Re: allocate memory in userspace (Answer)

From: Timo Benk (t_benk@web.de)
Date: Tue Jul 02 2002 - 05:24:42 EST

As stated by Andy Kleen this is not a good solution.
This is his post to the topic:

----<snip>----
> I am a kernel newbie and i am writing a module. I
> need to allocate some memory in userspace because
> i want to access syscalls like open(), lstat() etc.
> I need to call these methods in the kernel, and in
> my special case there is no other way, but i
> do not want to reimplement all the syscalls.
>
> I read that it should be possible, but i cannot
> find any example or recipe on how to do it.

mm_segment_t oldfs = get_fs();
set_fs(KERNEL_DS);
ret = sys_yoursyscall(kernelargs ...)
set_fs(oldfs);

Do not even think about using mmap or accessing sys_call_table for this.
Your other post was so tasteless that it would be good if you retracted
it with a followup because it would be very bad to have such an bad
example
in the l-k archives open to innocent search machine users uncommented.

-Andi
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
----<snap>-----

On Tue, Jul 02, 2002 at 09:51:17AM +0200, Timo Benk wrote:
> Hi,
>
> I found the following function in arch/i386/kernel/sys_i386.c:
>
> ---<snip>---
> /* common code for old and new mmaps */
> long do_mmap2
> (
> unsigned long addr, unsigned long len,
> unsigned long prot, unsigned long flags,
> unsigned long fd, unsigned long pgoff
> )
> {
> int error = -EBADF;
> struct file * file = NULL;
>
> flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);
> if (!(flags & MAP_ANONYMOUS)) {
> file = fget(fd);
> if (!file)
> goto out;
> }
>
> down_write(&current->mm->mmap_sem);
> error = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
> up_write(&current->mm->mmap_sem);
>
> if (file)
> fput(file);
> out:
> return error;
> }
> ---<snap>---
>
> the following code works for me(of course
> don't forget to munmap the memory).
> ---<snip>---
> char *userspace;
> char kernelspace[2048];
>
> userspace = (char*)do_mmap2
> (
> 0,
> 2048,
> PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_ANON,
> -1,
> 0
> );
>
> copy_to_user( userspace, "/dev/hda", 9 );
>
> copy_from_user( kernelspace, userspace, 9 );
> printk("%s\n",kernelspace);
> ---<snap>---
>
> Hope that helps any other struggling newbie:-)
>
> -timo
>
> --
> gpg key fingerprint = 6832 C8EC D823 4059 0CD1 6FBF 9383 7DBD 109E 98DC
> 

-- 
gpg key fingerprint = 6832 C8EC D823 4059 0CD1  6FBF 9383 7DBD 109E 98DC

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

This archive was generated by hypermail 2b29 : Sun Jul 07 2002 - 22:00:09 EST