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Date: Wed, 28 Jan 2009 09:28:42 -0700 (MST)
Message-Id: <20090128.092842.-1889956563.imp@bsdimp.com>
Subject: Re: [Qemu-devel] [PATCH] support >2TB SCSI disks
From: "M. Warner Losh" <imp@bsdimp.com>
In-Reply-To: <200901281230.29455.paul@codesourcery.com>
References: <20090127224619.3ae16173@bree.surriel.com>
	<200901281230.29455.paul@codesourcery.com>
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To: qemu-devel@nongnu.org, paul@codesourcery.com

In message: <200901281230.29455.paul@codesourcery.com>
            Paul Brook <paul@codesourcery.com> writes:
: > =A0 =A0 =A0case 0:
: > - =A0 =A0 =A0 =A0lba =3D buf[3] | (buf[2] << 8) | ((buf[1] & 0x1f) =
<< 16);
: > + =A0 =A0 =A0 =A0lba =3D (uint64_t) buf[3] | ((uint64_t) buf[2] << =
8) |
: > + =A0 =A0 =A0 =A0 =A0 =A0 =A0(((uint64_t) buf[1] & 0x1f) << 16);
: =

: This is not required, though I guess it's harmless.

Actually, I think it is required.  ANSI-C promotion rules say that a
char or unsigned char is promoted to an int when used in an
expression.  This causes the result to be a 32-bit number which is
sign-extended to a 64-bit number before being assigned to lba.

I've hit this "bug" in C before many times, and the patch specifically
called it out as a problem.

Warner