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.
--
mvh Henrik Austad
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