Lecture 4

Linking and loading

Lecture

Slides (PDF, PPTX).

Outline

• Linking (example)
• Relocatable object files
• Executable and Linkable Format (ELF)
• Shared libraries
• Linked libraries
• Library interpositioning
• Position-independent code
• Address space layout randomization

Examples

Tools for Manipulating Object Files

• ar - creates static libraries, and inserts, deletes, lists, and extracts members.
• strings - lists all of the printable strings contained in an object file.
• strip - deletes symbol table information from an object file.
• nm - lists the symbols defined in the symbol table of an object file.
• size - lists the names and sizes of the sections in an object file.
• readelf - displays the complete structure of an object file, including all of the information encoded in the ELF header. Subsumes the functionality of size and nm.
• objdump - the mother of all binary tools. Can display all of the information in an object file. Its most useful function is disassembling the binary instructions in the .text section.
• ldd - lists the shared libraries that an executable needs at run time.

Executable and Linkable Format (ELF)

main.o (relocatable object file):

riscv64-unknown-linux-gnu-gcc -c main.c -O1 -march=rv64g
riscv64-unknown-linux-gnu-objdump -r -d main.o

main.o:     file format elf64-littleriscv

Disassembly of section .text:

0000000000000000 <main>:
   0:	ff010113          	addi	sp,sp,-16
   4:	00113423          	sd	ra,8(sp)
   8:	00200593          	li	a1,2
   c:	00000537          	lui	a0,0x0
			c: R_RISCV_HI20	array
			c: R_RISCV_RELAX	*ABS*
  10:	00050513          	mv	a0,a0
			10: R_RISCV_LO12_I	array
			10: R_RISCV_RELAX	*ABS*
  14:	00000097          	auipc	ra,0x0
			14: R_RISCV_CALL	sum
			14: R_RISCV_RELAX	*ABS*
  18:	000080e7          	jalr	ra # 14 <main+0x14>
  1c:	00813083          	ld	ra,8(sp)
  20:	01010113          	addi	sp,sp,16
  24:	00008067          	ret

sum.o (relocatable object file):

tatarnikov@akos:~/sum$ riscv64-unknown-linux-gnu-gcc -c sum.c -O1 -march=rv64g
tatarnikov@akos:~/sum$ riscv64-unknown-linux-gnu-objdump -r -d sum.o

sum.o:     file format elf64-littleriscv

Disassembly of section .text:

0000000000000000 <sum>:
   0:	02b05a63          	blez	a1,34 <.L4>
			0: R_RISCV_BRANCH	.L4
   4:	00050793          	mv	a5,a0
   8:	fff5871b          	addiw	a4,a1,-1
   c:	02071693          	slli	a3,a4,0x20
  10:	01e6d713          	srli	a4,a3,0x1e
  14:	00450513          	addi	a0,a0,4
  18:	00a70733          	add	a4,a4,a0
  1c:	00000513          	li	a0,0

0000000000000020 <.L3>:
  20:	0007a683          	lw	a3,0(a5)
  24:	00a6853b          	addw	a0,a3,a0
  28:	00478793          	addi	a5,a5,4
  2c:	fee79ae3          	bne	a5,a4,20 <.L3>
			2c: R_RISCV_BRANCH	.L3
  30:	00008067          	ret

0000000000000034 <.L4>:
  34:	00000513          	li	a0,0
  38:	00008067          	ret

main (executable object file):

riscv64-unknown-linux-gnu-gcc -o main main.o sum.o
riscv64-unknown-linux-gnu-objdump -x -d main

main:     file format elf64-littleriscv

Disassembly of section .text:

   ...

0000000000010430 <main>:
   10430:	ff010113          	addi	sp,sp,-16
   10434:	00113423          	sd	ra,8(sp)
   10438:	00200593          	li	a1,2
   1043c:	83018513          	addi	a0,gp,-2000 # 12030 <array>
   10440:	010000ef          	jal	ra,10450 <sum>
   10444:	00813083          	ld	ra,8(sp)
   10448:	01010113          	addi	sp,sp,16
   1044c:	00008067          	ret

0000000000010450 <sum>:
   10450:	02b05a63          	blez	a1,10484 <sum+0x34>
   10454:	00050793          	mv	a5,a0
   10458:	fff5871b          	addiw	a4,a1,-1
   1045c:	02071693          	slli	a3,a4,0x20
   10460:	01e6d713          	srli	a4,a3,0x1e
   10464:	00450513          	addi	a0,a0,4 # 12004 <__TMC_END__+0x4>
   10468:	00a70733          	add	a4,a4,a0
   1046c:	00000513          	li	a0,0
   10470:	0007a683          	lw	a3,0(a5)
   10474:	00a6853b          	addw	a0,a3,a0
   10478:	00478793          	addi	a5,a5,4
   1047c:	fee79ae3          	bne	a5,a4,10470 <sum+0x20>
   10480:	00008067          	ret
   10484:	00000513          	li	a0,0
   10488:	00008067          	ret

Library Interpositioning

• Compile-time
• Link-time
• Load-time/run-time

Workshop

Outline

• Creating static libraries
• Creating shared libraries
• Library interpositioning
• Make scripts

Practice

• Part 1 (Static and shared libraries)
• Part 2 (Make files)
• Part 3 (CMake)

Homework

Write and submit to GitHub the program described below. An instruction on how to work with GitHub is here.

Task

1. Improve Task 1 from Part 1:
   • Provide link-time interpositioning for the fred and john functions
   • Provide load-time interpositioning for the bill and sam functions
   • Create a Make script to build the program

Note: interposed functions can just print a message like “fred is called”.

References

• Linking. Chapter 7 in [CSPP]
• Section 2.5. in [OSC]
• Chapters 41 and 42 in [TLPI]
• Address space layout randomization (Wikipedia)