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

Jeremy Fitzhardinge
2007-07-01T22:13:44Z

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@goop.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@xensource.com>
Cc: Adrian Bunk <bunk@stusta.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)