linux-kernel - Re: Checking a USB drive's capacity

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Date:	Mon, 22 Dec 2008 22:51:39 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	linux-kernel@...r.kernel.org
Cc:	Jens Axboe <axboe@...nel.dk>
Subject:  Re: Checking a USB drive's capacity

Alan Stern wrote:
> Jens:
> 
> As you may know, there are plenty of USB mass-storage devices that
> respond incorrectly to READ CAPACITY: They report the total number of
> blocks instead of the highest block number.  As a result, the kernel 
> thinks the drive has one more sector than it really does.
> 
> So far we have dealt with this problem by means of a blacklist, but
> this gets more and more unsatisfactory all the time.  We haven't been
> able to find any other way to cope, since a few devices are so badly
> behaved that they crash hard when you try to access the nonexistent
> "last" sector, requiring a replug or power cycle.
> 
> My new idea is to keep in the blacklist only those devices which do
> crash -- a relatively small number.  Everything else we should be able
> to detect safely at runtime, by testing if it's possible to read the
> last sector.
> 
> The question is, how and where?  The logical place for testing the
> capacity is near the end of sd_read_capacity().  However this code runs
> before any media accesses have occurred; the drive might not even be
> spun up yet.  Not to mention that it seems strange to read the last
> sector before reading the first!

It seems to me that the safest solution would be to mark USB storage 
devices as unsafe for last-sector partition table probing, like some 
kind of CAPACITY_DUBIOUS flag, causing it to be skipped on these 
devices, at least by default. After all, it would seem quite unusual to 
see anything other than a DOS-style partition table on a USB storage 
device (or else no partition table at all).

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