PoC for kas_alias Issue with compat_binfmt_elf.c

Introduction

This PoC addresses an issue encountered with the kas_alias solution, which deals with duplicated symbols in the Linux kernel. Specifically, the problem arises when working with the infamous compat_binfmt_elf.c file, which includes another C file (binfmt_elf.c), resulting in conflicting symbols.

The proposed solution employs the #line directive to modify the debug information included in the object file, effectively resolving the issue caused by the inclusion of compat_binfmt_elf.c.

Overview

kas_alias

kas_alias is a solution designed to handle duplicated symbols in the Linux kernel. During the kernel build process, it inserts and utilizes addr2line to create aliases for duplicated symbols. After processing, a duplicate symbol is assigned an alias consisting of the original symbol name concatenated with the full path filename and line number where it is defined.

Compatibility Issue with compat_binfmt.c

The problem arises when dealing with compat_binfmt_elf.c, which includes another C file (binfmt_elf.c). Due to this inclusion, symbols defined in compat_binfmt_elf.c are perceived by addr2line as if they are defined in binfmt_elf.c, resulting in conflicting symbols with identical names, including their paths and line numbers.

Files in the PoC

File	Function	Kernel equivalent
inc.c	file c included in the other c	binfmt_elf.c
incr.h	helper needed to implement preprocessor inc	NA
main.c	main	NA
main.h	header	NA
use.c	file that includes another c file	compat_binfmt_elf.c
test.sh	build the executables and demonstrates the poc	NA

Sample Run

~ $ ./test.sh 
rm -f main1  main2 *.o
gcc -g -c main.c -o main.o
gcc -g -c inc.c -o inc.o
gcc main.o inc.o -o main1
gcc -g -c use.c -o use.o
gcc main.o use.o -o main2
testing line where print_test is defined in main1:
print_test
/tmp/test_line/inc.c:10
testing line where print_test is defined in main2:
print_test
/tmp/test_line/inc.c:use.c:10

alessandrocarminati / kas_alias_vs_compat_binfmt_elf