Re: Early crash (was: Re: module: show version information forbuilt-in modules in sysfs)

[Geert Uytterhoeven]

On Tue, Feb 1, 2011 at 23:26, Dmitry Torokhov <dtor@xxxxxxxxxx> wrote:
> On Tue, Feb 01, 2011 at 02:03:23PM -0800, Geert Uytterhoeven wrote:
>> On Tue, Feb 1, 2011 at 22:09, Dmitry Torokhov <dtor@xxxxxxxxxx> wrote:
>> > On Tue, Feb 01, 2011 at 12:33:29PM -0800, Geert Uytterhoeven wrote:
>> >> On Mon, Jan 24, 2011 at 11:59, Linux Kernel Mailing List
>> >> <linux-kernel@xxxxxxxxxxxxxxx> wrote:
>> >> > Gitweb: Â Â http://git.kernel.org/linus/e94965ed5beb23c6fabf7ed31f625e66d7ff28de
>> >>
>> >> > Â Âmodule: show version information for built-in modules in sysfs
>> >> >
>> >> > Â ÂCurrently only drivers that are built as modules have their versions
>> >> > Â Âshown in /sys/module/<module_name>/version, but this information might
>> >> > Â Âalso be useful for built-in drivers as well. This especially important
>> >> > Â Âfor drivers that do not define any parameters - such drivers, if
>> >> > Â Âbuilt-in, are completely invisible from userspace.
>> >> >
>> >> > Â ÂThis patch changes MODULE_VERSION() macro so that in case when we are
>> >> > Â Âcompiling built-in module, version information is stored in a separate
>> >> > Â Âsection. Kernel then uses this data to create 'version' sysfs attribute
>> >> > Â Âin the same fashion it creates attributes for module parameters.
>> >>
>> >> This commit causes the crash below on m68k (ARAnyM).
>> >> Reverting this commit and its dependency
>> >> 3b90a5b292321b2acac3921f77046ae195aef53f
>> >> ("module: fix linker error for MODULE_VERSION when !MODULE and CONFIG_SYSFS=n")
>> >> makes it boot again.
>> >>
>> >
>> > Hi Geert,
>> >
>> > Does the follwing help by any chance?
>> >
>> > From d6fd4a6e0fc2d3f0a74962d4a6f663a46d230ecd Mon Sep 17 00:00:00 2001
>> > diff --git a/arch/m68knommu/kernel/vmlinux.lds.S b/arch/m68knommu/kernel/vmlinux.lds.S
>> > index ef33213..47e15eb 100644
>> > --- a/arch/m68knommu/kernel/vmlinux.lds.S
>> > +++ b/arch/m68knommu/kernel/vmlinux.lds.S
>>
>> The crash happened on m68k with MMU, not m68knommu.
>>
>
> Hmm, OK then. Could you please see if the crash happens if you return
> early in kernel/params.c::version_sysfs_builtin() ? Also, do you see

It does not crash if version_sysfs_builtin() returns early.

> anything in __modev section of your build?

"objdump -h" says:

| Sections:
| Idx Name          Size      VMA       LMA       File off  Algn
|   7 __modver      0000007c  002e0f84  002e0f84  002e0f84  2**2
|                   CONTENTS, ALLOC, LOAD, DATA

"nm vmlinux | grep __modver" says:

| 002e0f84 d __modver_version_attr
| 002e0fa8 d __modver_version_attr
| 00039026 T __modver_version_show
| 002e0f84 D __start___modver
| 002e0fca D __stop___modver

The section size (0x7c) is larger than __stop___modver -
__start___modver (0x46)?

Adding some debugging code (which increases the section size even more?) shows:

vattr = 002e1004
vattr->module_name = xz_dec
    mk = 00c2ee50
    err = 0
    kobject_uevent done
    kobject_put done
vattr = 002e1026
vattr->module_name = (null)
Unable to handle kernel NULL pointer dereference at virtual address 0000002c

So the second module in the list has no name. Why?
Aha, it's not NULL, but just < PAGE_SIZE (0x2c).

sizeof(struct module_version_attribute) = 34, which you can see from
the 2 consecutive vattr
pointers above. But the "aligned(sizeof(void *))" in the definition of
MODULE_VERSION() puts
the next module_version_attribute struct in the array at offset 36,
not offset 34!
On m68k, the alignment of 32-bit integrals is 2 bytes, not 4.

Removing the alignment, cfr. this (gmail-pasted hence
whitespace-corrupted) patch
fixes it:

--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -176,7 +176,7 @@ extern struct module __this_module;
                                             struct module *, char *);  \
        static struct module_version_attribute __modver_version_attr    \
        __used                                                          \
-    __attribute__ ((__section__ ("__modver"),aligned(sizeof(void *)))) \
+    __attribute__ ((__section__ ("__modver"))) \
        = {                                                             \
                .mattr  = {                                             \
                        .attr   = {                                     \

Why is the explicit alignment there? Can it be removed?
If not, you can move the alignment into the definition of struct
module_version_attribute,
so it will be honoured everywhere.

Gr{oetje,eeting}s,

            Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
             Â Â -- Linus Torvalds
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