Henrik Austad wrote:
> On Friday 01 August 2008 10:41:55 Prasad Joshi wrote:
>> Hi All,
>>
>> Here is the code for __do_clear_user, I am not getting how does it work.
>> Can any one please explain?
>
> I think the easiest way, would be to find a chart over all
> assembly-instructions and then go through the function step by step (see
> references below).
>
> a few notes: might_sleep() is a kernel-macro that yields if there are other
> tasks that have a job to do.
>
> And, even though I don't fully know what all instructions do, I assume it
> iterates over the memoryregion, zeroing it out as it goes.(as the comment
> suggests).
>
>> #define __do_clear_user(addr,size) \
>> do { \
>> int __d0; \
>> might_sleep(); \
>> __asm__ __volatile__( \
>> "0: rep; stosl\n" \
>> " movl %2,%0\n" \
>> "1: rep; stosb\n" \
>> "2:\n" \
>> ".section .fixup,\"ax\"\n" \
>> "3: lea 0(%2,%0,4),%0\n" \
>> " jmp 2b\n" \
>> ".previous\n" \
>> _ASM_EXTABLE(0b,3b) \
>> _ASM_EXTABLE(1b,2b) \
>>
>> : "=&c"(size), "=&D" (__d0) \
>> : "r"(size & 3), "0"(size / 4), "1"(addr), "a"(0)); \
>>
>> } while (0)
>>
>> /**
>> * clear_user: - Zero a block of memory in user space.
>> * @to: Destination address, in user space.
>> * @n: Number of bytes to zero.
>> *
>> * Zero a block of memory in user space.
>> *
>> * Returns number of bytes that could not be cleared.
>> * On success, this will be zero.
>> */
>> unsigned long
>> clear_user(void __user *to, unsigned long n)
>> {
>> might_sleep();
>> if (access_ok(VERIFY_WRITE, to, n))
>> __do_clear_user(to, n);
>> return n;
>> }
>> EXPORT_SYMBOL(clear_user);
>>
>>
>> clear_user() is the function which is calling __do_clear_user(), from the
>> context I can understand it is clearing the user space buffer. But how does
>> it work?
>
> Have a look here:
>
> http://www.ibm.com/developerworks/library/l-ia.html
> http://docs.sun.com/app/docs/doc/817-5477/
> http://oopweb.com/Assembly/Documents/ArtOfAssembly/Volume/toc.html
>
> Sorry for not giving you a 'this function does ...' kind of explaination, but
> hopefully you can gather some more insight from these pages.
>
I wonder why __do_clear_user can't be written in C.
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