Position-Independent Code
Position-independent code, or PIC, is the name of the code generation technique used in the PowerPC environments that allows the dynamic linker to load a region of code at a non-fixed virtual memory address. Without some form of position-independent code generation, the operating system would need to place all code you wanted to be shared at fixed addresses in virtual memory, which would make maintenance of the operating system remarkably difficult. For example, it would be nearly impossible to support shared libraries and frameworks because each one would need to be preassigned an address that could never change.
Mach-O position-independent code design is based on the observation that the __DATA segment is always located at a constant offset from the __TEXT segment. That is, the dynamic loader, when loading any Mach-O file, never moves a file’s __TEXT segment relative to its __DATA segment. Therefore, a function can use its own current address plus a fixed offset to determine the location of the data it wishes to access. All segments of a Mach-O file, not only the __TEXT and __DATA segments, are at fixed offsets relative to the other segments.
Eliminating Position-Independent Code References
Position-independent code is typically required for shared libraries and bundles to allow the dynamic loader to relocate them to different addresses at load time. However, it is not required for applications that typically reside at the same address in virtual memory. GCC 3.1 introduces a new option, called -mdynamic-no-pic. This option both reduces the code size of application executables and improves their performance by eliminating position-independent code references, while preserving indirect calls to shared libraries and indirection to undefined symbols. If you use Xcode to create your application, this option is enabled by default. For an example of dynamic code generated without PIC, see Listing 2.
Listing 2 shows an example of the position-independent code generated for the C code in Listing 1.
Listing 1 C source code example for position-independent code
struct s { int member1; int member2; }; |
struct s bar = {1,2}; |
int foo(void) |
{ |
return bar.member2; |
} |
Listing 2 Position-independent code generated from the C example (with addresses in the left column)
.text |
; The function foo |
.align 2 |
.globl _foo |
0x0 _foo: mflr r0 ; save the link register (LR) |
0x4 bcl 20,31,L1$pb ; Use the branch always instruction |
; that does not affect the link |
; register stack to get the address |
; of L1$pb into the LR. |
0x8 L1$pb: mflr r10 ; then move LR to r10 |
0xc mtlr r0 ; restore the previous LR |
; bar is located at L1$pc + distance |
0x10 addis r9,r10,ha16(_bar-L1$pb); L1$pb plus high 16 bits of distance |
0x14 la r9,lo16(_bar-L1$pb)(r9) ; plus low 16 of distance |
; => r9 now contains address of bar |
0x18 lwz r3,4(r9) ; return bar.member2 |
0x1c blr |
.data |
; The initialized structure bar |
.align 2 |
.globl _bar |
0x20 _bar: .long 1 ; member1’s initialized value |
0x24 .long 2 ; member2’s initialized value |
To calculate the address of _bar, the generated code adds the address of the L1$pb symbol (0x8) to the distance to bar. The distance to bar from the address of L1$pb is the value of the expression _bar - L1$pb, which is 0x18 (0x20 - 0x8).
Relocating Position-Independent Code
To support relocation of code in intermediate object files, Mach-O supports a section difference relocation entry format. Relocation entries are described in OS X ABI Mach-O File Format Reference.
Each of the add-immediate instructions is represented by two relocation entries. For the addis instruction (at address 0x10 in the example) the following tables list the two relocation entries. The fields of the first relocation entry (of type scattered_relocation_info) are:
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The values of the second relocation entry are:
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The first relocation entry for the la instruction (at address 0x14 in the example) is:
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The values of the second relocation entry are:
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Relocations in the x86-64 Environment
Relocations in the OS X x86-64 environment are different than relocations in other OS X environments and System V x86-64 (http://www.x86-64.org/documentation). The main differences are:.
Scattered relocations are not used
Compiler-generated code uses mostly external relocations
Mach Object (Mach-O), not Executable and Linkable Format (ELF), is used as the executable file format
This section describes how relocations are implemented in the OS X x86-64 environment.
When the assembler generates relocations, if the target label is a local label (it begins with L), the previous nonlocal label in the same section is used as the target of the external relocation. An addend (that is, the 4 in _foo + 4) is used with the distance from that nonlocal label to the target label. The assembler uses an internal relocation only when there is no previous nonlocal label in the section.
The addend is encoded in the instruction (Mach-O does not have RELA relocations). For PC-relative relocations, the addend is stored in the instruction. This practice is different than in other OS X environments, which encode the addend minus the current section offset. The x86-64 relocation types are described in OS X ABI Mach-O File Format Reference.
Listing 3 shows assembly instructions and the relocation and section content that they generate.
