LOOP_GET_STATUS(64) truncates pathnames to 64 chars (was Re: Bugin mkfs.btrfs?!)

From: Chris Samuel
Date: Mon Jan 24 2011 - 19:23:00 EST

* CC'd to linux-kernel in case they have any feedback on this.
* Long thread, trying to work out why mkfs.btrfs failed to
* make a filesystem on an encrypted loopback mount called
* /dev/loop2. Cause turned out to be mkfs.btrfs calling
* LOOP_GET_STATUS to find out if the block device was mounted
* and getting a truncated device name back and so it later
* fails when lstat() is called on the truncated device path.
* The long device name for the encrypted loopback mount was
* because /dev/disk/by-id/$ID was used when Felix created it
* to cope with devices moving around.

On 25/01/11 00:01, Felix Blanke wrote:

> you were talking about the LOOP_GET_STATUS function. I'm not
> quite sure where does it came from. Is it part of the kernel?
> Or does it come from the util-linux package?

It's in the kernel, and there is both LOOP_GET_STATUS (old
implementation) and LOOP_GET_STATUS64 (new implementation).

They return structures called loop_info and loop_info64
respectively and both are defined in include/linux/loop.h .

Sadly in both cases the lengths of paths are defined to be
LO_NAME_SIZE which is currently 64 and hence either
implementation will cause the problematic:

0x7fffa30b3cf0) = -1 ENOENT (No such file or directory)

I've CC'd this to the LKML in case they have any feedback on
this apparent problem with the API.

Chris Samuel : http://www.csamuel.org/ : Melbourne, VIC
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