Crt0 Process Analysis in newlib, newlibcrt0

Crt0 Process Analysis in newlib, newlibcrt0

Recently, I became interested in the newlib startup code crt0, So I analyzed its code. Crt0 source code is located in libgloss/arm/crt0.S, in order to be compatible with various ARM architecture, crt0.S has a large number of conditions to judge macro definition, for only concerned about ARMv7e-M I am very painful. In my hand, there is a mbed project based on STM32F412 using the crt0 startup method, refer to the crt0.o disassembly I can extract part of the code crt0.S and ARMv7e-M.

The disassembly of crt0.o is as follows:

08008220 <_mainCRTStartup>: 8008220:    4b15          ldr    r3, [pc, #84]    ; (8008278 <_mainCRTStartup+0x58>) 8008222:    2b00          cmp    r3, #0 8008224:    bf08          it    eq 8008226:    4b13          ldreq    r3, [pc, #76]    ; (8008274 <_mainCRTStartup+0x54>) 8008228:    469d          mov    sp, r3 800822a:    f5a3 3a80     sub.w    sl, r3, #65536    ; 0x10000 800822e:    2100          movs    r1, #0 8008230:    468b          mov    fp, r1 8008232:    460f          mov    r7, r1 8008234:    4813          ldr    r0, [pc, #76]    ; (8008284 <_mainCRTStartup+0x64>) 8008236:    4a14          ldr    r2, [pc, #80]    ; (8008288 <_mainCRTStartup+0x68>) 8008238:    1a12          subs    r2, r2, r0 800823a:    f01c fcd7     bl    8024bec <memset> 800823e:    4b0f          ldr    r3, [pc, #60]    ; (800827c <_mainCRTStartup+0x5c>) 8008240:    2b00          cmp    r3, #0 8008242:    d000          beq.n    8008246 <_mainCRTStartup+0x26> 8008244:    4798          blx    r3 8008246:    4b0e          ldr    r3, [pc, #56]    ; (8008280 <_mainCRTStartup+0x60>) 8008248:    2b00          cmp    r3, #0 800824a:    d000          beq.n    800824e <_mainCRTStartup+0x2e> 800824c:    4798          blx    r3 800824e:    2000          movs    r0, #0 8008250:    2100          movs    r1, #0 8008252:    0004          movs    r4, r0 8008254:    000d          movs    r5, r1 8008256:    480d          ldr    r0, [pc, #52]    ; (800828c <_mainCRTStartup+0x6c>) 8008258:    2800          cmp    r0, #0 800825a:    d002          beq.n    8008262 <_mainCRTStartup+0x42> 800825c:    480c          ldr    r0, [pc, #48]    ; (8008290 <_mainCRTStartup+0x70>) 800825e:    f00f f868     bl    8017332 <__wrap_atexit> 8008262:    f01c f805     bl    8024270 <__libc_init_array> 8008266:    0020          movs    r0, r4 8008268:    0029          movs    r1, r5 800826a:    f00f f821     bl    80172b0 <__wrap_main> 800826e:    f00f f85d     bl    801732c <__wrap_exit> 8008272:    bf00          nop 8008274:    00080000     .word    0x00080000 8008278:    20040000     .word    0x20040000 800827c:    00000000     .word    0x00000000 8008280:    080172a3     .word    0x080172a3 8008284:    20000c00     .word    0x20000c00 8008288:    2000ac58     .word    0x2000ac58 800828c:    08017333     .word    0x08017333 8008290:    00000000     .word    0x00000000

The extracted crt0.S code is as follows:

    FUNC_START  _mainCRTStartup    FUNC_START  _start/* Start by setting up a stack */    /*  Set up the stack pointer to a fixed value */    /*  Changes by toralf:        - Allow linker script to provide stack via __stack symbol - see          defintion of .Lstack        - Provide "hooks" that may be used by the application to add          custom init code - see .Lhwinit and .Lswinit          - Go through all execution modes and set up stack for each of them.          Loosely based on init.s from ARM/Motorola example code.              Note: Mode switch via CPSR is not allowed once in non-privileged            mode, so we take care not to enter "User" to set up its sp,            and also skip most operations if already in that mode. */    ldr r3, .Lstack    cmp r3, #0    it  eq    ldreq   r3, .LC0    /* Note: This 'mov' is essential when starting in User, and ensures we         always get *some* sp value for the initial mode, even if we          have somehow missed it below (in which case it gets the same         value as FIQ - not ideal, but better than nothing.) */    mov sp, r3.LC23:    /* Setup a default stack-limit in-case the code has been       compiled with "-mapcs-stack-check".  Hard-wiring this value       is not ideal, since there is currently no support for       checking that the heap and stack have not collided, or that       this default 64k is enough for the program being executed.       However, it ensures that this simple crt0 world will not       immediately cause an overflow event:  */    sub sl, r3, #64 << 10   /* Still assumes 256bytes below sl */    /* Zero the memory in the .bss section.  */    movs    a2, #0          /* Second arg: fill value */    mov fp, a2          /* Null frame pointer */    mov r7, a2          /* Null frame pointer for Thumb */        ldr a1, .LC1        /* First arg: start of memory block */    ldr a3, .LC2        subs    a3, a3, a1      /* Third arg: length of block */        bl  memset/* Changes by toralf: Taken from libgloss/m68k/crt0.S * initialize target specific stuff. Only execute these * functions it they exist. */    ldr r3, .Lhwinit    cmp r3, #0    beq .LC24    indirect_call r3.LC24:      ldr r3, .Lswinit    cmp r3, #0    beq .LC25    indirect_call r3.LC25:      movs    r0, #0      /*  no arguments  */    movs    r1, #0      /*  no argv either */    /* Some arm/elf targets use the .init and .fini sections       to create constructors and destructors, and for these       targets we need to call the _init function and arrange       for _fini to be called at program exit.  */    movs    r4, r0    movs    r5, r1e    /* Make reference to atexit weak to avoid unconditionally pulling in       support code.  Refer to comments in __atexit.c for more details.  */    ldr r0, .Latexit    cmp r0, #0    beq .Lweak_atexit    ldr r0, .Lfini    bl  atexit.Lweak_atexit:    bl  _init    movs    r0, r4    movs    r1, r5    bl  main    bl  exit        /* Should not return.  */        /* For Thumb, constants must be after the code since only        positive offsets are supported for PC relative addresses.  */.LC0:    .word   0x80000         /* Top of RAM on the PIE board.  */.Lstack:        .word   __stack.Lhwinit:       .word   ardware_init_hook.Lswinit:    .word   software_init_hook    /* Set up defaults for the above variables in the form of weak symbols       - so that application will link correctly, and get value 0 in       runtime (meaning "ignore setting") for the variables, when the user       does not provide the symbols. (The linker uses a weak symbol if,       and only if, a normal version of the same symbol isn't provided       e.g. by a linker script or another object file.) */      .weak __stack    .weak hardware_init_hook    .weak software_init_hook.LC1:  .word __bss_start__.LC2:  .word __bss_end__  .weak atexit.Latexit:  .word atexit  /* Weak reference _fini in case of lite exit.  */  .weak _fini.Lfini:  .word _fini

The crt0 startup process is as follows:

_ Stack ,__ bss_start _ and _ bss_end _ must be defined.

The hardware_init_hook and software_init_hook functions must be implemented before the main function.

Aiexit, exit, _ init and _ fini are generally related to the global structure and structure of C ++. This is analyzed in the next section.