Linux-4.14
Aarch64
在前面的分析中调用print_symbol("PC is at %s\n", instruction_pointer(regs))输出当前PC地址的时候，输出的的内容却是：PC is at demo_init+0xc/0x1000 [demo]
下面分析一下这个函数print_symbol。
1 static __printf(1, 2)
2 void __check_printsym_format(const char *fmt, ...)
3 {
4 }
5
6 static inline void print_symbol(const char *fmt, unsigned long addr)
7 {
8 __check_printsym_format(fmt, "");
9 __print_symbol(fmt, (unsigned long)
10 __builtin_extract_return_addr((void *)addr));
11 }
第8行，格式检查
第9行，__builtin_extract_return_addr((void *)addr)返回实际的addr，这里返回的还是addr，这个函数的说明可以参考GCC文档：
下面分析__print_symbol：
1 /* Look up a kernel symbol and print it to the kernel messages. */
2 void __print_symbol(const char *fmt, unsigned long address)
3 {
4 char buffer[KSYM_SYMBOL_LEN];
5
6 sprint_symbol(buffer, address);
7
8 printk(fmt, buffer);
9 }
第6行就是核心，这个函数完成了将address转换成对应的内核符号字符串，并将字符串存入buffer中
下面分析sprint_symbol:
1 /**
2 * sprint_symbol - Look up a kernel symbol and return it in a text buffer
3 * @buffer: buffer to be stored
4 * @address: address to lookup
5 *
6 * This function looks up a kernel symbol with @address and stores its name,
7 * offset, size and module name to @buffer if possible. If no symbol was found,
8 * just saves its @address as is.
9 *
10 * This function returns the number of bytes stored in @buffer.
11 */
12 int sprint_symbol(char *buffer, unsigned long address)
13 {
14 return __sprint_symbol(buffer, address, 0, 1);
15 }
根据注释，这个函数用于查找一个地址为address的内核符号，然后将查找到的符号名字，偏移，大小以及模块名存放到buffer中，如果没有找到的话，只是将address按字符串的格式存入buffer。
这里说明一下：demo_init+0xc/0x1000 [demo]
符号名字：demo_init
偏移：0xc
大小：0x1000
模块名：demo
上面这行的意思是：传入的address处于函数demo_init中，距离demo_init起始地址的偏移为0xC，demo_init函数占用的代码空间是0x1000。所在的内核模块是demo
下面分析__sprint_symbol：
1 /* Look up a kernel symbol and return it in a text buffer. */
2 static int __sprint_symbol(char *buffer, unsigned long address,
3 int symbol_offset, int add_offset)
4 {
5 char *modname;
6 const char *name;
7 unsigned long offset, size;
8 int len;
9
10 address += symbol_offset;
11 name = kallsyms_lookup(address, &size, &offset, &modname, buffer);
12 if (!name)
13 return sprintf(buffer, "0x%lx", address - symbol_offset);
14
15 if (name != buffer)
16 strcpy(buffer, name);
17 len = strlen(buffer);
18 offset -= symbol_offset;
19
20 if (add_offset)
21 len += sprintf(buffer + len, "+%#lx/%#lx", offset, size);
22
23 if (modname)
24 len += sprintf(buffer + len, " [%s]", modname);
25
26 return len;
27 }
上面的第11行的kallsyms_lookup就是根据address获取size，offset，modname
kallsyms_lookup：
1 /*
2 * Lookup an address
3 * - modname is set to NULL if it's in the kernel.
4 * - We guarantee that the returned name is valid until we reschedule even if.
5 * It resides in a module.
6 * - We also guarantee that modname will be valid until rescheduled.
7 */
8 const char *kallsyms_lookup(unsigned long addr,
9 unsigned long *symbolsize,
10 unsigned long *offset,
11 char **modname, char *namebuf)
12 {
13 const char *ret;
14
15 namebuf[KSYM_NAME_LEN - 1] = 0;
16 namebuf[0] = 0;
17
18 if (is_ksym_addr(addr)) {
19 unsigned long pos;
20
21 pos = get_symbol_pos(addr, symbolsize, offset);
22 /* Grab name */
23 kallsyms_expand_symbol(get_symbol_offset(pos),
24 namebuf, KSYM_NAME_LEN);
25 if (modname)
26 *modname = NULL;
27
28 ret = namebuf;
29 goto found;
30 }
31
32 /* See if it's in a module or a BPF JITed image. */
33 ret = module_address_lookup(addr, symbolsize, offset,
34 modname, namebuf);
35 if (!ret)
36 ret = bpf_address_lookup(addr, symbolsize,
37 offset, modname, namebuf);
38
39 found:
40 cleanup_symbol_name(namebuf);
41 return ret;
42 }
上面会从三个地方去查找符号，首先是内核中，如果没有找到，就从内核模块中查找，如果还是没有找到的话，最后就从bpf中查找。
下面分析第18~30行，即从内核中查找，其他的以后再分析。
第18行，判断addr是否位于内核的代码段
第21行，要分析get_symbol_pos需要用到内核代码编译时生成的的.tmp_kallsyms2.S，其中存放了符号信息。
大致说明一下这个文件:
这个文件是动态生成的，使用的工具是scripts/kallsyms.c，下面说明一下.tmp_kallsyms2.S中的变量作用：
kallsyms_offsets数组中存放的是每个符号距离_text地址的偏移量，对于一下System.map：
可以看到System.map中的符号地址减去_text的地址，就是kallsyms_offsets数组中的值。
kallsyms_relative_base中存放的是符号的基地址，这个值加上kallsyms_offsets数组中的offset就是符号的实际地址
kallsyms_num_syms存放的是内核符号的个数
kallsyms_names中存放的是每个符号的名字，每一行对应一个，不过这里为了压缩字符串，第一列表示后面的字节数，第二列开始表示的都是索引，索引的是kallsyms_token_index数组中的元素，而kallsyms_token_index数组中存放的也是索引，它索引的是kallsyms_token_table
kallsyms_token_index：
kallsyms_token_table：
在遍历kallsyms_names时为了加快索引速度，又引入了kallsyms_markers数组，这个数组每一个成员都是kallsyms_names中每256行的首地址，所以将来在根据address获得内核符号的索引下标后，将这个索引除以256，然后再在这个256行中找到对应的那行就快多了。
下面分析get_symbol_pos:
1 static unsigned long get_symbol_pos(unsigned long addr,
2 unsigned long *symbolsize,
3 unsigned long *offset)
4 {
5 unsigned long symbol_start = 0, symbol_end = 0;
6 unsigned long i, low, high, mid;
7
8 /* This kernel should never had been booted. */
9 if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE))
10 BUG_ON(!kallsyms_addresses);
11 else
12 BUG_ON(!kallsyms_offsets);
13
14 /* Do a binary search on the sorted kallsyms_addresses array. */
15 low = 0;
16 high = kallsyms_num_syms;
17
18 while (high - low > 1) {
19 mid = low + (high - low) / 2;
20 if (kallsyms_sym_address(mid) <= addr)
21 low = mid;
22 else
23 high = mid;
24 }
25
26 /*
27 * Search for the first aliased symbol. Aliased
28 * symbols are symbols with the same address.