Listing 3 Example assembly instructions and their corresponding relocations
call _foo |
r_type=X86_64_RELOC_BRANCH, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
E8 00 00 00 00 |
call _foo+4 |
r_type=X86_64_RELOC_BRANCH, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
E8 04 00 00 00 |
movq _foo@GOTPCREL(%rip), %rax |
r_type=X86_64_RELOC_GOT_LOAD, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
48 8B 05 00 00 00 00 |
pushq _foo@GOTPCREL(%rip) |
r_type=X86_64_RELOC_GOT, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
FF 35 00 00 00 00 |
movl _foo(%rip), %eax |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
8B 05 00 00 00 00 |
movl _foo+4(%rip), %eax |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
8B 05 04 00 00 00 |
movb $0x12, _foo(%rip) |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
C6 05 FF FF FF FF 12 |
movl $0x12345678, _foo(%rip) |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_foo |
C7 05 FC FF FF FF 78 56 34 12 |
.quad _foo |
r_type=X86_64_RELOC_UNSIGNED,r_length=3, r_extern=1,r_pcrel=0, r_symbolnum=_foo |
00 00 00 00 00 00 00 00 |
.quad _foo+4 |
r_type=X86_64_RELOC_UNSIGNED,r_length=3,r_extern=1,r_pcrel=0,r_symbolnum=_foo |
04 00 00 00 00 00 00 00 |
.quad _foo - _bar |
r_type=X86_64_RELOC_SUBTRACTOR,r_length=3,r_extern=1, r_pcrel=0,r_symbolnum=_bar |
r_type=X86_64_RELOC_UNSIGNED,r_length=3,r_extern=1, r_pcrel=0,r_symbolnum=_foo |
00 00 00 00 00 00 00 00 |
.quad _foo - _bar + 4 |
r_type=X86_64_RELOC_SUBTRACTOR,r_length=3, r_extern=1,r_pcrel=0,r_symbolnum=_bar |
r_type=X86_64_RELOC_UNSIGNED,r_length=3, r_extern=1,r_pcrel=0,r_symbolnum=_foo |
04 00 00 00 00 00 00 00 |
.long _foo - _bar |
r_type=X86_64_RELOC_SUBTRACTOR,r_length=2,r_extern=1,r_pcrel=0,r_symbolnum=_bar |
r_type=X86_64_RELOC_UNSIGNED,r_length=2,r_extern=1,r_pcrel=0,r_symbolnum=_foo |
00 00 00 00 |
lea L1(%rip), %rax |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=1, r_pcrel=1, r_symbolnum=_prev |
48 8d 05 12 00 00 00 |
// Assumes that _prev is the first nonlocal label 0x12 bytes before L1. |
lea L0(%rip), %rax |
r_type=X86_64_RELOC_SIGNED, r_length=2, r_extern=0, r_pcrel=1, r_symbolnum=3 |
48 8d 05 56 00 00 00 |
// Assumes that L0 is in third section, and has an address of 0x00000056 |
// in .o file, and no previous nonlocal label. |
.quad L1 |
r_type=X86_64_RELOC_UNSIGNED,r_length=3,r_extern=1,r_pcrel=0, r_symbolnum= _prev |
12 00 00 00 00 00 00 00 |
// Assumes that _prev is the first nonlocal label 0x12 bytes before L1. |
.quad L0 |
r_type=X86_64_RELOC_UNSIGNED,r_length=3, r_extern=0, r_pcrel=0, r_symbolnum= 3 |
56 00 00 00 00 00 00 00 |
// Assumes that L0 is in third section, and has address of 0x00000056 |
// in .o file, and no previous nonlocal label. |
.quad _foo - . |
r_type=X86_64_RELOC_SUBTRACTOR,r_length=3,r_extern=1,r_pcrel=0,r_symbolnum=_prev |
r_type=X86_64_RELOC_UNSIGNED,r_length=3,r_extern=1,r_pcrel=0,r_symbolnum=_foo |
EE FF FF FF FF FF FF FF |
// Assumes that _prev is the first nonlocal label 0x12 bytes |
// before this .quad |
.quad _foo - L1 |
r_type=X86_64_RELOC_SUBTRACTOR,r_length=3,r_extern=1,r_pcrel=0,r_symbolnum=_prev |
r_type=X86_64_RELOC_UNSIGNED,r_length=3,r_extern=1,r_pcrel=0,r_symbolnum=_foo |
EE FF FF FF FF FF FF FF |
// Assumes that _prev is the first nonlocal label 0x12 bytes before L1. |
.quad L1 - _prev |
// No relocations. This is an assembly time constant. |
12 00 00 00 00 00 00 00 |
// Assumes that _prev is the first nonlocal label 0x12 bytes before L |
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