linux-kernel - Re: AMD-IOMMU and problem with _

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Message-ID: <20150929150658.GJ3036@8bytes.org>
Date:	Tue, 29 Sep 2015 17:06:58 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Alexander Holler <holler@...oftware.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: AMD-IOMMU and problem with __init(data)?

As expected it is no bug in the AMD IOMMU driver, but in your code.

On Wed, Sep 23, 2015 at 09:04:31PM +0200, Alexander Holler wrote:
> struct _annotated_initcall {
> 	initcall_t initcall;
> 	unsigned driver_id;
> 	unsigned *dependencies;
> 	struct device_driver *driver;
> };

This struct gets aligned on a 32 bytes boundary.

> +#define ANNOTATED_INITCALLS						\
> +		VMLINUX_SYMBOL(__annotated_initcall_start) = .;		\
> +		*(.annotated_initcall.init)				\
> +		VMLINUX_SYMBOL(__annotated_initcall_end) = .;

But this section does not.

> +	ac = __annotated_initcall_start;
> +	pr_info("ac %p ID %u\n", ac, ac->driver_id);
> +	BUG_ON(ac->driver_id != 23);

So when you access __annotated_initcall_start here, you don't access the
first element of your array, but actually the zero padding before your
struct.

On my system the section was aligned on an 8 bytes boundary, which means
there were 24 bytes of padding before the symbol you try to access.

	Joerg

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