linux-kernel - Re: [PATCH 26/26] x86, pkeys: Documentation

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Date:	Fri, 2 Oct 2015 14:14:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Hansen <dave@...1.net>, Kees Cook <keescook@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation



On 02/10/2015 13:58, Linus Torvalds wrote:
> On Fri, Oct 2, 2015 at 7:49 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>> On 02/10/2015 00:48, Linus Torvalds wrote:
>>> It's quite likely that you will find that compilers put read-only
>>> constants in the text section, knowing that executable means readable.
>>
>> Not on x86 (because it has large immediates; RISC machines and s390 do
>> put large constants in the text section).
>>
>> But at the very least jump tables reside in the .text seection.
> 
> Yes, at least traditionally gcc put things like the jump tables for
> switch() statements immediately next to the code. That caused lots of
> pain on the P4, where the L1 I$ and D$ were exclusive. I think that
> caused gcc to then put the jump tables further away, and it might be
> in a separate section these days - but it might also just be
> "sufficiently aligned" that the L1 cache issue isn't in play any more.
> 
> Anyway, because of the P4 exclusive L1 I/D$ issue we can pretty much
> rest easy knowing that the data accesses and text accesses should be
> separated by at least one cacheline (maybe even 128 bytes - I think
> the L4 used 64-byte line size, but it was sub-sections of a 128-byte
> bigger line - but that might have been in the L2 only).
> 
> But I could easily see the compiler/linker still putting them in the
> same ELF segment.

You're entirely right, it puts them in .rodata actually.  But .rodata is
in the same segment as .text:

$ readelf --segments /bin/true
...
 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym
          .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init
          .plt .text .fini .rodata .eh_frame_hdr .eh_frame 
   03     .init_array .fini_array .jcr .data.rel.ro .dynamic .got .data .bss 
   04     .dynamic 
   05     .note.ABI-tag .note.gnu.build-id 
   06     .eh_frame_hdr 
   07     
   08     .init_array .fini_array .jcr .data.rel.ro .dynamic .got 


Paolo
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