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Date: Sat, 26 Apr 2008 10:53:10 +0430
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Subject: Re: CopyOut Size Limits
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Do not allocate my_type (in general, big data structures)
on kernel stack (e.g, use malloc(9)).


Adam wrote:
> Hi, I am writing a custom system call that needs to transfer 16kb of 
> data from the kernel to userspace. I am transferring the data out of 
> the kernel by using copyout.  This seems to work for a small struct of 
> data < 4k.
>
> int my_system_call(struct thread *td, struct my_system_call_args *uap)
> {
>     my_structtype_t my_type;
>
>     copyout(&my_type,uap->my_type,sizeof(my_type) ))!=0)
>     {
>         printf("\n copyout failed ret%d\n",error);
>         return error;
>     }
>
>     printf("exiting kernel %d\n",error);
>     return (0);
> }
>
> However once I expand my struct size beyond around 4k that I get a 
> "Fatal Double Fault."  It seems like I am overrunning the kernel 
> stack.  Does copyout use memory from the kernel stack?  What is the 
> limit for copyout?  Is there some way to allocate additional space?  
> Alternatively what is the appropriate method for transferring kbs of 
> data from kernel to userspace?
>
> Thanks,
> Adam
>
>
>
>
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>

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