Re: [PATCH] ioctl_list.2: BLKRASET/BLKRAGET take unsigned long

Arnd Bergmann
2017-02-15T12:04:17Z

On Wed, Feb 15, 2017 at 12:22 PM, Cyril Hrubis <chrubis-AlSwsSmVLrQ@public.gmane.org> wrote:
> [CCing linux api as well]
>> The BLKRASET/BLKRAGET ioctls() take unsigned long, if I pass int * to
>> the BLKRAGET ioctl on x86_64 (or on any other arch where sizeof(int) !=
>> sizeof(long)) the BLKRAGET ioctl will rewrite four bytes on the stack.
>>
>> If you look at block/ioctl.c in kernel sources you can clearly see that
>> BLKRAGET ioctl calls put_long().

BLKFRAGET as well, fwiw, see compat_blkdev_ioctl().
We might as well include them in the list.

>> I also wonder if it's OK to pass int value to ioctl() at all, the arg
>> value seems to be unsigned long in the syscall definition in fs/ioctl.c
>> and there does not seem to be any glibc magic around the syscall.

This shouldn't matter, if you pass an 'int' into a function that takes
a 'long', it will be extended if necessary. The question is more about
how it gets interpreted, and in this case it's done by assigning to

struct backing_dev_info {
...
unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
...
};

Large values will not be used in practice (these are capped by the
I/O sizes), but I guess you can set a value larger than UINT_MAX
and read it back later.

>> man2/ioctl_list.2 | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/man2/ioctl_list.2 b/man2/ioctl_list.2
>> index 0165c77..c8efd66 100644
>> --- a/man2/ioctl_list.2
>> +++ b/man2/ioctl_list.2
>> @@ -311,8 +311,8 @@ l l l l.
>> 0x0000125F BLKRRPART void
>> 0x00001260 BLKGETSIZE unsigned long *
>> 0x00001261 BLKFLSBUF void
>> -0x00001262 BLKRASET int
>> -0x00001263 BLKRAGET int *
>> +0x00001262 BLKRASET unsigned long
>> +0x00001263 BLKRAGET unsigned long *
>> 0x00000001 FIBMAP int * // I-O
>> 0x00000002 FIGETBSZ int *
>> 0x80086601 FS_IOC_GETFLAGS int *

Looks correct to me.

Arnd
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