lukas2511 · March 20, 2019 20:32 · Mar 20, 2019 · Mar 20, 2019 · Mar 20, 2019 · Mar 20, 2019
diff --git a/Tiny gcc compiled binary experiment b/Tiny gcc compiled binary experiment
diff --git a/strip.py b/strip.py
@@ -7,17 +7,17 @@
 
 is_64bit = binary[0x04] == 0x02
 
-elf_header_names = ['e_ident[EI_MAG]', 'e_ident[EI_CLASS]', 'e_ident[EI_DATA]', 'e_ident[EI_VERSION]', 'e_ident[EI_OSABI]', 'e_ident[EI_ABIVERSION]', 'e_type', 'e_machine', 'e_version', 'e_entry', 'e_phoff', 'e_shoff', 'e_flags', 'e_ehsize', 'e_phentsize', 'e_phnum', 'e_shentsize', 'e_shnum', 'e_shstrndx']
+elf_header_names = ['e_ident[EI_MAG]', 'e_ident[EI_CLASS]', 'e_ident[EI_DATA]', 'e_ident[EI_VERSION]', 'e_ident[EI_OSABI]', 'e_ident[EI_ABIVERSION]', 'e_ident[EI_RESERVED]', 'e_type', 'e_machine', 'e_version', 'e_entry', 'e_phoff', 'e_shoff', 'e_flags', 'e_ehsize', 'e_phentsize', 'e_phnum', 'e_shentsize', 'e_shnum', 'e_shstrndx']
 
 elf_header = {}
 program_header = {}
 
 if is_64bit:
-    elf_header_format = "<IBBBBBxxxxxxxHHIQQQIHHHHHH"
+    elf_header_format = "<IBBBBB7sHHIQQQIHHHHHH"
     program_header_format = "<IIQQQQQQ"
     program_header_names = ['p_type', 'p_flags', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_align']
 else:
-    elf_header_format = "<IBBBBBxxxxxxxHHIIIIIHHHHHH"
+    elf_header_format = "<IBBBBB7sHHIIIIIHHHHHH"
     program_header_format = "<IIIIIIII"
     program_header_names = ['p_type', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_flags', 'p_align']
 
@@ -38,6 +38,7 @@
 
 elf_header['e_entry'] -= program_header_length
 elf_header['e_phnum'] = 1
+elf_header['e_ident[EI_RESERVED]'] = b"METTIGL"
 
 program_header['p_offset'] -= program_header_length
 program_header['p_vaddr'] -= program_header_length

diff --git a/strip.py b/strip.py
@@ -51,9 +51,11 @@
 
 new_elf_header = struct.pack(elf_header_format, *elf_header_values)
 new_program_header = struct.pack(program_header_format, *program_header_values)
+new_binary = new_elf_header + new_program_header + program
 
 print("ELF header size: %d" % len(new_elf_header))
 print("Program header size: %d" % len(new_program_header))
 print("Program size: %d" % len(program))
+print("Total size: %d" % len(new_binary))
 
-open(sys.argv[1], "wb").write(new_elf_header + new_program_header + program)
+open(sys.argv[1], "wb").write(new_binary)
diff --git a/main.c b/main.c
@@ -1,11 +1,16 @@
+int main();
+
 void exit(int x) {
-	while(1) {
-		asm("movl %0, %%ebx" : : "r"(x) :);
-		asm("movl $1, %eax");
-		asm("int $0x80");
-	}
+	asm("movl %0, %%ebx" : : "r"(x) :);
+	asm("movl $1, %eax");
+	asm("int $0x80");
+	while(1);
 }
 
 void _start() {
-	exit(42);
+	exit(main());
+}
+
+int main() {
+	return 42;
 }
diff --git a/Makefile b/Makefile
@@ -1,5 +1,9 @@
-main: main.c Makefile strip.py
-	gcc -o main main.c -nodefaultlibs -nostdlib -m32 -nostartfiles -Os -s -static -Wl,--build-id=none -Wl,--nmagic
+BITS ?= 32
+
+.PHONY: main clean test
+
+main:
+	gcc -o main main.c -nodefaultlibs -nostdlib -m$(BITS) -nostartfiles -Os -s -static -Wl,--build-id=none -Wl,--nmagic
 	strip --remove-section=.comment --remove-section=.eh_frame --strip-all main
 	sstrip -z main
 	python strip.py main

diff --git a/main.c b/main.c
@@ -1,5 +1,11 @@
-#define EXIT(x) { __asm__("movl $1, %eax; movl $" x ", %ebx; int $0x80"); }
+void exit(int x) {
+	while(1) {
+		asm("movl %0, %%ebx" : : "r"(x) :);
+		asm("movl $1, %eax");
+		asm("int $0x80");
+	}
+}
 
 void _start() {
-	EXIT("42");
+	exit(42);
 }
diff --git a/strip.py b/strip.py
@@ -1,25 +1,59 @@
 #!/usr/bin/env python3
 
 import sys
+import struct
 
 binary = open(sys.argv[1], "rb").read()
 
-elf_header = bytearray(binary[0x00:0x00+0x34])
-program_header = bytearray(binary[0x34:0x34+0x20])
-program = bytearray(binary[0x74:])
+is_64bit = binary[0x04] == 0x02
 
