Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

[afzal mohammed <afzal.mohd.ma@gmail.com>]

Hi,

On Sat, Jun 13, 2020 at 02:15:52PM +0100, Russell King - ARM Linux admin wrote:
> On Sat, Jun 13, 2020 at 05:34:32PM +0530, afzal mohammed wrote:

> > i think C
> > library cuts any size read, write to page size (if it exceeds) &
> > invokes the system call.

> You can't make that assumption about read(2).  stdio in the C library
> may read a page size of data at a time, but programs are allowed to
> call read(2) directly, and the C library will pass such a call straight
> through to the kernel.  So, if userspace requests a 16k read via
> read(2), then read(2) will be invoked covering 16k.
> 
> As an extreme case, for example:
> 
> $ strace -e read dd if=/dev/zero of=/dev/null bs=1048576 count=1
> read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1048576) = 1048576

Okay. Yes, observed that dd is passing whatever is the 'bs' to
Kernel and from the 'dd' sources (of busybox), it is invoking read
system call directly passing 'bs', so it is the tmpfs read that is
splitting it to page size as mentioned by Arnd.

Regards
afzal