linux-kernel - Bug introduced in 3b93f911d5

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Message-Id: <435E586E-2045-4070-8FA7-8DF468280E3C@cam.ac.uk>
Date:	Fri, 8 Aug 2014 00:11:39 +0100
From:	Anton Altaparmakov <aia21@....ac.uk>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Bug introduced in 3b93f911d5

Hi Al,

Was just looking at __generic_file_write_iter() and found a bug in the code that you added in 3b93f911d5.

Consider the case where generic_file_direct_write() returns a partial write, i.e. written > 0 && written < count.

Also consider that the following generic_perform_write() fails with an error, i.e. status < 0.

This code then does something very bogus:

		if (unlikely(status < 0) && !written) {
			err = status;
			goto out;
		}
		iocb->ki_pos = pos + status;
		...
		endbyte = pos + status - 1;

The if condition is false as written is > 0 yet status is negative thus iocb->ki_pos is set to pos + status where status is negative thus ki_pos is actually set to "pos - random value".

And similar for "endbyte" being set to "pos - random value - 1", etc.

Doesn't seem like that is what you intended?

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
University of Cambridge Information Services, Roger Needham Building
7 JJ Thomson Avenue, Cambridge, CB3 0RB, UK

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