[RFC PATCH 0/6] Clean up arm linker scripts

[Sam Edwards]

On 2/28/24 04:15, Ilias Apalodimas wrote:
> On Wed, 28 Feb 2024 at 13:11, Peter Robinson <pbrobinson at gmail.com> wrote:
>>
>> On Wed, 28 Feb 2024 at 10:58, Ilias Apalodimas
>> <ilias.apalodimas at linaro.org> wrote:
>>>
>>> The arm linker scripts had a mix of symbols and C defined variables in an
>>> effort to emit relative references instead of absolute ones e.g [0].
>>> This has led to confusion over the years, ending up with mixed section
>>> definitions. Some sections being defined with overlays and different
>>> definitions between v7 and v8 architectures.
>>> For example __efi_runtime_rel_start/end is defined as a linker symbol for
>>> armv8 and a C variable in armv7.
>>>
>>> I am not sure if this used to be a compiler bug, but linker scripts nowadays
>>> can emit relative references, as long as the symbol definition is contained
>>> within the section definition. So let's switch most of the C defined variables
>>
>> Should we be setting/upping the minimum version of the linker version
>> as part of this?
> 
> The patch that added those as C-defined variables, was in 2013. So any
> linker that's not ancient history should emit those correctly.
> 

GNU ld fixed the problem on 2016-02-23, with this entry in the bfd 
changelog:

	* elflink.c (bfd_elf_record_link_assignment): Check for shared
	library, instead of PIC, and don't check PDE when making linker
	assigned symbol dynamic.

It looks like the change was first included in binutils 2.27, released 
2016-08-03. I don't know where the minimum linker version requirement is 
memorialized but there's a good chance that U-Boot already requires a 
version more recent than that. (Someone who knows where that is will 
have to check.)

In either case, I strongly agree: sections.c is an unnecessary 
workaround with any reasonably recent linker version, it's 
fickle/incompatible with non-GNU linkers such as LLD, and the marker 
symbols belong in the linker script. This patchset is a big step in the 
right direction!

> 
>>
>>> and clean up the arm sections.c file. There's still a few symbols remaining --
>>> __secure_start/end, __secure_stack_start/end and __end which can be cleaned up
>>> in a followup series.
>>>
>>> The resulting binary (tested in QEMU v7/v8) had no size differences apart from
>>> the emited sections and object types of those variables. I've also added prints
>>> throughout the U-Boot init sequence. The offsets and delta for 'end - start'
>>> section sizes is unchanged.
>>>
>>> For example on QEMU v8:
>>> $~ ./scripts/bloat-o-meter u-boot u-boot.new
>>> add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
>>> Function                                     old     new   delta
>>> Total: Before=798861, After=798861, chg +0.00%
>>>
>>> $~ readelf -sW u-boot | grep bss_s
>>>      12: 00000000001029b8     0 SECTION LOCAL  DEFAULT   12 .bss_start
>>>    8088: 0000000000000018     0 NOTYPE  GLOBAL DEFAULT    1 _bss_start_ofs
>>>    8376: 00000000001029b8     0 OBJECT  GLOBAL DEFAULT   12 __bss_start
>>> $~ readelf -sW u-boot.new | grep bss_s
>>>    8085: 0000000000000018     0 NOTYPE  GLOBAL DEFAULT    1 _bss_start_ofs
>>>    8373: 00000000001029b8     0 NOTYPE  GLOBAL DEFAULT   12 __bss_start
>>>
>>> For QEMU v7 the differences are a bit bigger but only affect the variables
>>> placed in the .bss section because that was defined as an OVERLAY in the
>>> existing linker script.
>>> For example:
>>> $~ nm u-boot | grep tftp_prev_block
>>> 000ca0dc ? tftp_prev_block
>>> $~ nm u-boot.new | grep tftp_prev_block
>>> 000e0a5c b tftp_prev_block -----> The symbol is now placed into .bss
>>>
>>> It's worth noting that since the efi regions are affected by the change, booting
>>> with EFI is preferable while testing. Booting the kernel only should be enough
>>> since the efi stub and the kernel proper do request boottime and runtime
>>> services respectively.
>>> Something along the lines of
>>>> virtio scan && load virtio 0 $kernel_addr_r Image && bootefi $kernel_addr_r
>>> will work for QEMU aarch64.
>>>
>>> Tested platforms:
>>> - QEMU aarch64
>>> - Xilinx kv260 kria starter kit & zynq
>>> - QEMU armv7
>>> - STM32MP157C-DK2
>>>
>>> [0] commit 3ebd1cbc49f0 ("arm: make __bss_start and __bss_end__ compiler-generated")
>>>
>>> Ilias Apalodimas (6):
>>>    arm: baltos: remove custom linker script
>>>    arm: clean up v7 and v8 linker scripts for bss_start/end
>>>    arm: fix __efi_runtime_rel_start/end definitions
>>>    arm: clean up v7 and v8 linker scripts for __rel_dyn_start/end
>>>    arm: fix __efi_runtime_start/end definitions
>>>    arm: move image_copy_start/end to linker symbols
>>>
>>>   arch/arm/cpu/armv8/u-boot-spl.lds         |  14 +--
>>>   arch/arm/cpu/armv8/u-boot.lds             |  45 ++------
>>>   arch/arm/cpu/u-boot.lds                   |  74 +++----------
>>>   arch/arm/lib/sections.c                   |  10 --
>>>   arch/arm/mach-rockchip/u-boot-tpl-v8.lds  |  22 +---
>>>   arch/arm/mach-zynq/u-boot.lds             |  72 +++---------
>>>   board/qualcomm/dragonboard820c/u-boot.lds |  41 ++-----
>>>   board/vscom/baltos/u-boot.lds             | 128 ----------------------
>>>   include/asm-generic/sections.h            |   3 +
>>>   lib/efi_loader/efi_runtime.c              |   1 +
>>>   10 files changed, 58 insertions(+), 352 deletions(-)
>>>   delete mode 100644 board/vscom/baltos/u-boot.lds
>>>
>>> --
>>> 2.37.2
>>>