[PATCH -next] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
liuyuntao (F)
liuyuntao12 at huawei.com
Tue Feb 27 00:06:56 PST 2024
On 2024/2/23 0:04, Arnd Bergmann wrote:
> On Thu, Feb 22, 2024, at 12:24, liuyuntao (F) wrote:
>>
>> The position of the caret has been moved below the right brace
>> of { KEEP(*(.vectors.bhb.loop8)) }, indicating that lld is treating
>> the entire `KEEP(*(.vectors))` as a file name. This could potentially be
>> a bug in lld. Perhaps we can temporarily
>> enable the DCE option only when option LD_IS_LLD is disabled,
>> like risc-v:
>>
>> `select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD`.
>
> I would really like to see this working with lld if at all
> possible, as it allows the combination of gc-sections with
> lto and CONFIG_TRIM_UNUSED_KSYMS.
>
> I experimented with lld myself now and I did get a booting
> kernel even without the the KEEP() on the vectors. I also
> see that this is the only use of OVERLAY in the kernel, so
> I hope that we can find a way to make it work with existing
> lld after all, either without the KEEP or without the OVERLAY.
>
> Did you see any problems without the KEEP() on the vectors?
>
> Arnd
Hi, Arnd. I have added a global symbol g_keep1 in .vectors, g_keep2 in
.vectors.bhb.loop8 and g_keep3 in .vectors.bhb.bpiall respectively. I
also added another section to reference these three global symbols, to
prevent these sections from being removed during linking with --gc-sections.
It worked,but there should be a better way to achieve it. the patch:
diff --git a/arch/arm/include/asm/vmlinux.lds.h
b/arch/arm/include/asm/vmlinux.lds.h
index f2ff79f740ab..d60f6e83a9f7 100644
--- a/arch/arm/include/asm/vmlinux.lds.h
+++ b/arch/arm/include/asm/vmlinux.lds.h
@@ -125,13 +125,13 @@
__vectors_lma = .; \
OVERLAY 0xffff0000 : NOCROSSREFS AT(__vectors_lma) { \
.vectors { \
- KEEP(*(.vectors)) \
+ *(.vectors) \
} \
.vectors.bhb.loop8 { \
- KEEP(*(.vectors.bhb.loop8)) \
+ *(.vectors.bhb.loop8) \
} \
.vectors.bhb.bpiall { \
- KEEP(*(.vectors.bhb.bpiall)) \
+ *(.vectors.bhb.bpiall) \
} \
} \
ARM_LMA(__vectors, .vectors); \
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 6150a716828c..84536e805da0 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -1075,6 +1075,9 @@ THUMB( .reloc ., R_ARM_THM_PC12,
.L__vector_swi )
W(b) vector_addrexcptn
W(b) vector_irq
W(b) vector_fiq
+ .global g_keep1
+ g_keep1:
+ .word 0
#ifdef CONFIG_HARDEN_BRANCH_HISTORY
.section .vectors.bhb.loop8, "ax", %progbits
@@ -1088,6 +1091,9 @@ THUMB( .reloc ., R_ARM_THM_PC12,
.L__vector_bhb_loop8_swi )
W(b) vector_addrexcptn
W(b) vector_bhb_loop8_irq
W(b) vector_bhb_loop8_fiq
+ .global g_keep2
+ g_keep2:
+ .word 0
.section .vectors.bhb.bpiall, "ax", %progbits
W(b) vector_rst
@@ -1100,6 +1106,9 @@ THUMB( .reloc ., R_ARM_THM_PC12,
.L__vector_bhb_bpiall_swi )
W(b) vector_addrexcptn
W(b) vector_bhb_bpiall_irq
W(b) vector_bhb_bpiall_fiq
+ .global g_keep3
+ g_keep3:
+ .word 0
#endif
.data
@@ -1108,3 +1117,8 @@ THUMB( .reloc ., R_ARM_THM_PC12,
.L__vector_bhb_bpiall_swi )
.globl cr_alignment
cr_alignment:
.space 4
+
+.section .keep_vectors, "ax", %progbits
+ LDR r0, =g_keep1
+ LDR r1, =g_keep2
+ LDR r2, =g_keep3
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 01a887c1141a..5cdfb4ba3ac4 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -62,6 +62,7 @@ SECTIONS
.text : { /* Real text segment */
_stext = .; /* Text and read-only data */
ARM_TEXT
+ KEEP(*(.keep_vectors))
}
#ifdef CONFIG_DEBUG_ALIGN_RODATA
--
2.34.1
> I would really like to see this working with lld if at all
> possible, as it allows the combination of gc-sections with
> lto and CONFIG_TRIM_UNUSED_KSYMS.
Then, I enabled config CONFIG_LTO_CLANG_THIN in arm32, but
came across a link failure using clang/lld:
following symbols must have non local/private scope:
free_mem_end_ptr
free_mem_ptr
malloc_count
malloc_ptr
output_data
in file arch/arm/boot/compressed/Makefile:
# We need to prevent any GOTOFF relocs being used with references
# to symbols in the .bss section since we cannot relocate them
# independently from the rest at run time. This can be achieved by
# ensuring that no private .bss symbols exist, as global symbols
# always have a GOT entry which is what we need.
# The .data section is already discarded by the linker script so no need
# to bother about it here.
check_for_bad_syms = \
bad_syms=$$($(NM) $@ | sed -n 's/^.\{8\} [bc] \(.*\)/\1/p') && \
[ -z "$$bad_syms" ] || \
( echo "following symbols must have non local/private scope:" >&2; \
echo "$$bad_syms" >&2; false )
Turn on the config CONFIG_LTO_CLANG_THIN , use nm to check the type of
these symbols as 'b', turn off the config type to 'B'. I tried to
explicitly declare these variables using
__attribute__((visibility("default")), but it didn't take effect on the
variable `output_data`.
ld.lld: warning:
.thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_ptr)
is being placed in '.data.':
ld.lld: warning:
.thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_ptr)
is being placed in '.data.'
ld.lld: warning:
.thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_count)
is being placed in '.dat'
following symbols must have non local/private scope:
output_data
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