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Date: Sat, 10 Mar 2018 14:29:30 +0100
From: Stephen Kitt <steve@....org>
To: Bart Van Assche <Bart.VanAssche@....com>
Cc: "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>, "hare@...e.com"
<hare@...e.com>, "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
"axboe@...nel.dk" <axboe@...nel.dk>, "linux-scsi@...r.kernel.org"
<linux-scsi@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "linux-block@...r.kernel.org"
<linux-block@...r.kernel.org>, "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] scsi: resolve COMMAND_SIZE at compile time
Hi Bart,
On Fri, 9 Mar 2018 22:47:12 +0000, Bart Van Assche <Bart.VanAssche@....com>
wrote:
> On Fri, 2018-03-09 at 23:33 +0100, Stephen Kitt wrote:
> > +/*
> > + * SCSI command sizes are as follows, in bytes, for fixed size commands,
> > per
> > + * group: 6, 10, 10, 12, 16, 12, 10, 10. The top three bits of an opcode
> > + * determine its group.
> > + * The size table is encoded into a 32-bit value by subtracting each
> > value
> > + * from 16, resulting in a value of 1715488362
> > + * (6 << 28 + 6 << 24 + 4 << 20 + 0 << 16 + 4 << 12 + 6 << 8 + 6 << 4 +
> > 10).
> > + * Command group 3 is reserved and should never be used.
> > + */
> > +#define COMMAND_SIZE(opcode) \
> > + (16 - (15 & (1715488362 >> (4 * (((opcode) >> 5) & 7)))))
>
> To me this seems hard to read and hard to verify. Could this have been
> written as a combination of ternary expressions, e.g. using a gcc statement
> expression to ensure that opcode is evaluated once?
That’s what I’d tried initially, e.g.
#define COMMAND_SIZE(opcode) ({ \
int index = ((opcode) >> 5) & 7; \
index == 0 ? 6 : (index == 4 ? 16 : index == 3 || index == 5 ? 12 : 10); \
})
But gcc still reckons that results in a VLA, defeating the initial purpose of
the exercise.
Does it help if I make the magic value construction clearer?
#define SCSI_COMMAND_SIZE_TBL ( \
(16 - 6) \
+ ((16 - 10) << 4) \
+ ((16 - 10) << 8) \
+ ((16 - 12) << 12) \
+ ((16 - 16) << 16) \
+ ((16 - 12) << 20) \
+ ((16 - 10) << 24) \
+ ((16 - 10) << 28))
#define COMMAND_SIZE(opcode) \
(16 - (15 & (SCSI_COMMAND_SIZE_TBL >> (4 * (((opcode) >> 5) & 7)))))
Regards,
Stephen
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