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

Michael Kerrisk (man-pages)
2017-04-10T15:21:34Z

On 02/15/2017 01:04 PM, Arnd Bergmann wrote:
> 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.

Thanks for reviewing this, Arnd.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/