Assembly function address table and data under the function or in data section-CodePudding

I have a question about putting data (address table or other data) in the .text section under its function or put in .data section? For example, I have a function like this :

extern int i0();
extern int i1();
extern int i2();
extern int i3();
extern int i4();
extern int i5();

void fff(int x) {
 switch (x) {
     case 0:
     i0();
     break;
     case 1:
     i1();
     break;
     case 2:
     i2();
     break;
     case 3:
     i3();
     break;
     case 4:
     i4();
     break;
     case 5:
     i5();
     break;
 }
}

here in assembly, this is my code:

fff:
        cmp     edi, 5
        ja      .L10
        mov     edi, edi
        xor     eax, eax
        jmp     [QWORD PTR .L4[0 rdi*8]]
.L4:
        .quad   .L9
        .quad   .L8
        .quad   .L7
        .quad   .L6
        .quad   .L5
        .quad   .L3
.L5:
        jmp     i4
.L3:
        jmp     i5
.L9:
        jmp     i0
.L8:
        jmp     i1
.L7:
        jmp     i2
.L6:
        jmp     i3
.L10:
        ret

Here I have .L4 which holds the jump addresses ... where should I put this .L4 table ? Under the fff function or I have to put it in the .data section ? What about static data ? For example, I have 2 QWORD for a function, I must put it in that function, or I must put those QWORDs in the data section ? Why ? I know that there will be no problem if I put it in .data section or under its function, but I want to know which way is better?

CodePudding user response：

The .data section is usually writable, and you would not want your jump table to be accidentally or maliciously overwritten. So .data isn't the best place for it.

.text would be fine; it is normally read-only. It doesn't really matter whether it's near the function or not. Many systems have a .rodata section which is read-only and not executable, which would be even better; it would help catch bugs or attacks which accidentally or deliberately try to execute the bytes of the jump table.

CodePudding user response：

Yes, you can put the table of pointers (.L4:) in .text section (if it won't be modified at run time) but I don't see a reason for double indirection to a set of jumps to external functions i0..i5. You can jump to those functions with immediate near jump E9xxxxxxxx which is 5 byte long; the linker will take care of xxxxxxxx completion. Just multiply the index x in rdi by five and jump to one of the five immediate jumps:

|                              |     global fff
|                              |     extern i0,i1,i2,i3,i4,i5
|00000000:4883FF05             |fff: cmp rdi, 5
|00000004:7725                 |     ja  .L10
|00000006:FFA4BF[0D000000]     |     jmp [.L4 rdi 4*rdi]
|0000000D:E9(00000000)         |.L4: jmp i0
|00000012:E9(00000000)         |     jmp i1
|00000017:E9(00000000)         |     jmp i2
|0000001C:E9(00000000)         |     jmp i3
|00000021:E9(00000000)         |     jmp i4
|00000026:E9(00000000)         |     jmp i5
|0000002B:C3                   |.L10:ret
|0000002C:                     |