On Wed, Nov 5, 2014 at 7:21 AM, Eric Rannaud <e@xxxxxxxxxxxxxxxx> wrote: > On Wed, Nov 5, 2014 at 12:27 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: >> On Mon, Nov 03, 2014 at 11:04:27AM -0800, Linus Torvalds wrote: >>> Oh, so you don't actually need any file contents at all? >>> >>> If that is actually a real usage, then maybe we should just say that >>> "O_TMPFILE|O_RDONLY" is fine, and remove the check that it has to be >>> writable. >> >> Wasn't this disallowed to prevent problems on old kernels that don't use >> O_TMPFILE? In that case we'd ignore the flag and would just get a file >> handle for the directory instead. > > Yes, that was the idea at first, as discussed at > http://article.gmane.org/gmane.linux.file-systems/76273, in commit > bb458c644. But soon after that, O_WRONLY was allowed (ba57ea64cb1), > and O_RDWR was removed from O_TMPFILE. Now only remain the explicit > checks in build_open_flags() that Linus mentioned. > > If we allow > fd = open("/tmp", O_TMPFILE|O_RDONLY, 0600) > it would be seen by an old kernel as > fd = open("/tmp", O_DIRECTORY|O_RDONLY, 0600) > which will succeed. > > But unlike the other cases the creative definition of O_TMPFILE was > meant to prevent, this does not create a security risk for anyone > implementing a secure tmpfile, as they would be asking for a writable > fd. I have no idea whether it's a security risk, but it's a serious "wtf?!?" risk. > > To implement an atomic open() with O_TMPFILE+flink, if neither > O_WRONLY nor O_RDWR is in flags, you would have to manually check with > fstat that fd is indeed a regular file and not a directory. At least > if you need to run on old kernels. > > If such a changes goes in, the man page for open(2) should talk about > what happens on old kernels (it already has an explanation for the > writable case). With my occasional-API-reviewer hat on, NAK NAK NAK. This is even more insane than the rest of the O_TMPFILE interface. If you want to support this kind of use case (which seems entirely reasonable to me), then just add a syscall, please. Also, you can, mostly, downgrade from O_RDWR to O_RDONLY. We could easily add a way to downgrade all the way, I think. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
