On 11/30/20 9:11 PM, Simon Glass wrote:
+Marek Vasut who originally wrote it
Hi Heinrich,
On Sun, 29 Nov 2020 at 23:20, Heinrich Schuchardt <xypron.g...@gmx.de> wrote:
Am 30. November 2020 02:53:36 MEZ schrieb Simon Glass <s...@chromium.org>:
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.
One possible fix is to force each separate linker_list to start on the
largest possible boundary that can be required by the compiler. However
that does not seem to work on x86_64, which uses 16-byte alignment in
this
case but needs 32-byte alignment.
So add a Kconfig option to handle this. Set the default value to 4 so
as to avoid changing platforms that don't need it.
Update the ll_entry_start() accordingly.
Signed-off-by: Simon Glass <s...@chromium.org>
---
arch/Kconfig | 11 +++++++
doc/api/linker_lists.rst | 62 ++++++++++++++++++++++++++++++++++++++++
include/linker_lists.h | 3 +-
3 files changed, 75 insertions(+), 1 deletion(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index 3aa99e08fce..aa8664212f1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -7,6 +7,17 @@ config HAVE_ARCH_IOREMAP
config NEEDS_MANUAL_RELOC
bool
+config LINKER_LIST_ALIGN
+ int
+ default 32 if SANDBOX
What is so special about the sandbox?
I'm not too sure, actually. Also, 32 seems to be larger than
__BIGGEST_ALIGNMENT__ so it is confusing.
Just evaluate if the host is 64 bit and use 8 or 4 accordingly?
+ default 8 if ARM64 || X86
Shouldn't the default be 8 on all 64 bit platforms? And 4 on all 32 bit
platforms?
Possibly, but who knows? One way to really get to the bottom of this
is to have a test that checks that the alignment is what it should be.
I spent half a day diagnosing this but not that much time thinking of
the best solution. If you have time to dig into it please let me know.
Regards,
Simon
If you change ll_entry_start() as below, the linker will complain
"lib/efi_driver/efi_uclass.c:309:
undefined reference to `bad_alignment'"
If you change value 4 to 8, it stops complaining for qemu-x86_64_defconfig.
#define ll_alignment(x) \
(__builtin_offsetof(struct {char a; x b;}, b))
void bad_alignment(void);
#define ll_entry_start(_type, _list) \
({ \
static char start[0] __aligned(4) __attribute__((unused, \
section(".u_boot_list_2_"#_list"_1"))); \
if (ll_alignment(_type) > 4) \
bad_alignment(); \
(_type *)&start; \
})
If the alignment is smaller than the limit, the compiler can remove the
bad_alignment() call due to the optimization setting -Os (or -O2).
For RISC-V 64bit you also need 8 byte alignment.
I suggest that you replace 4 by sizeof(long) and run the change through
Gitlab to validate that 8 bytes on 64-bit systems and 4 bytes on 32-bit
systems are sufficient alignment.
I did not check if any linker lists are accessed via other means then
ll_entry_start().
Best regards
Heinrich
