Linker-Generated Arrays

A linker list is constructed by grouping together linker input sections, each containing one entry of the list. Each input section contains a constant initialized variable which holds the entry’s content. Linker list input sections are constructed from the list and entry names, plus a prefix which allows grouping all lists together. Assuming _list and _entry are the list and entry names, then the corresponding input section name is

.u_boot_list_ + 2_ + @_list + _2_ + @_entry

and the C variable name is

_u_boot_list + _2_ + @_list + _2_ + @_entry

This ensures uniqueness for both input section and C variable name.

Note that the names differ only in the first character, “.” for the section and “_” for the variable, so that the linker cannot confuse section and symbol names. From now on, both names will be referred to as

%u_boot_list_ + 2_ + @_list + _2_ + @_entry

Entry variables need never be referred to directly.

The naming scheme for input sections allows grouping all linker lists into a single linker output section and grouping all entries for a single list.

Note the two ‘_2_’ constant components in the names: their presence allows putting a start and end symbols around a list, by mapping these symbols to sections names with components “1” (before) and “3” (after) instead of “2” (within). Start and end symbols for a list can generally be defined as

%u_boot_list_2_ + @_list + _1_...
%u_boot_list_2_ + @_list + _3_...

Start and end symbols for the whole of the linker lists area can be defined as

%u_boot_list_1_...
%u_boot_list_3_...

Here is an example of the sorted sections which result from a list “array” made up of three entries : “first”, “second” and “third”, iterated at least once.

.u_boot_list_2_array_1
.u_boot_list_2_array_2_first
.u_boot_list_2_array_2_second
.u_boot_list_2_array_2_third
.u_boot_list_2_array_3

If lists must be divided into sublists (e.g. for iterating only on part of a list), one can simply give the list a name of the form ‘outer_2_inner’, where ‘outer’ is the global list name and ‘inner’ is the sub-list name. Iterators for the whole list should use the global list name (“outer”); iterators for only a sub-list should use the full sub-list name (“outer_2_inner”).

Here is an example of the sections generated from a global list named “drivers”, two sub-lists named “i2c” and “pci”, and iterators defined for the whole list and each sub-list:

%u_boot_list_2_drivers_1
%u_boot_list_2_drivers_2_i2c_1
%u_boot_list_2_drivers_2_i2c_2_first
%u_boot_list_2_drivers_2_i2c_2_first
%u_boot_list_2_drivers_2_i2c_2_second
%u_boot_list_2_drivers_2_i2c_2_third
%u_boot_list_2_drivers_2_i2c_3
%u_boot_list_2_drivers_2_pci_1
%u_boot_list_2_drivers_2_pci_2_first
%u_boot_list_2_drivers_2_pci_2_second
%u_boot_list_2_drivers_2_pci_2_third
%u_boot_list_2_drivers_2_pci_3
%u_boot_list_2_drivers_3

Alignment issues

The linker script uses alphabetic sorting to group the different linker lists together. Each group has its own struct and potentially its own alignment. But when the linker packs the structs together it cannot ensure that a linker list starts on the expected alignment boundary.

For example, if the first list has a struct size of 8 and we place 3 of them in the image, that means that the next struct will start at offset 0x18 from the start of the linker_list section. If the next struct has a size of 16 then it will start at an 8-byte aligned offset, but not a 16-byte aligned offset.

With sandbox on x86_64, a reference to a linker list item using ll_entry_get() can force alignment of that particular linker_list item, if it is in the same file as the linker_list item is declared.

Consider this example, where struct driver is 0x80 bytes:

ll_entry_declare(struct driver, fred, driver)

...

void *p = ll_entry_get(struct driver, fred, driver)

If these two lines of code are in the same file, then the entry is forced to be aligned at the ‘struct driver’ alignment, which is 16 bytes. If the second line of code is in a different file, then no action is taken, since the compiler cannot update the alignment of the linker_list item.

In the first case, an 8-byte ‘fill’ region is added:

.u_boot_list_2_driver_2_testbus_drv
            0x0000000000270018       0x80 test/built-in.o
            0x0000000000270018                _u_boot_list_2_driver_2_testbus_drv
.u_boot_list_2_driver_2_testfdt1_drv
            0x0000000000270098       0x80 test/built-in.o
            0x0000000000270098                _u_boot_list_2_driver_2_testfdt1_drv
*fill*         0x0000000000270118        0x8
.u_boot_list_2_driver_2_testfdt_drv
            0x0000000000270120       0x80 test/built-in.o
            0x0000000000270120                _u_boot_list_2_driver_2_testfdt_drv
.u_boot_list_2_driver_2_testprobe_drv
            0x00000000002701a0       0x80 test/built-in.o
            0x00000000002701a0                _u_boot_list_2_driver_2_testprobe_drv

With this, the linker_list no-longer works since items after testfdt1_drv are not at the expected address.

Ideally we would have a way to tell gcc not to align structs in this way. It is not clear how we could do this, and in any case it would require us to adjust every struct used by the linker_list feature.

