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IOCPARM_MAX defines maximum size of a structure that can be passed

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directly to ioctl(2). Because of how ioctl command is build using _IO*()
macros we have only 13 bits to encode structure size. So the structure
can be up to 8kB-1.

Currently we define IOCPARM_MAX as PAGE_SIZE.

This is IMHO wrong for three main reasons:

1. It is confusing on archs with page size larger than 8kB (not really
   sure if we support such archs (sparc64?)), as even if PAGE_SIZE is
   bigger than 8kB, we won't be able to encode anything larger in ioctl
   command.

2. It is a waste. Why the structure can be only 4kB on most archs if we
   have 13 bits dedicated for that, not 12?

3. It shouldn't depend on architecture and page size. My ioctl command
   can work on one arch, but can't on the other?

Increase IOCPARM_MAX to 8kB and make it independed of PAGE_SIZE and
architecture it is compiled for. This allows to use all the bits on all the
archs for size. Note that this doesn't mean we will copy more on every ioctl(2)
call. No. We still copyin(9)/copyout(9) only exact number of bytes encoded in
ioctl command.

Practical use for this change is ZFS. zfs_cmd_t structure used for ZFS
ioctls is larger than 4kB.

Silence on:	arch@
MFC after:	1 month
This commit is contained in:
Pawel Jakub Dawidek 2010-04-01 16:21:35 +00:00
parent 38f2d636ca
commit 7fa5b1ae33
1 changed files with 3 additions and 2 deletions
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5
sys/sys/ioccom.h
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@ -38,12 +38,13 @@
* any in or out parameters in the upper word. The high 3 bits of the
* upper word are used to encode the in/out status of the parameter.
*/
#define IOCPARM_MASK 0x1fff /* parameter length, at most 13 bits */
#define IOCPARM_SHIFT 13 /* number of bits for ioctl size */
#define IOCPARM_MASK ((1 << IOCPARM_SHIFT) - 1) /* parameter length mask */
#define IOCPARM_LEN(x) (((x) >> 16) & IOCPARM_MASK)
#define IOCBASECMD(x) ((x) & ~(IOCPARM_MASK << 16))
#define IOCGROUP(x) (((x) >> 8) & 0xff)
#define IOCPARM_MAX PAGE_SIZE /* max size of ioctl, mult. of PAGE_SIZE */
#define IOCPARM_MAX (1 << IOCPARM_SHIFT) /* max size of ioctl */
#define IOC_VOID 0x20000000 /* no parameters */
#define IOC_OUT 0x40000000 /* copy out parameters */
#define IOC_IN 0x80000000 /* copy in parameters */