Date: Fri, 19 Jan 2018 10:18:53 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Jann Horn <jannh@...gle.com> Cc: Dan Williams <dan.j.williams@...el.com>, kernel list <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Catalin Marinas <catalin.marinas@....com>, "the arch/x86 maintainers" <x86@...nel.org>, Will Deacon <will.deacon@....com>, Russell King <linux@...linux.org.uk>, Ingo Molnar <mingo@...hat.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Andrew Morton <akpm@...ux-foundation.org>, Alan Cox <alan@...ux.intel.com> Subject: Re: [PATCH v4 02/10] asm/nospec, array_ptr: sanitize speculative array de-references On Fri, Jan 19, 2018 at 2:20 AM, Jann Horn <jannh@...gle.com> wrote: >> + \ >> + __u._ptr = _arr + (_i & _mask); \ >> + __u._bit &= _mask; \ > > AFAICS, if `idx` is out of bounds, you first zero out the index > (`_i & _mask`) and then immediately afterwards zero out > the whole pointer (`_u._bit &= _mask`). > Is there a reason for the `_i & _mask`, and if so, can you > add a comment explaining that? I think that's just leftovers from my original (untested) thing that also did the access itself. So that __u._bit masking wasn't masking the pointer, it was masking the value that was *read* from the pointer, so that you could know that an invalid access returned 0/NULL, not just the first value in the array. Linus
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