-elf_header[0x2c] = 1 # only 1 program header
-elf_header[0x18] = 0x54 # fix entrypoint
-program_header[0x04] = 0x54 # fix program offset in file
-program_header[0x08] = 0x54 # fix program offset in memory
-program_header[0x0C] = 0x54 # fix program offset in "physical" memory
+elf_header_names = ['e_ident[EI_MAG]', 'e_ident[EI_CLASS]', 'e_ident[EI_DATA]', 'e_ident[EI_VERSION]', 'e_ident[EI_OSABI]', 'e_ident[EI_ABIVERSION]', 'e_type', 'e_machine', 'e_version', 'e_entry', 'e_phoff', 'e_shoff', 'e_flags', 'e_ehsize', 'e_phentsize', 'e_phnum', 'e_shentsize', 'e_shnum', 'e_shstrndx']
 
-# remove remaining section headers
-program = program[:-program[::-1].index(0x2e)-1]
-program = program[:-program[::-1].index(0x2e)-1]
+elf_header = {}
+program_header = {}
 
-# strip nullbytes
-while program[-1] == 0x00:
-    program = program[:-1]
+if is_64bit:
+    elf_header_format = "<IBBBBBxxxxxxxHHIQQQIHHHHHH"
+    program_header_format = "<IIQQQQQQ"
+    program_header_names = ['p_type', 'p_flags', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_align']
+else:
+    elf_header_format = "<IBBBBBxxxxxxxHHIIIIIHHHHHH"
+    program_header_format = "<IIIIIIII"
+    program_header_names = ['p_type', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_flags', 'p_align']
 
-open(sys.argv[1], "wb").write(elf_header + program_header + program)
+elf_header_length = struct.calcsize(elf_header_format)
+program_header_length = struct.calcsize(program_header_format)
+
+elf_header_values = list(struct.unpack(elf_header_format, binary[0x00:0x00+elf_header_length]))
+program_header_values = list(struct.unpack(program_header_format, binary[elf_header_length:elf_header_length+program_header_length]))
+
+for i in range(len(elf_header_names)):
+    elf_header[elf_header_names[i]] = elf_header_values[i]
+
+for i in range(len(program_header_names)):
+    program_header[program_header_names[i]] = program_header_values[i]
+
+program_offset = elf_header_length + elf_header['e_phnum'] * program_header_length
+program = binary[program_offset:program_offset+program_header['p_filesz']]
+
+elf_header['e_entry'] -= program_header_length
+elf_header['e_phnum'] = 1
+
+program_header['p_offset'] -= program_header_length
+program_header['p_vaddr'] -= program_header_length
+program_header['p_paddr'] -= program_header_length
+
+for i, e_name in enumerate(elf_header_names):
+    elf_header_values[i] = elf_header[e_name]
+
+for i, p_name in enumerate(program_header_names):
+    program_header_values[i] = program_header[p_name]
+
+new_elf_header = struct.pack(elf_header_format, *elf_header_values)
+new_program_header = struct.pack(program_header_format, *program_header_values)
+
+print("ELF header size: %d" % len(new_elf_header))
+print("Program header size: %d" % len(new_program_header))
+print("Program size: %d" % len(program))
+
+open(sys.argv[1], "wb").write(new_elf_header + new_program_header + program)
diff --git a/.gitignore b/.gitignore
@@ -0,0 +1 @@
+main
diff --git a/Makefile b/Makefile
@@ -0,0 +1,12 @@
+main: main.c Makefile strip.py
+	gcc -o main main.c -nodefaultlibs -nostdlib -m32 -nostartfiles -Os -s -static -Wl,--build-id=none -Wl,--nmagic
+	strip --remove-section=.comment --remove-section=.eh_frame --strip-all main
+	sstrip -z main
+	python strip.py main
+
+clean:
+	rm main
+
+test: main
+	@stat --printf="Size: %s Byte\n" main
+	@./main; if [ $${?} = 42 ]; then echo "Seems good :)"; else echo "Seems broken :-/"; fi
diff --git a/main.c b/main.c
@@ -0,0 +1,5 @@
+#define EXIT(x) { __asm__("movl $1, %eax; movl $" x ", %ebx; int $0x80"); }
+
+void _start() {
+	EXIT("42");
+}
diff --git a/strip.py b/strip.py
@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+
+import sys
+
+binary = open(sys.argv[1], "rb").read()
+
+elf_header = bytearray(binary[0x00:0x00+0x34])
+program_header = bytearray(binary[0x34:0x34+0x20])
+program = bytearray(binary[0x74:])
+
+elf_header[0x2c] = 1 # only 1 program header
+elf_header[0x18] = 0x54 # fix entrypoint
+program_header[0x04] = 0x54 # fix program offset in file
+program_header[0x08] = 0x54 # fix program offset in memory
+program_header[0x0C] = 0x54 # fix program offset in "physical" memory
+
+# remove remaining section headers
+program = program[:-program[::-1].index(0x2e)-1]
+program = program[:-program[::-1].index(0x2e)-1]
+
+# strip nullbytes
+while program[-1] == 0x00:
+    program = program[:-1]
+
+open(sys.argv[1], "wb").write(elf_header + program_header + program)
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