Messages in this thread | | | Subject | Re: Address translation | Date | Wed, 22 Nov 2000 21:39:51 +0000 | From | Keir Fraser <> |
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> The reason that everyone else uses copy_{to,from}_user is that there > is no way to guarantee that the userspace pointer is valid. That > memory may have been swapped out. The copy macros are prepared to > fault the memory in. The rest of the kernel is not. > > Jeff
I may be wrong on this, but I thought that copy_{to,from}_user are only necessary if the address range you are accessing might cause a fault which Linux cannot handle (ie. one which would cause the application to segfault if it accessed that memory). If it is only a matter of paging the memory in (and you are _sure_ the address range is otherwise valid) I think the access macros are unnecessary. I would be *very* glad if someone could confirm this, or shoot me down. :)
For instance, a kernel module I am writing allocates some memory in the current process's address space as follows:
down(&mm->mmap_sem); s->table = (void **)get_unmapped_area(0, SIZEOF_TABLE); if ( s->table != NULL ) do_brk((unsigned long)s->table, SIZEOF_TABLE); up(&mm->mmap_sem);
Some questions: (1) In a "top half" thread, can I now access this memory without the access macros (since I know the address range is valid)? (2) Can I also access this memory from an interrupt/exception context, or must I lock it? (ie. can faults be handled from such a context) (3) Is the above code sensible at all, or barking? It took me a while to figure that the above would work, and I think/hope it is the most elegant way to share memory between kernel and a process.
Thanks in advance for any info! -- Keir Fraser
PS. Please cc me directly (kaf24@cl.cam.ac.uk) with any replies. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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