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

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

On Wed, Feb 15, 2017 at 12:22 PM, Cyril Hrubis [off-list ref] wrote:

quoted

[CCing linux api as well]

quoted

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.

quoted

quoted

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.

quoted

quoted

 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/