btrfs: readdir problem workaround for 32-bit off_t

btrfs: readdir problem workaround for 32-bit off_t

[Jan Engelhardt]

Hi,



I am seeing abnormal kernel<->userland interaction for range-exceeding 
offsets during readdir.
A suggested patch for btrfs is below, but I think there could also
be involvement of (read: a bug in) Glibc.
Would the copied parties please have a look, as it may spans both glibc 
and btrfs.


Thanks,
Jan

parent 81841fa96da5597a411f5f274a664f186b2d2549 (v2.6.29-rc2-21-g81841fa)
commit fa8ea6611fedecba8e5fdf2a5dfd82affa5a982e
Author: Jan Engelhardt <jengelh@medozas.de>
Date:   Sat Jan 17 20:26:25 2009 +0100

btrfs: readdir problem workaround for 32-bit off_t

Kernel is i586, as is the userland.

btrfs sets filp->f_pos == (2^63-1) when it has reached the
end of a directory. This is problematic.

In obtuse 32-bit mode (no _FILE_OFFSET_BITS), readdir(3) will not return 
the last entry.

However, when compiled with -D_FILE_OFFSET_BITS=64, it will return the 
last entry. But telldir() will return (uint32_t)-1, which does not quite 
match up with the 2^63-1.

	$ cat test.c
	#include <dirent.h>
	#include <stdio.h>

	int main(int argc, const char **argv)
	{
		DIR *r = opendir(argv[1]);
		struct dirent *de;

		while ((de = readdir(r)) != NULL)
			printf("%lld %s\n", (long long)telldir(r), de->d_name);
		return 0;
	}
	$ gcc test.c -o test -ggdb3
	$ ./test /tmp/z
	2 .
	2 ..
	3 a
	4 b
	5 lib

	$ gcc test.c -o test -ggdb3 -D_FILE_OFFSET_BITS=64
	$ ./test /tmp/z
	2 .
	2 ..
	3 a
	4 b
	5 lib
	4294967295 strace.log

This has deep effects and my boot hangs. (Got btrfs as the root fs.)

I have no idea who exactly of Glibc and btrfs is at fault with what,
but this change makes btrfs usable for the moment.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
---
 fs/btrfs/inode.c |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8adfe05..901be02 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3264,10 +3264,7 @@ skip:
 	}
 
 	/* Reached end of directory/root. Bump pos past the last item. */
-	if (key_type == BTRFS_DIR_INDEX_KEY)
-		filp->f_pos = INT_LIMIT(typeof(filp->f_pos));
-	else
-		filp->f_pos++;
+	filp->f_pos++;
 nopos:
 	ret = 0;
 err:
-- 
# Created with git-export-patch

Re: btrfs: readdir problem workaround for 32-bit off_t

[Chris Mason]

On Sat, 2009-01-17 at 20:39 +0100, Jan Engelhardt wrote:
> Hi,
> 
> 
> 
> I am seeing abnormal kernel<->userland interaction for range-exceeding 
> offsets during readdir.
> A suggested patch for btrfs is below, but I think there could also
> be involvement of (read: a bug in) Glibc.
> Would the copied parties please have a look, as it may spans both glibc 
> and btrfs.
> 

Thanks for looking into this.

Your patch is how the code originally looked, but it can lead to loops
in buggy userland (a git bug hit this).   My plan was to mask off the
limit the same way XFS now does.

This is a btrfs bug, not glibc.

-chris