[PATCH] linker_list: Fix ll_entry_get alignment

[Sean Anderson]

On 10/1/23 21:16, Simon Glass wrote:
> Hi Sean,
> 
> On Sat, 30 Sept 2023 at 09:23, Sean Anderson <seanga2 at gmail.com> wrote:
>>
>> On 9/30/23 10:36, Sean Anderson wrote:
>>> When ll_entry_get is used on a list entry ll_entry_declare'd in the same
>>> file, the lack of alignment on the access will override the
>>> ll_entry_declare alignment. This causes GCC to use the default section
>>> alignment of 32 bytes. As list entries are not necessarily 32-byte aligned,
>>> this will cause a gap in the linker list, corrupting further entries.
>>>
>>> As a specific example, get_fs_loader uses DM_DRIVER_GET(fs_loader) in the
>>> same file where U_BOOT_DRIVER(fs_loader) is present. This causes a crash
>>> when walking the driver list.
>>>
>>> Fix this by adding appropriate alignment to all accesses.
>>>
>>> Fixes: 42ebaae3a33 ("common: Implement support for linker-generated arrays")
>>> Signed-off-by: Sean Anderson <seanga2 at gmail.com>
>>> ---
>>>
>>>    include/linker_lists.h | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linker_lists.h b/include/linker_lists.h
>>> index f9a2ee0c762..e0c8a01b9ba 100644
>>> --- a/include/linker_lists.h
>>> +++ b/include/linker_lists.h
>>> @@ -209,7 +209,8 @@
>>>     */
>>>    #define ll_entry_get(_type, _name, _list)                           \
>>>        ({                                                              \
>>> -             extern _type _u_boot_list_2_##_list##_2_##_name;        \
>>> +             extern _type __aligned(4)                               \
>>> +                     _u_boot_list_2_##_list##_2_##_name;             \
>>>                _type *_ll_result =                                     \
>>>                        &_u_boot_list_2_##_list##_2_##_name;            \
>>>                _ll_result;                                             \
>>> @@ -229,7 +230,7 @@
>>>     * @_list: name of the list
>>>     */
>>>    #define ll_entry_ref(_type, _name, _list)                           \
>>> -     ((_type *)&_u_boot_list_2_##_list##_2_##_name)
>>> +     ((_type __aligned(4) *)&_u_boot_list_2_##_list##_2_##_name)
>>
>> OK, so this causes an error in clang. And it isn't really necessary
>> because the entry is already declared at this point.
>>
>> So I guess the right fix is to replace DM_DRIVER_GET with DM_DRIVER_REF in
>> get_fs_loader. But this seems like a really big footgun. You can use the
>> wrong one and there are no errors except at runtime. I wonder if we can add
>> a warning of some kind?
> 
> I can imagine having a runtime check, something like:
> 
> ll_check(sizeof(struct something))
> 
> which checks that the linker list (end - start) is a multiple of the
> struct size. Do you think that would find the problem?

Most of the time, yes.

> If so, then it could be perhaps be turned into a link-time check. This
> produces a list of the linker lists along with their individual
> members:
> 
> or ll in $(nm /tmp/b/coreboot/u-boot |grep u_boot_list_2 |sed
> 's/.*_u_boot_list_2_\(.*\)_2_.*/\1/' |uniq); do echo; echo "linker
> list: %ll"; nm /tmp/b/coreboot/u-boot |grep $ll; done
> 
> ...
> linker list: ut_str_test
> 011a9a20 D _u_boot_list_2_ut_str_test_2_str_dectoul
> 011a9a30 D _u_boot_list_2_ut_str_test_2_str_hextoul
> 011a9a40 D _u_boot_list_2_ut_str_test_2_str_itoa
> 011a9a50 D _u_boot_list_2_ut_str_test_2_str_simple_strtoul
> 011a9a60 D _u_boot_list_2_ut_str_test_2_str_simple_strtoull
> 011a9a70 D _u_boot_list_2_ut_str_test_2_str_trailing
> 011a9a80 D _u_boot_list_2_ut_str_test_2_str_upper
> 011a9a90 D _u_boot_list_2_ut_str_test_2_str_xtoa
> 011a9aa0 D _u_boot_list_2_ut_str_test_2_test_str_to_list
> ...
> 
> Then you can check that the address of each one increments by the same amount.
> 
> Maybe.

Yeah, this would be the best way to find errors in the current system.

But maybe ll_entry_get should look like

#define ll_entry_get(_type, _name, _list)				\
	({								\
		ll_entry_declare(_type, _name, _list);			\
		_type *_ll_result =					\
			&_u_boot_list_2_##_list##_2_##_name;		\
		_ll_result;						\
	})

(untested)

Regardless, I think a link-time check would be a good sanity check.

--Sean