Update no libc programming

1 files changed, 134 insertions, 13 deletions

diff --git a/md/notes/undefined_c/titles.md b/md/notes/undefined_c/titles.md
index 60e14a4..6053291 100644
--- a/md/notes/undefined_c/titles.md
+++ b/md/notes/undefined_c/titles.md
@@ -859,6 +859,7 @@ Return type, and function arguments arent exposed when object file is created.
 Functions can have aliases.
 
 __link_fun2.c__
+
 ```c
 static void fun2()  {
 	printf("hello 2\n");
@@ -925,29 +926,148 @@ hello 2
 So all of this plays role in linking object files.
 There is more interesting utilit called ld its doing things on lower level then gcc.
 
+
+### Extern
+
+### Attributes
+PASS
+### Creating shared library
+PASS
+### Create static libraries
+PASS
+### Join all objects together 
+PASS 
+### Compile with musl
+
+The libc is not the only option as standard c library, there is few others one of them is musl
+
 ```
-$ ldd ./link_elf
-	linux-vdso.so.1 (0x00007ffd8f5d0000)
-	libc.so.6 => /usr/lib/libc.so.6 (0x00007f96c0e00000)
-	/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f96c10d2000)
+$ musl-gcc hello_world.c -o hello_world
+$ file ./hello_world
+hello_world_musl: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-x86_64.so.1, not stripped
 ```
 
-We havent specified those options when we compiled executable, lets use **ld** for that.
+
+### Inspect elf files
+
+There is few utilities that help to check if elf file is ok.
+
+ldd show what kind of shared libraries elf will try to load
 
 ```
-ld -dynamic-linker /lib/ld-linux.so.2 link_elf.o link_fun1.o link_fun2.o -o link_elf
+$ ldd hello_world
+	linux-vdso.so.1 (0x00007fffcb2ae000)
+	libc.so.6 => /usr/lib/libc.so.6 (0x00007ffb80c00000)
+	/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007ffb80fb9000)
+
 ```
 
+Readelf allows to inspect content of elf files, headers and interpret values in headers.
+In few example above we allready used that feature to check content of compiled objectfiles.
 
+```
+$ readelf -s ./hello_world
+Symbol table '.symtab' contains 37 entries:
+   Num:    Value          Size Type    Bind   Vis      Ndx Name
+     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
+     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.c
+     2: 000000000000039c    32 OBJECT  LOCAL  DEFAULT    4 __abi_tag
+     3: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS init.c
+     4: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
+     5: 0000000000001070     0 FUNC    LOCAL  DEFAULT   14 deregister_tm_clones
+     6: 00000000000010a0     0 FUNC    LOCAL  DEFAULT   14 register_tm_clones
+     7: 00000000000010e0     0 FUNC    LOCAL  DEFAULT   14 __do_global_dtors_aux
+     8: 0000000000004030     1 OBJECT  LOCAL  DEFAULT   25 completed.0
+     9: 0000000000003df0     0 OBJECT  LOCAL  DEFAULT   20 __do_global_dtor[...]
+    10: 0000000000001130     0 FUNC    LOCAL  DEFAULT   14 frame_dummy
+    11: 0000000000003de8     0 OBJECT  LOCAL  DEFAULT   19 __frame_dummy_in[...]
+    12: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS hello_world.c
+    13: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
+    14: 00000000000020b0     0 OBJECT  LOCAL  DEFAULT   18 __FRAME_END__
+    15: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 
+    16: 0000000000003df8     0 OBJECT  LOCAL  DEFAULT   21 _DYNAMIC
+    17: 0000000000002010     0 NOTYPE  LOCAL  DEFAULT   17 __GNU_EH_FRAME_HDR
+    18: 0000000000004000     0 OBJECT  LOCAL  DEFAULT   23 _GLOBAL_OFFSET_TABLE_
+    19: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_mai[...]
+    20: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterT[...]
+    21: 0000000000004020     0 NOTYPE  WEAK   DEFAULT   24 data_start
+    22: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND puts@GLIBC_2.2.5
+    23: 0000000000004030     0 NOTYPE  GLOBAL DEFAULT   24 _edata
+    24: 0000000000001154     0 FUNC    GLOBAL HIDDEN    15 _fini
+    25: 0000000000004020     0 NOTYPE  GLOBAL DEFAULT   24 __data_start
+    26: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
+    27: 0000000000004028     0 OBJECT  GLOBAL HIDDEN    24 __dso_handle
+    28: 0000000000002000     4 OBJECT  GLOBAL DEFAULT   16 _IO_stdin_used
+    29: 0000000000004038     0 NOTYPE  GLOBAL DEFAULT   25 _end
+    30: 0000000000001040    38 FUNC    GLOBAL DEFAULT   14 _start
+    31: 0000000000004030     0 NOTYPE  GLOBAL DEFAULT   25 __bss_start
+    32: 0000000000001139    26 FUNC    GLOBAL DEFAULT   14 main
+    33: 0000000000004030     0 OBJECT  GLOBAL HIDDEN    24 __TMC_END__
+    34: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMC[...]
+    35: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@G[...]
+    36: 0000000000001000     0 FUNC    GLOBAL HIDDEN    12 _init
+```
 
-### Extern
-### Attributes
-### Creating shared library
-### Create static libraries
-### Join all objects together
-### Compile with musl
-### Inspect elf files
 ### No standard library
+
+Lets write hello world without libc.
+
+__noc.c__
+```c
+void _start() {
+
+}
+```
+
+```
+$ gcc -c noc.c
+$ ld -dynamic-linker /lib/ld-linux.so.2 noc.o -o noc
+$ file noc
+noc: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped
+```
+
+Next step to make it more working then segfaulting.
+
+```c
+void _start() {
+ asm ( \
+      "movl $1,%eax\n" \
+      "xor %ebx,%ebx\n" \
+      "int $128\n" \
+    );
+}
+```
+
+Now this is all about calling the syscalls
+
+Lets print the message
+```c
+signed int write(int fd, const void *buf, unsigned int size)
+{
+    signed int ret;
+    asm volatile
+    (
+        "syscall"
+        : "=a" (ret)
+        //                 EDI      RSI       RDX
+        : "0"(1), "D"(fd), "S"(buf), "d"(size)
+        : "rcx", "r11", "memory"
+    );
+    return ret;
+}
+
+void _start() {
+	write(1,"no libc",8);
+ asm ( \
+      "movl $1,%eax\n" \
+      "xor %ebx,%ebx\n" \
+      "int $128\n" \
+    );
+}
+```
+
+http://main.lv/writeup/making_c_executables_smaller.md
+
 ### Memory leaks
 ### Code coverage
 ### Profiling
@@ -955,6 +1075,7 @@ ld -dynamic-linker /lib/ld-linux.so.2 link_elf.o link_fun1.o link_fun2.o -o link
 ### Atomic
 ### Multithreading
 ### Write plugins
+### Preload library
 
 
 ## Embedding C