My question is about passing data from kernel to a user space program. I want to implement a system call "get_data(size, char *buff, char **meta_buf)". In this call, buff is allocated by user space program and its length is passed in the size argument. However, meta_buf is a variable length buffer that is allocated (in the user space program's vm pages) and filled by kernel. User space program will free this region.
(I cant allocate data in user space as user space program does not know size of the meta_buff. Also, user space program cannot allocate a fixed amount of memory and call the system call again and again to read the entire meta data. meta_data has to be returned in one single system call)
Don't attempt to allocate memory for userspace from the kernel - this is a huge violation of the kernel's abstraction layering. Instead, consider a few other options:
It's hard to say more without knowing why this has to be atomic. Actually allocating memory's going to need to be interruptible anyway (or you're unlikely to succeed), so it's unlikely that going out of and back into the kernel is going to hurt much. In fact, any write to userspace memory must be interruptible, as there's the potential for page faults requiring IO.
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