How does longjmp work?

Question

I need to understand HOW longjmp function works; I know what it does, but I need to know how it does it.

I tried to disas the code in gdb but I can't understand some steps. The code is:

0xb7ead420 <siglongjmp+0>:      push   %ebp
0xb7ead421 <siglongjmp+1>:      mov    %esp,%ebp
0xb7ead423 <siglongjmp+3>:      sub    $0x18,%esp
0xb7ead426 <siglongjmp+6>:      mov    %ebx,-0xc(%ebp)
0xb7ead429 <siglongjmp+9>:      call   0xb7e9828f <_Unwind_Find_FDE@plt+119>
0xb7ead42e <siglongjmp+14>:     add    $0x12bbc6,%ebx
0xb7ead434 <siglongjmp+20>:     mov    %esi,-0x8(%ebp)
0xb7ead437 <siglongjmp+23>:     mov    0xc(%ebp),%esi
0xb7ead43a <siglongjmp+26>:     mov    %edi,-0x4(%ebp)
0xb7ead43d <siglongjmp+29>:     mov    0x8(%ebp),%edi
0xb7ead440 <siglongjmp+32>:     mov    %esi,0x4(%esp)
0xb7ead444 <siglongjmp+36>:     mov    %edi,(%esp)
0xb7ead447 <siglongjmp+39>:     call   0xb7ead4d0
0xb7ead44c <siglongjmp+44>:     mov    0x18(%edi),%eax
0xb7ead44f <siglongjmp+47>:     test   %eax,%eax
0xb7ead451 <siglongjmp+49>:     jne    0xb7ead470 <siglongjmp+80>
0xb7ead453 <siglongjmp+51>:     test   %esi,%esi
0xb7ead455 <siglongjmp+53>:     mov    $0x1,%eax
0xb7ead45a <siglongjmp+58>:     cmove  %eax,%esi
0xb7ead45d <siglongjmp+61>:     mov    %esi,0x4(%esp)
0xb7ead461 <siglongjmp+65>:     mov    %edi,(%esp)
0xb7ead464 <siglongjmp+68>:     call   0xb7ead490
0xb7ead469 <siglongjmp+73>:     lea    0x0(%esi,%eiz,1),%esi
0xb7ead470 <siglongjmp+80>:     lea    0x1c(%edi),%eax
0xb7ead473 <siglongjmp+83>:     movl   $0x0,0x8(%esp)
0xb7ead47b <siglongjmp+91>:     mov    %eax,0x4(%esp)
0xb7ead47f <siglongjmp+95>:     movl   $0x2,(%esp)
0xb7ead486 <siglongjmp+102>:    call   0xb7ead890 <sigprocmask>
0xb7ead48b <siglongjmp+107>:    jmp    0xb7ead453 <siglongjmp+51>

Can someone briefly explain me the code, or indicate where I can find the source code in the system?

Answer 1

Here is the i386 code for longjmp, in the standard i386 ABI, without any crazy extensions for interaction with C++, exceptions, cleanup functions, signal mask, etc.:

    mov 4(%esp),%edx
    mov 8(%esp),%eax
    test %eax,%eax
    jnz 1f
    inc %eax
1:
    mov (%edx),%ebx
    mov 4(%edx),%esi
    mov 8(%edx),%edi
    mov 12(%edx),%ebp
    mov 16(%edx),%ecx
    mov %ecx,%esp
    mov 20(%edx),%ecx
    jmp *%ecx

Answer 2

Mostly, it restores the registers and stack as they were at the time of the corresponding setjmp(). There is some additional cleanup required (fixing signal handling and unwinding pending stack handlers), as well as returning a different value as the apparent return value of setjmp, but restoring the state is the essence of the operation.

For it to work, the stack cannot be below the point at which setjmp was called. Longjmp is a brutish way to just forget everything that has been called below it down to the same level in the call stack (or function call nesting sequence) mostly by simply setting the stack pointer to the same frame it was when setjmp was called.

For it to work cleanly, longjmp() calls all the exit handlers for intermediate functions, so they can delete variables, and whatever other cleanup is normally done when a function returns. Resetting the stack to a point less deep releases all the auto variables but if one of those is a FILE *, the file needs to be closed and the i/o buffer freed too.

Answer 3

I think you need to see Procedure Activation Records and Call Stacks and Setjmp.h 's jmp_buf's structure.

Quoted from Expert C Programming: Deep C Secrets:

Setjmp saves a copy of the program counter and the current pointer to the top of the stack. This saves some initial values, if you like. Then longjmp restores these values effectively transferring control and resetting the state back to where you were when you did the save. It's termed "unwinding the stack", because you unroll activation records from the stack until you get to the saved one.

Have a look at page 153 also here.

The stackframe will be highly dependent on the machine and the executable, but the idea is the same.

