When I'm building the OS, I can't use stdlib and pic. At this point the GCC optimized switch statement will cause a fatal error.It looks like gcc uses a fast jump lookup table stored in rodata when it encounters a switch with a large number of consecutive-valued options, but this table doesn't seem to be available on the kernel , ld didn't set the table correctly(GDB confirms that).
The trying of indirect jump with incorrect address (about 0x8?fffffff???????) will crash the entire system.
because the format transform from ELF to binary cannot make sure the entry point stay at the front of .text, i use a "jmpq $kinit " tiny program in a single section to make the kernel entry easier to found by bootloader
void _entry kinit(uint32_t _mbtp)
{
...
struct mbt_tag_header *p = (void *)(uintptr_t)_mbtp;
...
p++; // skip the head part
while (p->type != 0)
{
switch (p->type)
{
case MBT_BASIC_MEM:
break;
...(more than 3 options)
default:
break;
}
p = (struct mbt_tag_header *)((char *)p + p->size);
p = ((uintptr_t)p % 8) ? (struct mbt_tag_header *)((char *)p + 8 - ((uintptr_t)p % 8)) : p;
}
...
while (1)
;
}
gcc generates the following assembly code：
Disassembly of section .text:
init.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <kinit>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 81 ec a0 00 00 00 sub $0xa0,%rsp
b: 89 bd 6c ff ff ff mov %edi,-0x94(%rbp)
11: 48 c7 85 70 ff ff ff movq $0x7830,-0x90(%rbp)
18: 30 78 00 00
1c: 48 c7 85 78 ff ff ff movq $0x0,-0x88(%rbp)
23: 00 00 00 00
27: 48 c7 45 80 00 00 00 movq $0x0,-0x80(%rbp)
2e: 00
2f: 48 c7 45 88 00 00 00 movq $0x0,-0x78(%rbp)
36: 00
37: 48 c7 45 90 00 00 00 movq $0x0,-0x70(%rbp)
3e: 00
3f: 48 c7 45 98 00 00 00 movq $0x0,-0x68(%rbp)
46: 00
47: 48 c7 45 a0 00 00 00 movq $0x0,-0x60(%rbp)
4e: 00
4f: 48 c7 45 a8 00 00 00 movq $0x0,-0x58(%rbp)
56: 00
57: 48 c7 45 b0 00 00 00 movq $0x0,-0x50(%rbp)
5e: 00
5f: 48 c7 45 b8 00 00 00 movq $0x0,-0x48(%rbp)
66: 00
67: 48 c7 45 c0 00 00 00 movq $0x0,-0x40(%rbp)
6e: 00
6f: 48 c7 45 c8 00 00 00 movq $0x0,-0x38(%rbp)
76: 00
77: 48 c7 45 d0 00 00 00 movq $0x0,-0x30(%rbp)
7e: 00
7f: 48 c7 45 d8 00 00 00 movq $0x0,-0x28(%rbp)
86: 00
87: 48 c7 45 e0 00 00 00 movq $0x0,-0x20(%rbp)
8e: 00
8f: c7 45 e8 00 00 00 00 movl $0x0,-0x18(%rbp)
96: c6 45 ec 00 movb $0x0,-0x14(%rbp)
9a: 8b 85 6c ff ff ff mov -0x94(%rbp),%eax
a0: 48 89 45 f8 mov %rax,-0x8(%rbp)
a4: 48 b8 00 00 00 00 00 movabs $0x0,%rax
ab: 00 00 00
ae: ff d0 callq *%rax
b0: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
b7: 00 00 00
ba: b8 00 00 00 00 mov $0x0,%eax
bf: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
c6: 00 00 00
c9: ff d2 callq *%rdx
cb: 48 83 45 f8 08 addq $0x8,-0x8(%rbp)
d0: e9 a7 01 00 00 jmpq 27c <kinit+0x27c>
d5: 48 8b 45 f8 mov -0x8(%rbp),%rax
d9: 8b 00 mov (%rax),%eax
db: 83 f8 0f cmp $0xf,%eax
de: 0f 87 5e 01 00 00 ja 242 <kinit+0x242>
e4: 89 c0 mov %eax,%eax
e6: 48 8d 14 c5 00 00 00 lea 0x0(,%rax,8),%rdx
ed: 00
ee: 48 b8 00 00 00 00 00 movabs $0x0,%rax
f5: 00 00 00
f8: 48 01 d0 add %rdx,%rax
fb: 48 8b 00 mov (%rax),%rax
fe: ff e0 jmpq *%rax
