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Date: Mon, 22 Aug 2016 18:33:28 -0500 From: Josh Poimboeuf <jpoimboe@...hat.com> To: Kees Cook <keescook@...omium.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Andy Lutomirski <luto@...capital.net>, Linus Torvalds <torvalds@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>, Brian Gerst <brgerst@...il.com>, Peter Zijlstra <peterz@...radead.org>, Frederic Weisbecker <fweisbec@...il.com>, Byungchul Park <byungchul.park@....com>, Nilay Vaish <nilayvaish@...il.com> Subject: Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack_frames() to use the new unwinder On Mon, Aug 22, 2016 at 03:27:19PM -0500, Josh Poimboeuf wrote: > On Fri, Aug 19, 2016 at 04:55:22PM -0500, Josh Poimboeuf wrote: > > On Fri, Aug 19, 2016 at 11:27:18AM -0700, Kees Cook wrote: > > > On Thu, Aug 18, 2016 at 6:06 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote: > > > > Convert arch_within_stack_frames() to use the new unwinder. > > > > > > > > This also changes some existing behavior: > > > > > > > > - Skip checking of pt_regs frames. > > > > - Warn if it can't reach the grandparent's stack frame. > > > > - Warn if it doesn't unwind to the end of the stack. > > > > > > > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com> > > > > > > All the stuff touching usercopy looks good to me. One question, > > > though, in looking through the unwinder. It seems like it's much more > > > complex than just the frame-hopping that the old > > > arch_within_stack_frames() did, but I'm curious to hear what you think > > > about its performance. We'll be calling this with every usercopy that > > > touches the stack, so I'd like to be able to estimate the performance > > > impact of this replacement... > > > > Yeah, good point. I'll take some measurements from before and after and > > get back to you. > > I took some before/after measurements by enclosing the affected > functions with ktime calls to get the total time spent in each function, > and did a "find /usr >/dev/null" to trigger a bunch of user copies. > > copy_to/from_user check_object_size arch_within_stack_frames > before: 13ms 6.8ms 0.61ms > after: 17ms 11ms 4.6ms > > The unwinder port made arch_within_stack_frames() *much* (8x) slower > than its current simple implementation, and added about 30% (4ms) to the > total copy_to/from_user() run time. > > Note that hardened usercopy itself is already quite slow: it made user > copies about 52% slower. With the unwinder port, that worsened to ~65%. FWIW, I think I messed up my math summary here. Hardened usercopy was roughly 110% slower than normal usercopy (i.e., it took more than twice as long) with 52% of the usercopy time being consumed by check_object_size(). With the unwinder, that worsened to 180% slower -- with 65% of the usercopy time being consumed by check_object_size(). -- Josh
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