From: Joe Perches <joe@perches.com>
To: "Tobin C. Harding" <me@tobin.cc>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: "kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>,
KVM list <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Tycho Andersen <tycho@docker.com>,
"Roberts, William C" <william.c.roberts@intel.com>,
Tejun Heo <tj@kernel.org>,
Jordan Glover <Golden_Miller83@protonmail.ch>,
Greg KH <gregkh@linuxfoundation.org>,
Petr Mladek <pmladek@suse.com>, Ian Campbell <ijc@hellion.org.uk>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Chris Fries <cfries@google.com>,
Dave Weinstein <olorin@google.com>,
Daniel Micay <danielmicay@gmail.com>,
Djalal Harouni <tixxdz@gmail.com>
Subject: [kernel-hardening] Re: [PATCH 0/3] add %pX specifier
Date: Tue, 10 Oct 2017 20:27:14 -0700 [thread overview]
Message-ID: <1507692434.3552.41.camel@perches.com> (raw)
In-Reply-To: <20171010233209.GB2049@eros>
On Wed, 2017-10-11 at 10:32 +1100, Tobin C. Harding wrote:
> On Tue, Oct 10, 2017 at 04:15:01PM -0700, Linus Torvalds wrote:
> > On Tue, Oct 10, 2017 at 4:09 PM, Tobin C. Harding <me@tobin.cc> wrote:
> > >
> > > I did not understand the code (specifically why the right shift of 16 twice?)
> >
> > It's a traditional trick to get the upper 32 bits.
> >
> > So it basically splits the (possibly 64-bit) pointer into the lower 32
> > bits and the upper 32 bits for a hash such as "jhash()" that takes
> > data that is "unsigned int".
> >
> > (NOTE! Using jhash here is not acceptable, since it's not
> > cryptographically safe, but think of it as an example of a hash that
> > takes 32-bit input).
> >
> > Doing ">> 32" is undefined on 32-bit architectures, and wouldn't work.
> >
> > But doing >> 16 >> 16 is a fine way to say "shift right by 32 on a
> > 64-bit architecture" while also being well-defined on a 32-bit one.
> >
> > Linus
>
> Awesome, thanks.
Another way is using the upper_32_bits() macro.
It's perhaps a bit more readable.
WARNING: multiple messages have this Message-ID (diff)
From: Joe Perches <joe@perches.com>
To: "Tobin C. Harding" <me@tobin.cc>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: "kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>,
KVM list <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Tycho Andersen <tycho@docker.com>,
"Roberts, William C" <william.c.roberts@intel.com>,
Tejun Heo <tj@kernel.org>,
Jordan Glover <Golden_Miller83@protonmail.ch>,
Greg KH <gregkh@linuxfoundation.org>,
Petr Mladek <pmladek@suse.com>, Ian Campbell <ijc@hellion.org.uk>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Chris Fries <cfries@google.com>,
Dave Weinstein <olorin@google.com>,
Da
Subject: Re: [PATCH 0/3] add %pX specifier
Date: Tue, 10 Oct 2017 20:27:14 -0700 [thread overview]
Message-ID: <1507692434.3552.41.camel@perches.com> (raw)
In-Reply-To: <20171010233209.GB2049@eros>
On Wed, 2017-10-11 at 10:32 +1100, Tobin C. Harding wrote:
> On Tue, Oct 10, 2017 at 04:15:01PM -0700, Linus Torvalds wrote:
> > On Tue, Oct 10, 2017 at 4:09 PM, Tobin C. Harding <me@tobin.cc> wrote:
> > >
> > > I did not understand the code (specifically why the right shift of 16 twice?)
> >
> > It's a traditional trick to get the upper 32 bits.
> >
> > So it basically splits the (possibly 64-bit) pointer into the lower 32
> > bits and the upper 32 bits for a hash such as "jhash()" that takes
> > data that is "unsigned int".
> >
> > (NOTE! Using jhash here is not acceptable, since it's not
> > cryptographically safe, but think of it as an example of a hash that
> > takes 32-bit input).
> >
> > Doing ">> 32" is undefined on 32-bit architectures, and wouldn't work.
> >
> > But doing >> 16 >> 16 is a fine way to say "shift right by 32 on a
> > 64-bit architecture" while also being well-defined on a 32-bit one.
> >
> > Linus
>
> Awesome, thanks.
Another way is using the upper_32_bits() macro.
It's perhaps a bit more readable.