100: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
107: 00 00 00
10a: b8 00 00 00 00 mov $0x0,%eax
10f: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
116: 00 00 00
119: ff d2 callq *%rdx
11b: 48 8b 45 f8 mov -0x8(%rbp),%rax
11f: 48 83 c0 0c add $0xc,%rax
123: 48 89 c7 mov %rax,%rdi
126: b8 00 00 00 00 mov $0x0,%eax
12b: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
132: 00 00 00
135: ff d2 callq *%rdx
137: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
13e: 00 00 00
141: b8 00 00 00 00 mov $0x0,%eax
146: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
14d: 00 00 00
150: ff d2 callq *%rdx
152: e9 ec 00 00 00 jmpq 243 <kinit+0x243>
157: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
15e: 00 00 00
161: b8 00 00 00 00 mov $0x0,%eax
166: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
16d: 00 00 00
170: ff d2 callq *%rdx
172: 48 8b 45 f8 mov -0x8(%rbp),%rax
176: 48 83 c0 0c add $0xc,%rax
17a: 48 89 c7 mov %rax,%rdi
17d: b8 00 00 00 00 mov $0x0,%eax
182: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
189: 00 00 00
18c: ff d2 callq *%rdx
18e: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
195: 00 00 00
198: b8 00 00 00 00 mov $0x0,%eax
19d: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
1a4: 00 00 00
1a7: ff d2 callq *%rdx
1a9: e9 95 00 00 00 jmpq 243 <kinit+0x243>
1ae: 48 bf 00 00 00 00 00 movabs $0x0,%rdi
1b5: 00 00 00
1b8: b8 00 00 00 00 mov $0x0,%eax
1bd: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
1c4: 00 00 00
1c7: ff d2 callq *%rdx
1c9: 48 8b 45 f8 mov -0x8(%rbp),%rax
1cd: 48 8b 40 08 mov 0x8(%rax),%rax
1d1: 89 c2 mov %eax,%edx
1d3: 48 8d 85 70 ff ff ff lea -0x90(%rbp),%rax
1da: 89 d6 mov %edx,%esi
1dc: 48 89 c7 mov %rax,%rdi
1df: 48 b8 00 00 00 00 00 movabs $0x0,%rax
1e6: 00 00 00
1e9: ff d0 callq *%rax
1eb: 48 89 c7 mov %rax,%rdi
1ee: b8 00 00 00 00 mov $0x0,%eax
1f3: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
1fa: 00 00 00
1fd: ff d2 callq *%rdx
1ff: 48 8b 45 f8 mov -0x8(%rbp),%rax
203: 48 8b 40 08 mov 0x8(%rax),%rax
207: 48 89 c2 mov %rax,%rdx
20a: 48 b8 00 00 00 00 00 movabs $0x0,%rax
211: 00 00 00
214: 48 89 50 08 mov %rdx,0x8(%rax)
218: 48 8b 45 f8 mov -0x8(%rbp),%rax
21c: 8b 40 14 mov 0x14(%rax),%eax
21f: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
226: 00 00 00
229: 89 42 10 mov %eax,0x10(%rdx)
22c: 48 8b 45 f8 mov -0x8(%rbp),%rax
230: 8b 40 18 mov 0x18(%rax),%eax
233: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
23a: 00 00 00
23d: 89 42 14 mov %eax,0x14(%rdx)
240: eb 01 jmp 243 <kinit+0x243>
242: 90 nop
243: 48 8b 45 f8 mov -0x8(%rbp),%rax
247: 8b 40 04 mov 0x4(%rax),%eax
24a: 89 c0 mov %eax,%eax
24c: 48 01 45 f8 add %rax,-0x8(%rbp)
250: 48 8b 45 f8 mov -0x8(%rbp),%rax
254: 83 e0 07 and $0x7,%eax
257: 48 85 c0 test %rax,%rax
25a: 74 18 je 274 <kinit+0x274>
25c: 48 8b 45 f8 mov -0x8(%rbp),%rax
260: 83 e0 07 and $0x7,%eax
263: ba 08 00 00 00 mov $0x8,%edx
268: 48 29 c2 sub %rax,%rdx
26b: 48 8b 45 f8 mov -0x8(%rbp),%rax
26f: 48 01 d0 add %rdx,%rax
272: eb 04 jmp 278 <kinit+0x278>
274: 48 8b 45 f8 mov -0x8(%rbp),%rax