29 */
30 while (low && kallsyms_sym_address(low-1) == kallsyms_sym_address(low))
31 --low;
32
33 symbol_start = kallsyms_sym_address(low);
34
35 /* Search for next non-aliased symbol. */
36 for (i = low + 1; i < kallsyms_num_syms; i++) {
37 if (kallsyms_sym_address(i) > symbol_start) {
38 symbol_end = kallsyms_sym_address(i);
39 break;
40 }
41 }
42
43 /* If we found no next symbol, we use the end of the section. */
44 if (!symbol_end) {
45 if (is_kernel_inittext(addr))
46 symbol_end = (unsigned long)_einittext;
47 else if (IS_ENABLED(CONFIG_KALLSYMS_ALL))
48 symbol_end = (unsigned long)_end;
49 else
50 symbol_end = (unsigned long)_etext;
51 }
52
53 if (symbolsize)
54 *symbolsize = symbol_end - symbol_start;
55 if (offset)
56 *offset = addr - symbol_start;
57
58 return low;
59 }
第18~24，根据addr查找kallsyms_offsets，获取addr在哪两个符号之间。这里用到了二分法的查找方式，最后addr就位于索引为low和high的两个符号之间，其实就是位于索引为low的函数内部
第30，在kallsyms_offsets中可以看到有很多符号的地址是相同的，这行用于获取相同address的符号中的第一个对应的索引，即low
第33，获取索引为low的符号的地址symbol_start
第36~41，获取紧接着比symbol_start大的一个符号地址，symbol_end
第54行，获取地址为symbol_start内核函数的占用的空间的大小
第56行，获取address相对于symbol_start的偏移量
第58行，返回address所在的内核函数的首地址对应的索引号
接着分析kallsyms_lookup：
第21行，获取了address所在的内核函数的首地址对应的索引号
第23行，get_symbol_offset获取pos对应的内核符号字符串的地址相对于kallsyms_names的偏移量，可以结合之前对.tmp_kallsyms2.S的分析理解
1 /*
2 * Find the offset on the compressed stream given and index in the
3 * kallsyms array.
4 */
5 static unsigned int get_symbol_offset(unsigned long pos)
6 {
7 const u8 *name;
8 int i;
9
10 /*
11 * Use the closest marker we have. We have markers every 256 positions,
12 * so that should be close enough.
13 */
14 name = &kallsyms_names[kallsyms_markers[pos >> 8]];
15
16 /*
17 * Sequentially scan all the symbols up to the point we're searching
18 * for. Every symbol is stored in a [<len>][<len> bytes of data] format,
19 * so we just need to add the len to the current pointer for every
20 * symbol we wish to skip.
21 */
22 for (i = 0; i < (pos & 0xFF); i++)
23 name = name + (*name) + 1;
24
25 return name - kallsyms_names;
26 }
kallsyms_expand_symbol：
1 /*
2 * Expand a compressed symbol data into the resulting uncompressed string,
3 * if uncompressed string is too long (>= maxlen), it will be truncated,
4 * given the offset to where the symbol is in the compressed stream.
5 */
6 static unsigned int kallsyms_expand_symbol(unsigned int off,
7 char *result, size_t maxlen)
8 {
9 int len, skipped_first = 0;
10 const u8 *tptr, *data;
11
12 /* Get the compressed symbol length from the first symbol byte. */
13 data = &kallsyms_names[off];
14 len = *data;
15 data++;
16
17 /*
18 * Update the offset to return the offset for the next symbol on
19 * the compressed stream.
20 */
21 off += len + 1;
22
23 /*
24 * For every byte on the compressed symbol data, copy the table
25 * entry for that byte.
26 */
27 while (len) {
28 tptr = &kallsyms_token_table[kallsyms_token_index[*data]];
29 data++;
30 len--;
31
32 while (*tptr) {
33 if (skipped_first) {
34 if (maxlen <= 1)
35 goto tail;
36 *result = *tptr;
37 result++;
38 maxlen--;
39 } else
40 skipped_first = 1;
41 tptr++;
42 }
43 }
44
45 tail:
46 if (maxlen)
47 *result = '\0';
48
49 /* Return to offset to the next symbol. */
50 return off;
51 }
最后会将转换得到的内核符号的字符串名字拷贝到namebuf中。
完。
