2009/10/20 vinit dhatrak <vinit.dhatrak@xxxxxxxxx>
Probably this code works because as per linux code, __copy_from_userOn Tue, Oct 20, 2009 at 4:38 PM, Rajat Jain <Rajat.Jain@xxxxxxxxxxxx> wrote:
>
> Hi,
>
>> Thank you for your reply.
>> it's interesting, my modified kernel image is run ok on my
>> hardware(arm926ejs). i test mounting ramfs and nfs, they are all ok.
>> are they occasional?
>>
>> sorry, i don't comprehend your explanation about it
>> In my opinion, if it's possible that the content of
>> parameters isn't in memory at the time of the call, the
>> "sys_mount" can't get them also.
>>
>> could u explain it in detail? Thanks
>
> OK. So here is it. Not all memory used by user space actually needs to
> be in RAM all the time. It may be swapped out to disk since the actual
> memory in use in a system is much more than its RAM size. When a piece
> of memory that is currently swapped out on disk needs to be accessed, it
> needs to be brought back into RAM memory - this is done by the page
> fault handler. But consider that the Disk IO is a very slow process, and
> thus it is relatively a very huge time for the kernel. For this reason,
> any memory that is accessed by the kernel needs to be locked down in RAM
> so that it cannot be swapped out.
>
> Secondly, the 4GB virtual address space is split up into user space and
> kernel space code (3G/1G split generally). User space cannot access
> kernel space virtual addresses and vice versa. Thus the user space
> pointer cannot be dereferenced in the kernel.
>
> Thus, any user data that needs to be accessed firstly needs to be copied
> into kernel address space. This done generally by copy_from_user()
> function or its varians that sys_mount() uses:
>
> exact_copy_from_user((void *)page, data, size);
> strncpy_from_user(page, filename, len);
>
> Now, how you code works comes as a surprize to me though...
>
> Thanks,
>
> Rajat
>
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>
is defined as memcpy for arm architecture. Correct me if I am wrong
here,
http://lxr.linux.no/#linux+v2.6.31/arch/arm/include/asm/uaccess.h#L393
-Vinit
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