278: 48 89 45 f8 mov %rax,-0x8(%rbp)
27c: 48 8b 45 f8 mov -0x8(%rbp),%rax
280: 8b 00 mov (%rax),%eax
282: 85 c0 test %eax,%eax
284: 0f 85 4b fe ff ff jne d5 <kinit+0xd5>
28a: 48 b8 00 00 00 00 00 movabs $0x0,%rax
291: 00 00 00
294: ff d0 callq *%rax
296: 48 b8 00 00 00 00 00 movabs $0x0,%rax
29d: 00 00 00
2a0: ff d0 callq *%rax
2a2: eb fe jmp 2a2 <kinit+0x2a2>
Linked version: (-g to be easier to read)
fffffff00020003e <kinit>:
#include <device/con/con.h>
#include <nst/printk.h>
#include <nst/types.h>
extern char* num2str32(char*,uint32_t);
void _entry kinit(uint32_t _mbtp)
{
fffffff00020003e: 55 push %rbp
fffffff00020003f: 48 89 e5 mov %rsp,%rbp
fffffff000200042: 48 81 ec a0 00 00 00 sub $0xa0,%rsp
fffffff000200049: 89 bd 6c ff ff ff mov %edi,-0x94(%rbp)
char buf[125] = "0x";
...
struct mbt_tag_header *p = (void *)(uintptr_t)_mbtp;
fffffff0002000d8: 8b 85 6c ff ff ff mov -0x94(%rbp),%eax
fffffff0002000de: 48 89 45 f8 mov %rax,-0x8(%rbp)
...(printk function calls)
p++; // skip the head part
fffffff000200109: 48 83 45 f8 08 addq $0x8,-0x8(%rbp)
while (p->type != 0)
fffffff00020010e: e9 a7 01 00 00 jmpq fffffff0002002ba <kinit+0x27c>
{
switch (p->type)
fffffff000200113: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff000200117: 8b 00 mov (%rax),%eax
fffffff000200119: 83 f8 0f cmp $0xf,%eax
fffffff00020011c: 0f 87 5e 01 00 00 ja fffffff000200280 <kinit+0x242>
fffffff000200122: 89 c0 mov %eax,%eax
fffffff000200124: 48 8d 14 c5 00 00 00 lea 0x0(,%rax,8),%rdx
fffffff00020012b: 00
fffffff00020012c: 48 b8 68 2e 20 00 f0 movabs $0xfffffff000202e68,%rax
fffffff000200133: ff ff ff
fffffff000200136: 48 01 d0 add %rdx,%rax
fffffff000200139: 48 8b 00 mov (%rax),%rax
fffffff00020013c: ff e0 jmpq *%rax
{
case MBT_BASIC_MEM:
break;
case MBT_CMD:
...(as well)
default:
break;
fffffff000200280: 90 nop
}
p = (struct mbt_tag_header *)((char *)p + p->size);
fffffff000200281: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff000200285: 8b 40 04 mov 0x4(%rax),%eax
fffffff000200288: 89 c0 mov %eax,%eax
fffffff00020028a: 48 01 45 f8 add %rax,-0x8(%rbp)
p = ((uintptr_t)p % 8) ? (struct mbt_tag_header *)((char *)p + 8 - ((uintptr_t)p % 8)) : p;
fffffff00020028e: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff000200292: 83 e0 07 and $0x7,%eax
fffffff000200295: 48 85 c0 test %rax,%rax
fffffff000200298: 74 18 je fffffff0002002b2 <kinit+0x274>
fffffff00020029a: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff00020029e: 83 e0 07 and $0x7,%eax
fffffff0002002a1: ba 08 00 00 00 mov $0x8,%edx
fffffff0002002a6: 48 29 c2 sub %rax,%rdx
fffffff0002002a9: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff0002002ad: 48 01 d0 add %rdx,%rax
fffffff0002002b0: eb 04 jmp fffffff0002002b6 <kinit+0x278>
fffffff0002002b2: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff0002002b6: 48 89 45 f8 mov %rax,-0x8(%rbp)
while (p->type != 0)
fffffff0002002ba: 48 8b 45 f8 mov -0x8(%rbp),%rax
fffffff0002002be: 8b 00 mov (%rax),%eax
fffffff0002002c0: 85 c0 test %eax,%eax
fffffff0002002c2: 0f 85 4b fe ff ff jne fffffff000200113 <kinit+0xd5>
}
...
