Re: [4/5] Add option for hardlinkable cache of extracted blobs

Dear diary, on Sun, Apr 17, 2005 at 09:25:17PM CEST, I got a letter where Paul Jackson <pj@sgi.com> told me that...

Does this distinction have any effect when doing F_OK?

On Sun, 17 Apr 2005, Petr Baudis wrote:

Actually, the documentation I've got says:

"F_OK requests checking whether merely testing for the existence of the
 file would be allowed (this depends on the permissions of the directories
 in the path to the file, as given in path-name.)"

So it shouldn't complain about a filename which you're allowed to try to stat, even if there's nothing there. And it would depend on the privs of the wrong user in looking at the path.

	-Daniel
*This .sig left intentionally blank*

Dear diary, on Sun, Apr 17, 2005 at 10:03:46PM CEST, I got a letter where Daniel Barkalow <barkalow@iabervon.org> told me that...

The documentation I've got says:

"R_OK,  W_OK  and  X_OK request checking whether the file exists and has
 read, write and execute permissions, respectively.  F_OK just requests
 checking for the existence of the file."

And IEEE1003.1 agrees: http://www.opengroup.org/onlinepubs/009695399/functions/access.html

Petr wrote:

You don't exactly say it, but I'm guessing that you think that this documentation is stating that F_OK checks for the existance of the file _regardless_ of path access permissions.

No so.  Write your own little test program, and/or read the kernel source.

Even if the file exists, if its directory entry is not accessible to the _real_ uid/gid, access F_OK will fail. If the problem is a lack of seach permissions on some directory in the path, the errno will be EACCES.

Dear diary, on Mon, Apr 18, 2005 at 03:35:08AM CEST, I got a letter where Paul Jackson <pj@sgi.com> told me that...

Ok, I stand corrected; and when giving the access(2) manual page a second look, it could imply that too. It has some room for more crystal-clearness, though. ;-)

Thanks,

Pasky wrote:

True indeed ;).

On Sun, Apr 17, 2005 at 04:03:46PM -0400, Daniel Barkalow wrote:

Isn't it the case that with selinux, various objects may be hidden depending on their accessibility? I wonder if this has an effect here.

(or what about any other security model?)

It may not be pretty, but it seems to have worked fine!

Here's my history log (with intermediate checking removed - I was being pretty anal ;):

	rsync -avz --ignore-existing master.kernel.org:/home/rmk/linux-2.6-rmk.git/ .git/
	rsync -avz --ignore-existing master.kernel.org:/home/rmk/linux-2.6-rmk.git/HEAD .git/MERGE-HEAD
	merge-base $(cat .git/HEAD) $(cat .git/MERGE-HEAD)
	for i in e7905b2f22eb5d5308c9122b9c06c2d02473dd4f $(cat .git/HEAD) $(cat .git/MERGE-HEAD); do cat-file commit $i | head -1; done
	read-tree -m cf9fd295d3048cd84c65d5e1a5a6b606bf4fddc6 9c78e08d12ae8189f3bd5e03accc39e3f08e45c9 a43c4447b2edc9fb01a6369f10c1165de4494c88
	write-tree 
	commit-tree 7792a93eddb3f9b8e3115daab8adb3030f258ce6 -p $(cat .git/HEAD) -p $(cat .git/MERGE-HEAD)
	echo 5fa17ec1c56589476c7c6a2712b10c81b3d5f85a > .git/HEAD 
	fsck-cache --unreachable 5fa17ec1c56589476c7c6a2712b10c81b3d5f85a

which looks really messy, because I really wanted to do each step slowly by hand, so those magic revision numbers are just cut-and-pasted from the results that all the previous stages had printed out.

NOTE! As expected, this merge had absolutely zero file-level clashes, which is why I could just do the "read-tree -m" followed by a write-tree. But it's a real merge: I had some extra commits in my tree that were not in Russell's tree, and obviously vice versa.

Also note! The end result is not actually written back to the corrent working directory, so to see what the merge result actually is, there's another final phase:

	read-tree 7792a93eddb3f9b8e3115daab8adb3030f258ce6
	update-cache --refresh
	checkout-cache -f -a

which just updates the current working directory to the results. I'm _not_ caring about old dirty state for now - the theory was to get this thing working first, and worry about making it nice to use later.

A second note: a real "merge" thing should notice that if the "merge-base" output ends up being one of the inputs (it one side is a strict subset of the other side), then the merge itself should never be done, and the script should just update directly to which-ever is non-common HEAD.

But as far as I can tell, this really did work out correctly and 100% according to plan. As a result, if you update to my current tree, the top-of-tree commit should be:

	cat-file commit $(cat .git/HEAD)

	tree 7792a93eddb3f9b8e3115daab8adb3030f258ce6
	parent 8173055926cdb8534fbaed517a792bd45aed8377
	parent df4449813c900973841d0fa5a9e9bc7186956e1e
	author Linus Torvalds <torvalds@ppc970.osdl.org> 1113774444 -0700
	committer Linus Torvalds <torvalds@ppc970.osdl.org> 1113774444 -0700

	Merge with master.kernel.org:/home/rmk/linux-2.6-rmk.git - ARM changes

	First ever true git merge. Let's see if it actually works.

Yehaa! It did take basically zero time, btw. Except for my bunbling about, and the first "rsync the objects from rmk's directory" part (which wasn't horrible, it just wasn't instantaneous like the other phases).

Btw, to see the output, you really want to have a "git log" that sorts by date. I had an old "gitlog.sh" that did the old recursive thing, and while it shows the right thing, the ordering ended up making it be very non-obvious that rmk's changes had been added recently, since they ended up being at the very bottom.

			Linus

I'm not sure what 'nothing there' means to you.

To me, it means 'no file there', so no you would not be allowed to stat it - and should fail ENOENT.

Yup.

Petr wrote:

Well, yeah. If only one of real or effective id's could traverse the path (execute perm on directories), then you'd get the wrong answer.