Hello,sorry to bother you, but I have some confusion about the RELA relocation.
As we know, RELA means relocation entries with addends.Then my question is:
If we use RELA,can we say that the offset will be 100 percent put into addend?
Considering the following testcase:
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#include <stdlib.h>
#define __test_section __attribute__((__section__(".data.test_section")))
static unsigned long flags __test_section;
static int *test __test_section;
unsigned long test1(void)
{
test = NULL;
return 0;
}
unsigned long test2(void)
{
//const volatile unsigned long *addr = &flags;
const unsigned long *addr = &flags;
return *addr;
}
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My toolchain is aarch64_be-linux-gnu cross compiler based on gcc-4.9.3 and binutils-2.24.
Execute the following command:
aarch64_be-linux-gnu-gcc -c test.c -O2 -dp
aarch64_be-linux-gnu-readelf -r test.o > readelf_r
The relocation information is as follows:
We can see that the offset of the global variable "flags" is put into Addend,and as my understanding,this is the right way of RELA relocations
*********************************************************************************
Relocation section '.rela.text' at offset 0x538 contains 4 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000000 000600000113 R_AARCH64_ADR_PRE 0000000000000000 .data.test_section + 0
000000000008 00060000011e R_AARCH64_LDST64_ 0000000000000000 .data.test_section + 0
000000000010 000600000113 R_AARCH64_ADR_PRE 0000000000000000 .data.test_section + 8
000000000014 00060000011e R_AARCH64_LDST64_ 0000000000000000 .data.test_section + 8
*********************************************************************************
But when "addr" is prefixed with the keyword "volatile" ,then the relocation would look like this:
*********************************************************************************
Relocation section '.rela.text' at offset 0x538 contains 4 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000000 000600000113 R_AARCH64_ADR_PRE 0000000000000000 .data.test_section + 0
000000000008 00060000011e R_AARCH64_LDST64_ 0000000000000000 .data.test_section + 0
000000000010 000600000113 R_AARCH64_ADR_PRE 0000000000000000 .data.test_section + 0
000000000014 000600000115 R_AARCH64_ADD_ABS 0000000000000000 .data.test_section + 0
*********************************************************************************
The addend is zero.
Get the assemble output of the two situations with the following command:
aarch64_be-linux-gnu-gcc -S test.c -O2 -dp -o test.s
the assemble output without "volatile":
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test2:
adrp x0, .LANCHOR0+8 // 6 *movdi_aarch64/10 [length = 4]
ldr x0, [x0,#:lo12:.LANCHOR0+8] // 12 *movdi_aarch64/5 [length = 4]
ret // 20 *do_return [length = 4]
.size test2, .-test2
.section .data.test_section,"aw",%progbits
.align 3
.LANCHOR0 = . + 0
.type test, %object
.size test, 8
************************************************************************************
the assemble output with "volatile":
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test2:
adrp x0, .LANCHOR0 // 5 *movdi_aarch64/10 [length = 4]
add x0, x0, :lo12:.LANCHOR0 // 6 add_losym_di [length = 4]
ldr x0, [x0,8] // 7 *movdi_aarch64/5 [length = 4]
ret // 20 *do_return [length = 4]
.size test2, .-test2
.section .data.test_section,"aw",%progbits
.align 3
.LANCHOR0 = . + 0
.type test, %object
.size test, 8
************************************************************************************