while (1)
fffffff0002002e0: eb fe jmp fffffff0002002e0 <kinit+0x2a2>
GCC -S output
.file "init.c"
.text
.section .rodata
.LC0:
.string "nST kernel\r\n"
.LC1:
.string "CMD line:"
.LC2:
.string "\r\n"
.LC3:
.string "loaded by:"
.LC4:
.string "frame buffer @"
.text
.globl kinit
.type kinit, @function
kinit:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
pushq %r15
pushq %rbx
subq $160, %rsp
.cfi_offset 15, -24
.cfi_offset 3, -32
.L16:
leaq .L16(%rip), %rbx
movabsq $_GLOBAL_OFFSET_TABLE_-.L16, %r11
addq %r11, %rbx
movl %edi, -164(%rbp)
movq $30768, -160(%rbp)
...
movl -164(%rbp), %eax
movq %rax, -24(%rbp)
movq %rbx, %r15
movabsq $printk_init@PLTOFF, %rax
addq %rbx, %rax
call *%rax
movabsq $.LC0@GOTOFF, %rax
leaq (%rbx,%rax), %rdi
movq %rbx, %r15
movl $0, %eax
movabsq $printk@PLTOFF, %rdx
addq %rbx, %rdx
call *%rdx
addq $8, -24(%rbp)
jmp .L2
.L13:
movq -24(%rbp), %rax
movl (%rax), %eax
cmpl $15, %eax
ja .L15
movl %eax, %eax
leaq 0(,%rax,8), %rdx
leaq .L5(%rip), %rax
movq (%rdx,%rax), %rax
leaq .L5(%rip), %rdx
addq %rdx, %rax
jmp *%rax
.section .rodata
.align 8
.align 4
.L5:
.quad .L15-.L5
.quad .L9-.L5
.quad .L8-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L6-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.quad .L15-.L5
.text
.L9:
movabsq $.LC1@GOTOFF, %rax
leaq (%rbx,%rax), %rdi
movq %rbx, %r15
...
jmp .L10
.L8:
movabsq $.LC3@GOTOFF, %rax
leaq (%rbx,%rax), %rdi
movq %rbx, %r15
movl $0, %eax
movabsq $printk@PLTOFF, %rdx
addq %rbx, %rdx
call *%rdx
movq -24(%rbp), %rax
addq $12, %rax
movq %rax, %rdi
movq %rbx, %r15
movl $0, %eax
movabsq $printk@PLTOFF, %rdx
addq %rbx, %rdx
call *%rdx
movabsq $.LC2@GOTOFF, %rax
leaq (%rbx,%rax), %rdi
movq %rbx, %r15
movl $0, %eax
movabsq $printk@PLTOFF, %rdx
addq %rbx, %rdx
call *%rdx
jmp .L10
.L6:
movabsq $.LC4@GOTOFF, %rax
leaq (%rbx,%rax), %rdi
movq %rbx, %r15
movl $0, %eax
...