WARNING: multiple messages have this Message-ID (diff)
From: Joe Perches <joe@perches.com>
To: "Tobin C. Harding" <me@tobin.cc>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: "kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>,
KVM list <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Tycho Andersen <tycho@docker.com>,
"Roberts, William C" <william.c.roberts@intel.com>,
Tejun Heo <tj@kernel.org>,
Jordan Glover <Golden_Miller83@protonmail.ch>,
Greg KH <gregkh@linuxfoundation.org>,
Petr Mladek <pmladek@suse.com>, Ian Campbell <ijc@hellion.org.uk>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Chris Fries <cfries@google.com>,
Dave Weinstein <olorin@google.com>,
Daniel Micay <danielmicay@gmail.com>,
Djalal Harouni <tixxdz@gmail.com>
Subject: Re: [PATCH 0/3] add %pX specifier
Date: Tue, 10 Oct 2017 20:27:14 -0700 [thread overview]
Message-ID: <1507692434.3552.41.camel@perches.com> (raw)
In-Reply-To: <20171010233209.GB2049@eros>
On Wed, 2017-10-11 at 10:32 +1100, Tobin C. Harding wrote:
> On Tue, Oct 10, 2017 at 04:15:01PM -0700, Linus Torvalds wrote:
> > On Tue, Oct 10, 2017 at 4:09 PM, Tobin C. Harding <me@tobin.cc> wrote:
> > >
> > > I did not understand the code (specifically why the right shift of 16 twice?)
> >
> > It's a traditional trick to get the upper 32 bits.
> >
> > So it basically splits the (possibly 64-bit) pointer into the lower 32
> > bits and the upper 32 bits for a hash such as "jhash()" that takes
> > data that is "unsigned int".
> >
> > (NOTE! Using jhash here is not acceptable, since it's not
> > cryptographically safe, but think of it as an example of a hash that
> > takes 32-bit input).
> >
> > Doing ">> 32" is undefined on 32-bit architectures, and wouldn't work.
> >
> > But doing >> 16 >> 16 is a fine way to say "shift right by 32 on a
> > 64-bit architecture" while also being well-defined on a 32-bit one.
> >
> > Linus
>
> Awesome, thanks.
Another way is using the upper_32_bits() macro.
It's perhaps a bit more readable.
next prev parent reply other threads:[~2017-10-11 3:27 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-10 23:09 [kernel-hardening] [PATCH 0/3] add %pX specifier Tobin C. Harding
2017-10-10 23:09 ` Tobin C. Harding
2017-10-10 23:09 ` Tobin C. Harding
2017-10-10 23:15 ` [kernel-hardening] " Linus Torvalds
2017-10-10 23:15 ` Linus Torvalds
2017-10-10 23:15 ` Linus Torvalds
2017-10-10 23:32 ` [kernel-hardening] " Tobin C. Harding
2017-10-10 23:32 ` Tobin C. Harding
2017-10-10 23:32 ` Tobin C. Harding
2017-10-11 3:27 ` Joe Perches [this message]
2017-10-11 3:27 ` Joe Perches
2017-10-11 3:27 ` Joe Perches
2017-10-11 20:11 ` [kernel-hardening] " Jason A. Donenfeld
2017-10-11 20:11 ` Jason A. Donenfeld
2017-10-11 20:11 ` Jason A. Donenfeld
2017-10-11 21:29 ` [kernel-hardening] " Linus Torvalds
2017-10-11 21:29 ` Linus Torvalds
2017-10-11 21:29 ` Linus Torvalds
2017-10-11 22:11 ` [kernel-hardening] " Jason A. Donenfeld
2017-10-11 22:11 ` Jason A. Donenfeld
2017-10-11 22:11 ` Jason A. Donenfeld
2017-10-12 18:37 ` [kernel-hardening] " Linus Torvalds
2017-10-12 18:37 ` Linus Torvalds
2017-10-12 18:37 ` Linus Torvalds
2017-10-10 23:16 ` [kernel-hardening] " Linus Torvalds
2017-10-10 23:16 ` Linus Torvalds
2017-10-10 23:16 ` Linus Torvalds
2017-10-10 23:31 ` [kernel-hardening] " Tobin C. Harding
2017-10-10 23:31 ` Tobin C. Harding
2017-10-10 23:31 ` Tobin C. Harding
2017-10-13 17:54 ` [kernel-hardening] " Roberts, William C
2017-10-13 17:54 ` Roberts, William C
2017-10-13 17:54 ` Roberts, William C
2017-10-16 2:09 ` [kernel-hardening] " Tobin Harding
2017-10-16 2:09 ` Tobin Harding
2017-10-11 20:09 ` [kernel-hardening] " Jason A. Donenfeld
2017-10-11 20:09 ` Jason A. Donenfeld
2017-10-11 20:09 ` Jason A. Donenfeld
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