[PATCH 1/2] linker_lists: Fix end-marker alignment to prevent padding

[Simon Glass]

Hi Rasmus,

On Mon, 23 Mar 2026 at 03:56, Rasmus Villemoes <ravi at prevas.dk> wrote:
>
> On Sat, Mar 21 2026, Simon Glass <sjg at chromium.org> wrote:
>
> > From: Simon Glass <simon.glass at canonical.com>
> >
> > Change the alignment of end markers in ll_entry_end() and ll_end_decl()
> > from __aligned(4) and __aligned(CONFIG_LINKER_LIST_ALIGN) respectively
> > to __aligned(1).
> >
> > The linker places zero-size end markers at aligned boundaries based on
> > what follows them. When the next list's start marker has a high alignment
> > requirement (e.g., 32 bytes), padding gets inserted before the end
> > marker. This causes the byte span (end - start) to not be an exact
> > multiple of the struct size.
> >
> > The compiler optimises pointer subtraction (end - start) using
> > magic-number multiplication for division. This optimisation only produces
> > correct results when the byte span is an exact multiple of the struct
> > size. With padding, the result is garbage (e.g., -858993444 instead of
> > 15).
> >
> > By using __aligned(1), the end marker is placed immediately after the
> > last entry with no padding, ensuring (end - start) equals exactly (n *
> > sizeof) where n is the number of entries.
>
> So I'm wondering why that is guaranteed. I mean, the linker is placing
> these sections one after another in order
>
>
>   2_foo_2_last_foo   size sizeof(struct foo), alignment max(4, alignof(struct foo))
>   2_foo_3            size 0, alignment 4 (1 with your patch)
>   2_bar_1            size 0, alignment CONFIG_LINKER_LIST_ALIGN
>   2_bar_2_first_bar  size sizeof(struct bar), alignment max(4, alignof(struct bar))
>
> So clearly the end of last_foo does have 4-byte alignment, yet it is
> observed that the linker sometimes makes 2_foo_3's address coincide with
> 2_bar_1's address?
>
> What I don't understand is that it seems that the linker could place the
> zero-size object 2_foo_3 at any 4-byte aligned address between the end
> of 2_foo_2_last_foo and 2_bar_1. And the same seems to be true when one
> changes it to have even smaller alignment requirement.
>
> So why does an align(1) stop the linker from placing that 0-size section
> at the same address as 2_bar_1, or even force it (as we need) to put it
> at the first possible address, i.e. immediately after last_foo?

My commit message was a bit confusing - alignment of symbol is not
based on what follows an item, just on the item itself (despite
appearances to the contrary).

My understanding of this is that the linker processes input sections
sequentially within the SORT(_u_boot_list*) output section, placing
each at the first address that satisfies its alignment. So the
location counter advances forward only by the minimum needed. But this
isn't specific to alignment 1.

I've used __aligned(1) in order to make it clear we don't want any
alignment. In all current cases, __aligned(4) would be OK too since
the structs we use are always 4-byte-aligned. We just want the end
marker to go at the current location-counter, i.e. immediately after
the last entry. I suppose another way of saying this is that we want
the end marker to be a 'multiple of the struct size' higher than the
start marker.

With ll_end_decl() using __aligned(CONFIG_LINKER_LIST_ALIGN), the
result depends on the struct size and the number of items in the list.
On sandbox the value is 32. If the last entry ends at, say, 0x103c
(4-byte aligned but not 32-byte aligned), the linker must advance to
0x1040 to place the end marker. So then there is a 4-byte gap, i.e.
(end - start) not a multiple of sizeof(struct), and the compiler's
magic-number division optimisation fails.

> Unless
> alignment 1 is somehow special-cased to mean "place as early as
> possible", I can't see how this should provide any better guarantees
> than what we already have.

Yes, we could use 4, see above. I think that would be confusing
though. We cannot use CONFIG_LINKER_LIST_ALIGN as above.

>
> So I don't oppose the patch at all, but I'd really like to understand
> how it actually works.

BTW I spent quite a lot of time analysing what was actually going on
and even wrote a Python script to check the build. There is a blog
post about all of this too (search for "Linker Lists" and magic
numbers).

Regards,
Simon