch below to make Linux > O_PATH a valid O_SEARCH implementation. Rich, you said you wanted to > look over it? I think the below looks correct, but it's not complete. The *at functions also need to use FMODE_EXEC rather than rechecking +x permissions at the time of the operation. > For O_EXEC my interpretation is that we basically just need this new > execveat syscall + a patch to add FMODE_EXEC and enforce it. So we > wouldn't even need the O_PATH|3 hack. But unless someone more familar > with the arcane details of the Posix language verifies it I'm tempted to > give up trying to help to implent these flags :( O_EXEC/O_SEARCH cannot be equal to O_PATH, because of differing semantics on open. With O_NOFOLLOW, O_PATH yields a file descriptor referring to the symlink itself. With O_EXEC or O_SEARCH, O_NOFOLLOW is required to make open fail if the target is a symlink. It would be a serious regression to eliminate the ability of O_PATH to open symlinks like this. Note that enforcing O_NOFOLLOW failure on symlinks can be implemented in userspace instead of (or in addition to, for better behavior with old kernels) kernelspace, but it still requires a different value from O_PATH or userspace would be eliminating access to an important O_PATH feature. Further, O_PATH|3 was the best value I could find to yield nearly reasonable fallback behavior on most old kernels. Simply using 3 fails to open directories and files to which the caller does not have write permission (mode 3 is a nearly-undocumented hack for opening devices for ioctl-only read-write access, it seems). On pre-O_PATH kernels, using O_PATH|3 would fallback to this failing case, yielding spurious failure-to-open for all O_SEARCH and some O_EXEC operations, but those kernels are old enough to be irrelevant to most users anyway. On kernels that do have O_PATH, using O_PATH|3 ignores the 3 and yields the current O_PATH semantics, which are nearly correct. Of course O_PATH|1 or O_PATH|2 would also work in principle, as would adding a completely new bit in addition to O_PATH, but these all seem less desirable. Rich
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