解析代码 .global reset_got
.type reset_got, %function ;
.type fixup_gdt_reloc, %function ;
reset_got:
pie_fixup:
ldr r0, =pie_fixup
ldr r1, =PAGE_START_MASK
and r0, r0, r1
#mov_imm r1, \_pie_fixup_size
add r1, r1, r0
bl fixup_gdt_reloc
/* Relocation codes */
#define R_ARM_RELATIVE 23
fixup_gdt_reloc:
mov r6, r0
mov r7, r1
#if ENABLE_ASSERTIONS
/* Test if the limits are 4K aligned */
orr r0, r0, r1
mov r1, #(PAGE_SIZE_MASK)
tst r0, r1
ASM_ASSERT(eq)
#endif
/*
* Calculate the offset based on return address in lr.
* Assume that this function is called within a page at the start of
* fixup region.
*/
ldr r1, =PAGE_START_MASK
and r2, lr, r1
subs r0, r2, r6 /* Diff(S) = Current Address - Compiled Address */
beq 3f /* Diff(S) = 0. No relocation needed */
ldr r1, =__GOT_START__
add r1, r1, r0
ldr r2, =__GOT_END__
add r2, r2, r0
/*
* GOT is an array of 32_bit addresses which must be fixed up as
* new_addr = old_addr + Diff(S).
* The new_addr is the address currently the binary is executing from
* and old_addr is the address at compile time.
*/
1: ldr r3, [r1]
/* Skip adding offset if address is < lower limit */
cmp r3, r6
blo 2f
/* Skip adding offset if address is > upper limit */
cmp r3, r7
bhi 2f
add r3, r3, r0
str r3, [r1]
2: add r1, r1, #4
cmp r1, r2
blo 1b
/* Starting dynamic relocations. Use ldr to get RELA_START and END */
3: ldr r1, =__RELA_START__
add r1, r1, r0
ldr r2, =__RELA_END__
add r2, r2, r0
/*
* According to ELF-32 specification, the RELA data structure is as
* follows:
* typedef struct {
* Elf32_Addr r_offset;
* Elf32_Xword r_info;
* } Elf32_Rela;
*
* r_offset is address of reference
* r_info is symbol index and type of relocation (in this case
* code 23 which corresponds to R_ARM_RELATIVE).
*
* Size of Elf32_Rela structure is 8 bytes.
*/
/* Skip R_ARM_NONE entry with code 0 */
1: ldr r3, [r1, #4]
ands r3, r3, #0xff
beq 2f
#if ENABLE_ASSERTIONS
/* Assert that the relocation type is R_ARM_RELATIVE */
cmp r3, #R_ARM_RELATIVE
ASM_ASSERT(eq)
#endif
ldr r3, [r1] /* r_offset */
add r3, r0, r3 /* Diff(S) + r_offset */
ldr r4, [r3]
/* Skip adding offset if address is < lower limit */
cmp r4, r6
blo 2f
/* Skip adding offset if address is >= upper limit */
cmp r4, r7
bhs 2f
add r4, r0, r4
str r4, [r3]
2: add r1, r1, #8
cmp r1, r2
blo 1b
bx lr
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本文详细解释了在计算机科学中如何通过不同后缀（B, H, S）来表示字节、半字和有符号数的传输过程，深入浅出地介绍了其在编程实践中的应用。
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