On Mon, Jan 11, 2016 at 7:26 PM, weixiangyu <weixiangyu@xxxxxxxxxx> wrote:
> 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?
No. Your example shows that. What are you trying to do really? It
is more complex than the below code.
Take an example of:
unsigned long test3(void)
{
return test[flags+flags1];
}
You will get the following code:
test3:
adrp x0, .LANCHOR0
add x1, x0, :lo12:.LANCHOR0
ldr x2, [x1,16]
ldr x1, [x1,8]
ldr x0, [x0,#:lo12:.LANCHOR0]
add x1, x2, x1
ldrsw x0, [x0,x1,lsl 2]
ret
Where you have three different variables at .LANCHOR0, LANCHOR0+8 and
LANCHOR0+16.
Are you trying to patch the code afterwards? Also you might not want
section anchors you can disable that -fno-section-anchors and you get
more relocations (three R_AARCH64_ADR_PRE) :
test3:
adrp x0, flags1
ldr x2, [x0,#:lo12:flags1]
adrp x0, flags
ldr x1, [x0,#:lo12:flags]
adrp x0, test
ldr x0, [x0,#:lo12:test]
add x1, x2, x1
ldrsw x0, [x0,x1,lsl 2]
ret
Thanks,
Andrew
>
> Considering the following testcase:
>
> **************************************************************
> #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;
> }
>
> **************************************************************
> 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":
> ************************************************************************************
> 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":
> ************************************************************************************
> 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
> ************************************************************************************
>
>
>
>
