linux-kernel - Re: vdso.so mislinked by buggy linker was Re: Linus 2.6.23-rc1

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Date:	Mon, 23 Jul 2007 08:45:17 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Andre Noll <maan@...temlinux.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: vdso.so mislinked by buggy linker was Re: Linus 2.6.23-rc1

On Mon, Jul 23, 2007 at 01:56:20AM +0200, Andi Kleen wrote:
> On Monday 23 July 2007 01:38:40 Andre Noll wrote:
> [readded linux-kernel, Linus]
> 
> >   [Nr] Name              Type             Address           Offset
> >        Size              EntSize          Flags  Link  Info  Align
> >   [ 0]                   NULL             0000000000000000  00000000
> >        0000000000000000  0000000000000000           0     0     0
> >   [ 1] .hash             HASH             ffffffffff700120  00000120
> >        00000000000000b4  0000000000000004   A       2     0     8
> >   [ 2] .dynsym           DYNSYM           ffffffffff7001d8  000001d8
> >        0000000000000270  0000000000000018   A       3    12     8
> >   [ 3] .dynstr           STRTAB           ffffffffff700448  00000448
> >        0000000000000059  0000000000000000   A       0     0     1
> >   [ 4] .gnu.version      VERSYM           ffffffffff7004a2  000004a2
> >        0000000000000034  0000000000000002   A       2     0     2
> >   [ 5] .gnu.version_d    VERDEF           ffffffffff7004d8  000004d8
> >        0000000000000038  0000000000000000   A       3     2     8
> >   [ 6] .text             PROGBITS         ffffffffff700c00  00100bab
>                                                               ^^^^^^^^
> >        00000000000002e4  0000000000000000  AX       0     0     64
> 
> It puts .text at 1MB. Your vdso file must be huge? 
> 
> It looks like it ignores the
> -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
> options passed to it. The AMD64 ABI has a 1MB minimum page size, but
> these options are supposed to disable it.
> 
> Not sure how to work around this, but having an 1+MB vdso would be incredibly
> wasteful. What version is it? Perhaps we just drop support for this. I can't
> think of a workaround currently.

Looking at vdso.lds.S, if you change just VDSO_TEXT_OFFSET to 0xc00 and
don't tweak the linker script, then you jump backwards with the dot, you
should even get a linker warning about it:

  . = VDSO_PRELINK + VDSO_TEXT_OFFSET;

  .text           : { *(.text) }                :text
  .text.ptr       : { *(.text.ptr) }            :text
  . = VDSO_PRELINK + 0x900;

Guess that 0x900 should have been VDSO_TEXT_OFFSET + 0x400 or something
similar.  Also note that it is highly desirable to fit the whole vdso into
one page, so increasing VDSO_TEXT_OFFSET etc. offsets too much is just
wasting memory.  From the above dump, VDSO_TEXT_OFFSET 0x500 is too low,
but 0x600 should work, assuming .data section is moved 0x100 higher as well.

	Jakub
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