Read data to kernel buffer by blkdev_read_iter

Read data to kernel buffer by blkdev_read_iter

[Ivan Safonov]

Hi,
I want to read data by kernel module from a block device like this:

static int module_init(void)
{
	const size_t len = 0x1000;
	struct file *file;
	char *buf;
	struct iovec iovec;
	struct kiocb kiocb;
	struct iov_iter iov_iter;
	ssize_t rcount;

	buf = vmalloc(len);
	file = filp_open("/dev/sda", 0, 0);

	iovec.iov_base = buf;
	iovec.iov_len = len;

	kiocb.ki_filp = file;
	kiocb.ki_pos = 0x200000000;
	kiocb.ki_complete = NULL;
	kiocb.ki_flags = IOCB_DIRECT;
	kiocb.ki_hint = 0;

	iov_iter_init(&iov_iter, READ, &iovec, 1, len);

	rcount = blkdev_read_iter(&kiocb, &iov_iter);

	filp_close(file, current);
	vfree(buf);

	return rcount < 0 ? rcount : 0;
}

but blkdev_read_iter always return -EFAULT. I tried to localize the error:

blkdev_read_iter
   generic_file_read_iter
     blkdev_direct_IO
       __blkdev_direct_IO_simple
         bio_iov_iter_get_pages
           __bio_iov_iter_get_pages
             iov_iter_get_pages
               get_user_pages_fast
                 ...
                   find_vma

find_vma return NULL for vmalloc'ed block.

Kernel version is 4.19.16.

Is this behavior valid for iov_iter with type ITER_KVEC?

--
Ivan Safonov.

Re: Read data to kernel buffer by blkdev_read_iter

[Bart Van Assche]

On 3/22/19 6:06 AM, Ivan Safonov wrote:
> Kernel version is 4.19.16.
> 
> Is this behavior valid for iov_iter with type ITER_KVEC?

Are you familiar with the ITER_KVEC flag?

Bart.

Re: Read data to kernel buffer by blkdev_read_iter

[Ivan Safonov]

On 3/22/19 4:42 PM, Bart Van Assche wrote:
> On 3/22/19 6:06 AM, Ivan Safonov wrote:
>> Kernel version is 4.19.16.
>>
>> Is this behavior valid for iov_iter with type ITER_KVEC?
> 
> Are you familiar with the ITER_KVEC flag?

No, I did not find a clear description of this flag.

In the previous message I misled you: iov_iter has ITER_KVEC type only 
if set_fs(get_ds()) present before iov_iter_init(). In this case 
_without_ IOCB_DIRECT blkdev_read_iter() work fine. With IOCB_DIRECT 
blkdev_read_iter() return -EFAULT.

Re: Read data to kernel buffer by blkdev_read_iter

[Bart Van Assche]

On Fri, 2019-03-22 at 17:57 +0300, Ivan Safonov wrote:
> In the previous message I misled you: iov_iter has ITER_KVEC type only 
> if set_fs(get_ds()) present before iov_iter_init(). In this case 
> _without_ IOCB_DIRECT blkdev_read_iter() work fine. With IOCB_DIRECT 
> blkdev_read_iter() return -EFAULT.

Have you tried to store a physical address in iov_base instead of a virtual
address?

Bart.

Re: Read data to kernel buffer by blkdev_read_iter

[Ivan Safonov]

On 3/22/19 6:36 PM, Bart Van Assche wrote:
> On Fri, 2019-03-22 at 17:57 +0300, Ivan Safonov wrote:
>> In the previous message I misled you: iov_iter has ITER_KVEC type only
>> if set_fs(get_ds()) present before iov_iter_init(). In this case
>> _without_ IOCB_DIRECT blkdev_read_iter() work fine. With IOCB_DIRECT
>> blkdev_read_iter() return -EFAULT.
> 
> Have you tried to store a physical address in iov_base instead of a virtual
> address?
I tried: already -EFAULT.

vm_area_struct description says: ranges consist of virtual addresses:
/*
  * This struct defines a memory VMM memory area. There is one of these
  * per VM-area/task.  A VM area is any part of the process virtual memory
  * space that has a special rule for the page-fault handlers (ie a shared
  * library, the executable area etc).
  */

So, the physical addresses could not solve this problem.

I think there should be no memory checks for the kernel allocated 
memory, or the blkdev_read_iter() is not available to work with 
vmalloc'ed memory at all.

--
Ivan Safonov.