jmp .L10
.L15:
nop
.L10:
movq -24(%rbp), %rax
movl 4(%rax), %eax
movl %eax, %eax
addq %rax, -24(%rbp)
movq -24(%rbp), %rax
andl $7, %eax
testq %rax, %rax
je .L11
movq -24(%rbp), %rax
andl $7, %eax
movl $8, %edx
subq %rax, %rdx
movq -24(%rbp), %rax
addq %rdx, %rax
jmp .L12
.L11:
movq -24(%rbp), %rax
.L12:
movq %rax, -24(%rbp)
.L2:
movq -24(%rbp), %rax
movl (%rax), %eax
testl %eax, %eax
jne .L13
movq %rbx, %r15
movabsq $display_init@PLTOFF, %rax
addq %rbx, %rax
call *%rax
movq %rbx, %r15
movabsq $console_init@PLTOFF, %rax
addq %rbx, %rax
call *%rax
.L14:
jmp .L14
.cfi_endproc
.LFE3:
.size kinit, .-kinit
.ident "GCC: (Debian 10.2.1-6) 10.2.1 20210110"
.section .note.GNU-stack,"",@progbits
That's the .rodata:
Contents of section .rodata:
fffffff000202e20 00460c00 00000000 450c0000 00000000 .F......E.......
fffffff000202e30 6e535420 6b65726e 656c0d0a 00434d44 nST kernel...CMD
fffffff000202e40 206c696e 653a000d 0a006c6f 61646564 line:....loaded
fffffff000202e50 2062793a 00667261 6d652062 75666665 by:.frame buffe
fffffff000202e60 72204000 00000000 80022000 f0ffffff r @....... .....
fffffff000202e70 3e012000 f0ffffff 95012000 f0ffffff >. ....... .....
fffffff000202e80 80022000 f0ffffff 80022000 f0ffffff .. ....... .....
fffffff000202e90 80022000 f0ffffff 80022000 f0ffffff .. ....... .....
fffffff000202ea0 80022000 f0ffffff ec012000 f0ffffff .. ....... .....
fffffff000202eb0 80022000 f0ffffff 80022000 f0ffffff .. ....... .....
fffffff000202ec0 80022000 f0ffffff 80022000 f0ffffff .. ....... .....
fffffff000202ed0 80022000 f0ffffff 80022000 f0ffffff .. ....... .....
fffffff000202ee0 80022000 f0ffffff c70c2000 f0ffffff .. ....... .....
fffffff000202ef0 da0c2000 f0ffffff ed0c2000 f0ffffff .. ....... .....
fffffff000202f00 000d2000 f0ffffff 130d2000 f0ffffff .. ....... .....
fffffff000202f10 260d2000 f0ffffff 390d2000 f0ffffff &. .....9. .....
fffffff000202f20 4c0d2000 f0ffffff 5f0d2000 f0ffffff L. ....._. .....
fffffff000202f30 6f0d2000 f0ffffff 7f0d2000 f0ffffff o. ....... .....
fffffff000202f40 8f0d2000 f0ffffff 9f0d2000 f0ffffff .. ....... .....
fffffff000202f50 af0d2000 f0ffffff bf0d2000 f0ffffff .. ....... .....
fffffff000202f60 cf0d2000 f0ffffff
the kernel linker script:
ENTRY(kinit)
SECTIONS
{
. = 0xfffffff000000000;
. += 2M;
.init :
{
*(.init)
}
.text :
{
__kernel_start = .;
*(.text)
}
.data :
{
*(.data)
}
.rodata :
{
*(.rodata)
}
.bss :
{
*(.bss)
}
}
CMDLINE:-c -Wall -mcmodel=large -nostdinc -nostdlib -I ~/nst/include -g -fno-pic
That's because:
in mcmodel=large compliers assumes all sizes and distances might need a full 64 bits. Using GOT table (when -fno-pic, it will create a local offset table in .rodata). Which will not work with broken GOT table.
Solution:
Add following codes into your ld linker script
.got :
{
*(.got)
*(.igot)
}
.got.plt :
{
*(.got.plt)
*(.igot.plt)
}
https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.