Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 23 Aug 2016 14:47:36 -0400 | Subject | Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack_frames() to use the new unwinder |
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On Mon, Aug 22, 2016 at 9:27 PM, Kees Cook <keescook@chromium.org> wrote: > > I need to re-check the copy_*_user changes, but on several > architectures, the bounds checking is only triggered for non > built-in-const sizes, so these kinds of pointless checks shouldn't > happen.
They definitely happen at least on x86.
"stat()" is one common user of fixed-sized structures being copied. There are tons of others, but 'stat()' is the one I've seen in my profiles before as being noticeable. It's been critical enough that I have occasionally tried to play with making it avoid the "copy to temporary struct, then copy_to_user() the whole struct" and just do it field-by-field. But it gets nasty with the padding fields etc, so it's never been done.
Not doing the access size checks for constant-sized copies (at least when they are "sufficiently small" constants) would probably be the right thing to do, and then depend on gcc just getting the static case right warning-wise. Which isn't apparently getting done right now either, but oh well..
Linus
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