linux-kernel - Re: 2.6.22-rc6-mm1: Xen: WARNING: Absolute relocations present

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Date:	Sun, 01 Jul 2007 15:13:34 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Adrian Bunk <bunk@...sta.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Andi Kleen <ak@...e.de>, chrisw@...s-sol.org,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	Vivek Goyal <vgoyal@...ibm.com>
Subject: Re: 2.6.22-rc6-mm1: Xen: WARNING: Absolute relocations present

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>
>   
>>>> Adrian Bunk wrote:
>>>>
>>>>         
>>>>> <--  snip  -->
>>>>>
>>>>> ...
>>>>>   RELOCS  arch/i386/boot/compressed/vmlinux.relocs
>>>>> WARNING: Absolute relocations present
>>>>> Offset     Info     Type     Sym.Value Sym.Name
>>>>> c0101f80 020c6501   R_386_32 00000000  xen_irq_disable_direct_reloc
>>>>> c0101f9a 0221ea01   R_386_32 00000000  xen_save_fl_direct_reloc
>>>>>           
> Actually I have to ask.  How in the world are these absolute symbols
> ok from a relocation perspective.
>
> If the kernel is not running at 0xc0100000 the offset value looks like
> it will be completely bogus.
>   

Notice they're absolute 0.

> Maybe those are ok, but if you could please explain why those are
> false positives I would appreciate it. (Especially in your patch
> description).
>
> If these are indeed false positives the patch looks ok.
>   

Subject: xen: suppress abs symbol warnings for unused reloc pointers

arch/i386/xen/xen-asm.S defines some small pieces of code which are
used to implement a few paravirt_ops.  They're designed so they can be
used either in-place, or be inline patched into their callsites if
there's enough space.

Some of those operations need to make calls out (specifically, if you
re-enable events [interrupts], and there's a pending event at that
time).  These calls need the call instruction to be relocated if the
code is patched inline.  In this case xen_foo_reloc is a section-relative
symbol which points to xen_foo's required relocation.

Other operations have no need of a relocation, and so their
corresponding xen_bar_reloc is absolute 0.  These are the cases which
are triggering the warning.

This patch adds those symbols to the list of safe abs symbols.

Signed-off-by: Jeremy Fitzhardinge <jeremy@...source.com>
Cc: Adrian Bunk <bunk@...sta.de>

---
 arch/i386/boot/compressed/relocs.c |    2 ++
 1 file changed, 2 insertions(+)

===================================================================
--- a/arch/i386/boot/compressed/relocs.c
+++ b/arch/i386/boot/compressed/relocs.c
@@ -31,6 +31,8 @@ static const char* safe_abs_relocs[] = {
 		"__kernel_rt_sigreturn",
 		"__kernel_sigreturn",
 		"SYSENTER_RETURN",
+		"xen_irq_disable_direct_reloc",
+		"xen_save_fl_direct_reloc",
 };
 
 static int is_safe_abs_reloc(const char* sym_name)


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