On Fri, Aug 21, 2020 at 1:04 PM Andy Shevchenko
[off-list ref] wrote:

On Fri, Aug 21, 2020 at 01:51:19PM +0300, Andy Shevchenko wrote:

quoted

On Fri, Aug 21, 2020 at 10:59:19AM +0200, Bartosz Golaszewski wrote:

quoted

On Fri, Aug 21, 2020 at 10:35 AM Andy Shevchenko
[off-list ref] wrote:

quoted

On Thu, Aug 20, 2020 at 08:51:08PM +0200, Bartosz Golaszewski wrote:

...

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+static struct devres *to_devres(void *data)
+{
+     return data - ALIGN(sizeof(struct devres), ARCH_KMALLOC_MINALIGN);
+}
+
+static size_t devres_data_size(size_t total_size)
+{
+     return total_size - ALIGN(sizeof(struct devres), ARCH_KMALLOC_MINALIGN);
+}

quoted

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The data pointer in struct devres is defined as:

    u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];

And this value (assigned the value of ARCH_DMA_MINALIGN) varies from
one arch to another. I wasn't really sure if offsetof() would work for
every case so I went with something very explicit.

I have checked with a small program simulating to_devres() with your variant,
offsetof() and container_of().

The result is this: if MINALIGN < sizeof(long) and since struct is unpacked the
offsetof(), and thus container_of(), gives correct result, while ALIGN()
approach mistakenly moves pointer too back.

...

quoted

I think you need to change this to use container_of() and offsetof().

To be clear, there is probably no real problem, except unlikely possible
MINALIGN=4 on 64-bit arch, but for sake of the correctness.

Thanks for taking the time to check it. I'll switch to container_of()
for (hopefully) the last iteration.

Bart

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