linux-kernel - Re: [RFC] LKMM: Add volatile

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Sun, 6 Jun 2021 13:40:21 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Sun, Jun 06, 2021 at 11:04:49AM -0700, Linus Torvalds wrote:
>     if (READ_ONCE(a)) {
>         barrier();
>         WRITE_ONCE(b,1);
>    } else {
>         barrier();
>         WRITE_ONCE(b, 1);
>     }
> 
> and currently because gcc thinks "same exact code", it will actually
> optimize this to (pseudo-asm):
> 
>     LD A
>     "empty asm"
>     ST $1,B
> 
> which is very much NOT equivalent to
> 
>     LD A
>     BEQ over
>     "empty asm"
>     ST $1,B
>     JMP join
> 
> over:
>     "empty asm"
>     ST $1,B
> 
> join:
> 
> and that's the whole point of the barriers.

You didn't use a barrier with these semantics though.  There is nothing
in that code that guarantees a branch.

> See, but it VIOLATES the semantics of the code.

The code violates your expectations of the code.

> You can't join those two empty asm's (and then remove the branch),
> because the semantics of the code really aren't the same any more if
> you do. Truly.

You truly should have written a branch in tthe asm if you truly wanted
a branch instruction.


Segher