ARM ELF 对象内的函数大小不正确 [英] Incorrect function size inside ARM ELF object
问题描述
目标文件的readelf输出:
readelf output of the object file:
Symbol table '.symtab' contains 15 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 FILE LOCAL DEFAULT ABS fp16.c
2: 00000000 0 SECTION LOCAL DEFAULT 1
3: 00000000 0 SECTION LOCAL DEFAULT 3
4: 00000000 0 SECTION LOCAL DEFAULT 4
5: 00000000 0 NOTYPE LOCAL DEFAULT 1 $t
6: 00000001 194 FUNC LOCAL DEFAULT 1 __gnu_f2h_internal
7: 00000010 0 NOTYPE LOCAL DEFAULT 5 $d
8: 00000000 0 SECTION LOCAL DEFAULT 5
9: 00000000 0 SECTION LOCAL DEFAULT 7
10: 000000c5 78 FUNC GLOBAL HIDDEN 1 __gnu_h2f_internal
11: 00000115 4 FUNC GLOBAL HIDDEN 1 __gnu_f2h_ieee
12: 00000119 4 FUNC GLOBAL HIDDEN 1 __gnu_h2f_ieee
13: 0000011d 4 FUNC GLOBAL HIDDEN 1 __gnu_f2h_alternative
14: 00000121 4 FUNC GLOBAL HIDDEN 1 __gnu_h2f_alternative
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00000000 000034 000124 00 AX 0 0 4
[ 2] .rel.text REL 00000000 00058c 000010 08 9 1 4
[ 3] .data PROGBITS 00000000 000158 000000 00 WA 0 0 1
[ 4] .bss NOBITS 00000000 000158 000000 00 WA 0 0 1
[ 5] .debug_frame PROGBITS 00000000 000158 00008c 00 0 0 4
[ 6] .rel.debug_frame REL 00000000 00059c 000060 08 9 5 4
[ 7] .ARM.attributes ARM_ATTRIBUTES 00000000 0001e4 00002f 00 0 0 1
[ 8] .shstrtab STRTAB 00000000 000213 000051 00 0 0 1
[ 9] .symtab SYMTAB 00000000 00041c 0000f0 10 10 10 4
[10] .strtab STRTAB 00000000 00050c 00007e 00 0 0 1
Relocation section '.rel.text' at offset 0x58c contains 2 entries:
Offset Info Type Sym.Value Sym. Name
0000011a 00000a66 R_ARM_THM_JUMP11 000000c5 __gnu_h2f_internal
00000122 00000a66 R_ARM_THM_JUMP11 000000c5 __gnu_h2f_internal
Relocation section '.rel.debug_frame' at offset 0x59c contains 12 entries:
Offset Info Type Sym.Value Sym. Name
00000014 00000802 R_ARM_ABS32 00000000 .debug_frame
00000018 00000202 R_ARM_ABS32 00000000 .text
00000040 00000802 R_ARM_ABS32 00000000 .debug_frame
00000044 00000202 R_ARM_ABS32 00000000 .text
00000050 00000802 R_ARM_ABS32 00000000 .debug_frame
00000054 00000202 R_ARM_ABS32 00000000 .text
00000060 00000802 R_ARM_ABS32 00000000 .debug_frame
00000064 00000202 R_ARM_ABS32 00000000 .text
00000070 00000802 R_ARM_ABS32 00000000 .debug_frame
00000074 00000202 R_ARM_ABS32 00000000 .text
00000080 00000802 R_ARM_ABS32 00000000 .debug_frame
00000084 00000202 R_ARM_ABS32 00000000 .text
.text 部分结构，据我所知:
.text 部分的大小为 0x124
.text section has size of 0x124
0x0: unknown byte
0x1-0xC3: __gnu_f2h_internal
0xC3-0xC5: two unknown bytes between those functions (btw what are those?)
0xC5-0x113: __gnu_h2f_internal
0x113-0x115: two unknown bytes between those functions
0x115-0x119: __gnu_f2h_ieee
0x119-0x11D: __gnu_h2f_ieee
0x11D-0x121: __gnu_f2h_alternative
0x121-0x125: __gnu_h2f_alternative // section is only 0x124, what happened to the missing byte?
注意到节大小是 0x124，最后一个函数以 0x125 结束，丢失的字节是怎么回事?
Notice that the section size is 0x124 and the last function end in 0x125, what happend to the missing byte?
谢谢.
推荐答案
从技术上讲，您的缺失字节"就是 0x0 处的那个.
Technically, your "missing byte" is the one right there at 0x0.
请注意，您正在查看符号的 值，即运行时函数地址(如果您的 .text 部分 VMA 不是 0，这将更加清晰).由于它们是 Thumb 函数，因此地址设置了位 0，以便处理器在调用它们时切换到 Thumb 模式；这些指令的实际位置仍然是半字对齐的，即 0x0、0xc4、0x114 等，因为它们不能以其他方式执行(如果 PC 未对齐，您会认为这是错误的).根据ARMELF 规范 说到 STT_FUNC 符号以获得与该符号对应的指令的实际 VMA，然后减去该部分的开头，您应该具有与目标文件本身相同的相对偏移量.
Note that you're looking at the value of the symbol, i.e. the runtime function address (this would be a lot clearer if your .text section VMA wasn't 0). Since they're Thumb functions, the addresses have bit 0 set such that the processor will switch to Thumb mode when calling them; the actual locations of those instructions are still halfword-aligned, i.e. 0x0, 0xc4, 0x114, etc. since they couldn't be executed otherwise (you'd take a fault for a misaligned PC). Strip off bit 0 as per what the ARM ELF spec says about STT_FUNC symbols to get the actual VMA of the instruction corresponding to that symbol, then subtract the start of the section and you should have the same relative offset as within the object file itself.
<offset in section> = (<symbol value> & ~1) - <section VMA>
某些函数后的额外半字填充只是确保每个符号都是字对齐的 - 这可能有多种原因，但第一个想到的是 adr 指令不会如果他们不正常工作.
The extra halfword padding after some functions just ensures each symbol is word-aligned - there are probably various reasons for this, but the first one that comes to mind is that the adr instruction wouldn't work properly if they weren't.
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