On Mon, Apr 20, 2020 at 03:34:57PM -0700, Kees Cook wrote: > On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote: > > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote: > > > + /* > > > + * Since the compiler chooses a 4 bit alignment for the stack, > > > + * let's save one additional bit (9 total), which gets us up > > > + * near 5 bits of entropy. > > > + */ > > > + choose_random_kstack_offset(get_random_int() & 0x1FF); > > > > Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff > > is 9 bits, and that is 4+5 but so what? > > Er, well, yes. I guess I was just trying to explain why there were 9 > bits saved here and to document what I was seeing the compiler actually > doing with the values. (And it serves as a comparison to the x86 comment > which is explaining similar calculations in the face of x86_64 vs ia32.) > > Would something like this be better? > > /* > * Since the compiler uses 4 bit alignment for the stack (1 more than > * x86_64), let's try to match the 5ish-bit entropy seen in x86_64, > * instead of having needlessly lower entropy. As a result, keep the > * low 9 bits. > */ Yes, thank you! I was missing the comparison to x86_64 and so the one "additional" bit didn't make sense to me. With the new comment: Acked-by: Will Deacon <will@xxxxxxxxxx> I'm assuming you're merging this via some other tree, but let me know if you need anything else from me. Cheers, Will
