Hi Heinrich,
On Wed, 9 Dec 2020 at 11:22, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
On 12/1/20 4:58 PM, Simon Glass wrote:
Hi Heinrich,
On Mon, 30 Nov 2020 at 15:56, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
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.glpk@gmx.de wrote:
Am 30. November 2020 02:53:36 MEZ schrieb Simon Glass sjg@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 sjg@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
intdefault 32 if SANDBOXWhat 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 || X86Shouldn'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().
Thanks. You would think that would be enough, but somehow ll_entry_get() seems to make things worse.
I'm not sure how to do sizeof(long) in Kconfig
Linux defines a symbol 64BIT in arch/ for all architectures, e.g.
arch/x86/Kconfig:3:config 64BIT arch/x86/Kconfig-4- bool "64-bit kernel" if "$(ARCH)" = "x86" arch/x86/Kconfig-5- default "$(ARCH)" != "i386"
U-Boot has it only for two architectures:
arch/mips/Kconfig:501:config 64BIT arch/riscv/Kconfig:152:config 64BIT
Yes indeed.
But I haven't actually found documentation that explains what is going on with alignment. It is hard to define rules when we don't know the behaviour.
https://docs.adacore.com/live/wave/binutils-stable/html/ld/ld.html
Regards, Simon