第一种是借助于kprobe机制，通过kprobe机制中会调用kallsyms_lookup_name函数并设置到kprobe结构体中返回的原理找到我们需要的函数地址
内核中调用逻辑简化代码如下：
如上逻辑可以看到其实就是调用的kallsyms_lookup_name函数来获取地址存到我们的addr中
第二种方式是借助于内核调试器提供的能力间接调用kallsyms_lookup_name函数获取地址
在内核中有这样一个函数并且这个函数是导出的
int register_kprobe(struct kprobe *p)
{
int ret;
struct kprobe *old_p;
struct module *probed_mod;
kprobe_opcode_t *addr;
addr = kprobe_addr(p);
if (IS_ERR(addr))
return PTR_ERR(addr);
p->addr = addr;
。。。。。。
}
static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
{
return _kprobe_addr(p->addr, p->symbol_name, p->offset);
}
static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
const char *symbol_name, unsigned int offset)
{
if ((symbol_name && addr) || (!symbol_name && !addr))
goto invalid;
if (symbol_name) {
addr = kprobe_lookup_name(symbol_name, offset);
if (!addr)
return ERR_PTR(-ENOENT);
}
。。。。。
}
kprobe_opcode_t * __weak kprobe_lookup_name(const char *name,
unsigned int __unused)
{
return ((kprobe_opcode_t *)(kallsyms_lookup_name(name)));
}
int register_kprobe(struct kprobe *p)
{
int ret;
struct kprobe *old_p;
struct module *probed_mod;
kprobe_opcode_t *addr;
addr = kprobe_addr(p);
if (IS_ERR(addr))
return PTR_ERR(addr);
p->addr = addr;
。。。。。。
}
static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
{
return _kprobe_addr(p->addr, p->symbol_name, p->offset);
}
static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
const char *symbol_name, unsigned int offset)
{
if ((symbol_name && addr) || (!symbol_name && !addr))
goto invalid;
if (symbol_name) {
addr = kprobe_lookup_name(symbol_name, offset);
if (!addr)
return ERR_PTR(-ENOENT);
}
。。。。。
}
kprobe_opcode_t * __weak kprobe_lookup_name(const char *name,
unsigned int __unused)
{
return ((kprobe_opcode_t *)(kallsyms_lookup_name(name)));
}
extern int kdbgetsymval(const char *, kdb_symtab_t *);
int kdbgetsymval(const char *symname, kdb_symtab_t *symtab)
{
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: symname=%s, symtab=%px\n", symname,
symtab);
memset(symtab, 0, sizeof(*symtab));
symtab->sym_start = kallsyms_lookup_name(symname);
if (symtab->sym_start) {
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: returns 1, "
"symtab->sym_start=0x%lx\n",
symtab->sym_start);
return 1;
}
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: returns 0\n");
return 0;
}
EXPORT_SYMBOL(kdbgetsymval);
extern int kdbgetsymval(const char *, kdb_symtab_t *);
int kdbgetsymval(const char *symname, kdb_symtab_t *symtab)
{
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: symname=%s, symtab=%px\n", symname,
symtab);
memset(symtab, 0, sizeof(*symtab));
symtab->sym_start = kallsyms_lookup_name(symname);
if (symtab->sym_start) {
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: returns 1, "
"symtab->sym_start=0x%lx\n",
symtab->sym_start);
return 1;
}
if (KDB_DEBUG(AR))
kdb_printf("kdbgetsymval: returns 0\n");
return 0;
}
EXPORT_SYMBOL(kdbgetsymval);
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kallsyms.h>
#include <linux/kprobes.h>
#include <linux/cdev.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("ch");
typedef struct __ksymtab {
unsigned long value;
const char *mod_name;
unsigned long mod_start;
unsigned long mod_end;
const char *sec_name;
unsigned long sec_start;
unsigned long sec_end;
const char *sym_name;
unsigned long sym_start;
unsigned long sym_end;
} kdb_symtab_t;
extern int kdbgetsymval(const char *, kdb_symtab_t *);
struct char_device_struct {
struct char_device_struct *next;
unsigned int major;
unsigned int baseminor;
int minorct;
char name[64];
struct cdev *cdev;
};
int noop_pre(struct kprobe* p, struct pt_regs* regs) {return 0;}
static struct kprobe kp = {
.symbol_name = "kallsyms_lookup_name",
};
unsigned long (*kallsyms_lookup_name_func)(const char* name) = NULL;
static int __init m_init(void) {
kp.pre_handler = noop_pre;
int ret = register_kprobe(&kp);
if (ret != 0) {
printk("find failed\n");
return ret;
}
kallsyms_lookup_name_func = (void*)kp.addr;
printk("lookup func addr is %p\n", kp.addr);
unregister_kprobe(&kp);
unsigned long chrdevs_addr = kallsyms_lookup_name_func("chrdevs");
printk("chardevs addr is 0x%p\n", chrdevs_addr);
kdb_symtab_t val;
kdbgetsymval("chrdevs", &val);
printk("second method get chrdevs addr is 0x%p\n", val.sym_start);
return 0;
}
static void __exit m_exit(void) {
}
module_init(m_init);
module_exit(m_exit);
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