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Message-ID: <20180309232933.14e39858@heffalump.sk2.org>
Date: Fri, 9 Mar 2018 23:29:33 +0100
From: Stephen Kitt <steve@....org>
To: Jens Axboe <axboe@...nel.dk>, "James E.J. Bottomley"
<jejb@...ux.vnet.ibm.com>, "Martin K. Petersen"
<martin.petersen@...cle.com>, Hannes Reinecke <hare@...e.com>
Cc: linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
linux-kernel@...r.kernel.org, Kernel Hardening
<kernel-hardening@...ts.openwall.com>
Subject: VLA removal, device_handler and COMMAND_SIZE
Hi,
I’ve been looking into removing some VLAs from device_handler drivers,
prompted by https://lkml.org/lkml/2018/3/7/621
The uses in question here are quite straightforward, e.g. in
drivers/scsi/device_handler/scsi_dh_alua.c:
u8 cdb[COMMAND_SIZE(MAINTENANCE_IN)];
There’s no trivial way of keeping the compiler happy with -Wvla though here,
at least not while keeping the behaviour strictly identical. I’ve come up
with two approaches, and I’m curious whether they’re appropriate or if
there’s a better way...
The first approach is to use MAX_COMMAND_SIZE instead; this wastes a few
bytes on the stack here and there, and stays reasonably maintainable.
The second approach might be symptomatic of a twisted mind, and involves
replacing COMMAND_SIZE so that it can be calculated at compile time when the
opcode is known:
/*
* 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)))))
This has the side-effect of making some of the call sites more complex, and
the macro itself isn’t the most maintainer-friendly. It does mean we can drop
BLK_SCSI_REQUEST from drivers/target/Kconfig so it’s not all bad...
Both patches will follow in reply to this email, I’ll let more familiar
developers judge which is appropriate (if any).
Regards,
Stephen
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