The simplest fix seems to be to force each separate linker_list to start on the largest possible boundary that can be required by the compiler. This is the purpose of CONFIG_LINKER_LIST_ALIGN

llsym(_type, _name, _list)

Access a linker-generated array entry

Parameters

_type
Data type of the entry
_name
Name of the entry
_list
name of the list. Should contain only characters allowed in a C variable name!
ll_entry_declare(_type, _name, _list)

Declare linker-generated array entry

Parameters

_type
Data type of the entry
_name
Name of the entry
_list
name of the list. Should contain only characters allowed in a C variable name!

Description

This macro declares a variable that is placed into a linker-generated array. This is a basic building block for more advanced use of linker- generated arrays. The user is expected to build their own macro wrapper around this one.

A variable declared using this macro must be compile-time initialized.

Special precaution must be made when using this macro:

  1. The _type must not contain the “static” keyword, otherwise the entry is generated and can be iterated but is listed in the map file and cannot be retrieved by name.
  2. In case a section is declared that contains some array elements AND a subsection of this section is declared and contains some elements, it is imperative that the elements are of the same type.
  3. In case an outer section is declared that contains some array elements AND an inner subsection of this section is declared and contains some elements, then when traversing the outer section, even the elements of the inner sections are present in the array.

Example

ll_entry_declare(struct my_sub_cmd, my_sub_cmd, cmd_sub) = {
        .x = 3,
        .y = 4,
};
ll_entry_declare_list(_type, _name, _list)

Declare a list of link-generated array entries

Parameters

_type
Data type of each entry
_name
Name of the entry
_list
name of the list. Should contain only characters allowed in a C variable name!

Description

This is like ll_entry_declare() but creates multiple entries. It should be assigned to an array.

ll_entry_declare_list(struct my_sub_cmd, my_sub_cmd, cmd_sub) = {
     { .x = 3, .y = 4 },
     { .x = 8, .y = 2 },
     { .x = 1, .y = 7 }
};
ll_entry_start(_type, _list)

Point to first entry of linker-generated array

Parameters

_type
Data type of the entry
_list
Name of the list in which this entry is placed

Description

This function returns (_type *) pointer to the very first entry of a linker-generated array placed into subsection of .u_boot_list section specified by _list argument.

Since this macro defines an array start symbol, its leftmost index must be 2 and its rightmost index must be 1.

Example

struct my_sub_cmd *msc = ll_entry_start(struct my_sub_cmd, cmd_sub);
ll_entry_end(_type, _list)

Point after last entry of linker-generated array

Parameters

_type
Data type of the entry
_list
Name of the list in which this entry is placed (with underscores instead of dots)

Description

This function returns (_type *) pointer after the very last entry of a linker-generated array placed into subsection of .u_boot_list section specified by _list argument.

Since this macro defines an array end symbol, its leftmost index must be 2 and its rightmost index must be 3.

Example

struct my_sub_cmd *msc = ll_entry_end(struct my_sub_cmd, cmd_sub);
ll_entry_count(_type, _list)

Return the number of elements in linker-generated array

Parameters

_type
Data type of the entry
_list
Name of the list of which the number of elements is computed

Description

This function returns the number of elements of a linker-generated array placed into subsection of .u_boot_list section specified by _list argument. The result is of an unsigned int type.

Example

int i;
const unsigned int count = ll_entry_count(struct my_sub_cmd, cmd_sub);
struct my_sub_cmd *msc = ll_entry_start(struct my_sub_cmd, cmd_sub);
for (i = 0; i < count; i++, msc++)
        printf("Entry ``i``, x=``i`` y=``i``\n", i, msc->x, msc->y);
ll_entry_get(_type, _name, _list)

Retrieve entry from linker-generated array by name

Parameters

_type
Data type of the entry
_name
Name of the entry
_list
Name of the list in which this entry is placed

Description

This function returns a pointer to a particular entry in linker-generated array identified by the subsection of u_boot_list where the entry resides and it’s name.

Example

ll_entry_declare(struct my_sub_cmd, my_sub_cmd, cmd_sub) = {
        .x = 3,
        .y = 4,
};
...
struct my_sub_cmd *c = ll_entry_get(struct my_sub_cmd, my_sub_cmd, cmd_sub);
ll_start(_type)

Point to first entry of first linker-generated array

Parameters

_type
Data type of the entry

Description

This function returns (_type *) pointer to the very first entry of the very first linker-generated array.

Since this macro defines the start of the linker-generated arrays, its leftmost index must be 1.

Example

struct my_sub_cmd *msc = ll_start(struct my_sub_cmd);
ll_end(_type)

Point after last entry of last linker-generated array

Parameters

_type
Data type of the entry

Description

This function returns (_type *) pointer after the very last entry of the very last linker-generated array.

Since this macro defines the end of the linker-generated arrays, its leftmost index must be 3.

Example

struct my_sub_cmd *msc = ll_end(struct my_sub_cmd);