linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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:	Wed, 09 Jul 2008 14:06:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> H. Peter Anvin wrote:
>> Thinking about this some more, I don't know if it would make sense to put the
>> x86-64 stack canary at the *end* of the percpu area, and otherwise use
>> negative offsets.  That would make sure they were readily reachable from
>> %rip-based references from within the kernel text area.
>
> If we can move the canary then a whole pile of options open up.  But the problem
> is that we can't.

But we can pick an arbitrary point where %gs points at.

Hmm.  This whole thing is even sillier then I thought.
Why can't we access per cpu vars as:
%gs:(per_cpu__var - __per_cpu_start) ?

If we can subtract constants and allow the linker to perform that resolution
at link.  A zero based per cpu segment becomes a moot issue.

We may need to change the definition of PERCPU in vmlinux.lds.h to
#define PERCPU(align)							\
	. = ALIGN(align);						\
-	__per_cpu_start = .;					\
	.data.percpu  : AT(ADDR(.data.percpu) - LOAD_OFFSET) {		\
+		__per_cpu_start = .;					\
		*(.data.percpu)						\
		*(.data.percpu.shared_aligned)				\
+		__per_cpu_end = .;					\
+	}
-	}								\
-	__per_cpu_end = .;


So that the linker knows  __per_cpu_start and __per_cpu_end are in the same section
but otherwise it sounds entirely reasonable.  Just slightly trickier math at link
time.

Eric
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/