linux-kernel - Re: [PATCH 00/20] objtool: UACCESS validation v3

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Date:   Thu, 7 Mar 2019 09:00:49 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Anvin <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 00/20] objtool: UACCESS validation v3

On Thu, Mar 7, 2019 at 8:47 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> This "fixes" it, and also seems to help -Os make much code:

Yeah, considering that this __trace_if() macro from hell is doing an
'if()' on the result of that inner thing, it makes sense to *not*  use
that "looks simpler and shorter" array sequence, but depend on the
compiler then noticing that the conditionals are the same and joining
the two branches together.

The compiler has to do much more work to either generate the actual
dynamic array thing, or notice that the _index_ of the array matches
the _conditional_ on the branch, and undo that thing.

But that macro really is the macro from hell regardless.

Do people really use CONFIG_PROFILE_ALL_BRANCHES?

                Linus