Answer 4

In Windows X64 MASM

.code

my_jmp_buf STRUCT

        _Frame QWORD ?;
        _Rbx QWORD ?;
        _Rsp QWORD ?;
        _Rbp QWORD ?;
        _Rsi QWORD ?;
        _Rdi QWORD ?;
        _R12 QWORD ?;
        _R13 QWORD ?;
        _R14 QWORD ?;
        _R15 QWORD ?;
        _Rip QWORD ?;
        _MxCsr DWORD ?;
        _FpCsr WORD ?;
        _Spare WORD ?;
        _Xmm6 XMMWORD ?;
        _Xmm7 XMMWORD ?;
        _Xmm8 XMMWORD ?;
        _Xmm9 XMMWORD ?;
        _Xmm10 XMMWORD ?;
        _Xmm11 XMMWORD ?;
        _Xmm12 XMMWORD ?;
        _Xmm13 XMMWORD ?;
        _Xmm14 XMMWORD ?;
        _Xmm15 XMMWORD ?;

my_jmp_buf ENDS


;extern "C" int my_setjmp(jmp_buf env);
public my_setjmp

my_setjmp PROC

    mov rax, [rsp] ;save ip 
    mov (my_jmp_buf ptr[rcx])._Rip, rax

    lea rax, [rsp + 8] ;save sp before call this function
    mov (my_jmp_buf ptr[rcx])._Rsp, rax
    mov (my_jmp_buf ptr[rcx])._Frame, rax

    ;save gprs
    mov (my_jmp_buf ptr[rcx])._Rbx,rbx  
    mov (my_jmp_buf ptr[rcx])._Rbp,rbp  
    mov (my_jmp_buf ptr[rcx])._Rsi,rsi  
    mov (my_jmp_buf ptr[rcx])._Rdi,rdi  
    mov (my_jmp_buf ptr[rcx])._R12,r12  
    mov (my_jmp_buf ptr[rcx])._R13,r13  
    mov (my_jmp_buf ptr[rcx])._R14,r14  
    mov (my_jmp_buf ptr[rcx])._R15,r15  

    ;save fp and xmm
    stmxcsr     (my_jmp_buf ptr[rcx])._MxCsr
    fnstcw      (my_jmp_buf ptr[rcx])._FpCsr
    movdqa      (my_jmp_buf ptr[rcx])._Xmm6,xmm6  
    movdqa      (my_jmp_buf ptr[rcx])._Xmm7,xmm7  
    movdqa      (my_jmp_buf ptr[rcx])._Xmm8,xmm8  
    movdqa      (my_jmp_buf ptr[rcx])._Xmm9,xmm9  
    movdqa      (my_jmp_buf ptr[rcx])._Xmm10,xmm10
    movdqa      (my_jmp_buf ptr[rcx])._Xmm11,xmm11
    movdqa      (my_jmp_buf ptr[rcx])._Xmm12,xmm12
    movdqa      (my_jmp_buf ptr[rcx])._Xmm13,xmm13
    movdqa      (my_jmp_buf ptr[rcx])._Xmm14,xmm14
    movdqa      (my_jmp_buf ptr[rcx])._Xmm15,xmm15

    xor         rax,rax  
    ret  

my_setjmp ENDP


;extern "C" void my_longjmp(jmp_buf env,  int value);

public my_longjmp

my_longjmp PROC

    ;restore fp and xmm
    movdqa      xmm15,(my_jmp_buf ptr[rcx])._Xmm15
    movdqa      xmm14,(my_jmp_buf ptr[rcx])._Xmm14
    movdqa      xmm13,(my_jmp_buf ptr[rcx])._Xmm13
    movdqa      xmm12,(my_jmp_buf ptr[rcx])._Xmm12
    movdqa      xmm11,(my_jmp_buf ptr[rcx])._Xmm11
    movdqa      xmm10,(my_jmp_buf ptr[rcx])._Xmm10
    movdqa      xmm9,(my_jmp_buf ptr[rcx])._Xmm9
    movdqa      xmm8,(my_jmp_buf ptr[rcx])._Xmm8
    movdqa      xmm7,(my_jmp_buf ptr[rcx])._Xmm7
    movdqa      xmm6,(my_jmp_buf ptr[rcx])._Xmm6

    fldcw      (my_jmp_buf ptr[rcx])._FpCsr
    ldmxcsr    (my_jmp_buf ptr[rcx])._MxCsr


    ;restore gprs
    mov r15, (my_jmp_buf ptr[rcx])._R15 
    mov r14, (my_jmp_buf ptr[rcx])._R14
    mov r13, (my_jmp_buf ptr[rcx])._R13  
    mov r12, (my_jmp_buf ptr[rcx])._R12  
    mov rdi, (my_jmp_buf ptr[rcx])._Rdi  
    mov rsi, (my_jmp_buf ptr[rcx])._Rsi  
    mov rbp, (my_jmp_buf ptr[rcx])._Rbp  
    mov rbx, (my_jmp_buf ptr[rcx])._Rbx



    ;retore sp
    mov rsp, (my_jmp_buf ptr[rcx])._Rsp

    ;restore ip
    mov rcx, (my_jmp_buf ptr[rcx])._Rip; must be the last instruction as rcx modified

    ;return value
    mov rax, rdx 

   jmp rcx
my_longjmp ENDP

END