> From: Linus Torvalds
> Sent: 07 October 2019 04:12
...
> In this case, I think it's done a few callers up in i915_gem_pread_ioctl():
>
> if (!access_ok(u64_to_user_ptr(args->data_ptr),
> args->size))
> return -EFAULT;
>
> but honestly, trying to optimize away another "access_ok()" is just
> not worth it. I'd rather have an extra one than miss one.
You don't really want an extra access_ok() for every 'word' of a copy.
Some copies have to be done a word at a time.
And the checks someone added to copy_to/from_user() to detect kernel
buffer overruns must kill performance when the buffers are way down the stack
or in kmalloc()ed space.
David
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