src/wrapper/kallsyms.c

   1 /* SPDX-License-Identifier: (GPL-2.0-only OR LGPL-2.1-only)
   2  *
   3  * wrapper/kallsyms.c
   4  *
   5  * Wrapper around kallsyms. Using kprobes to get its address when available.
   6  *
   7  * Can we mainline LTTng already so we don't have to waste our time doing this
   8  * kind of hack ?
   9  *
  10  * Copyright (C) 2020 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  11  */
  12
  13 #include <linux/kprobes.h>
  14 #include <linux/module.h>
  15 #include <wrapper/kallsyms.h>
  16
  17 #if (LTTNG_LINUX_VERSION_CODE >= LTTNG_KERNEL_VERSION(5,7,0))
  18
  19 #ifndef CONFIG_KPROBES
  20 # error "LTTng-modules requires CONFIG_KPROBES on kernels >= 5.7.0"
  21 #endif
  22
  23 static
  24 unsigned long (*kallsyms_lookup_name_sym)(const char *name);
  25
  26 static
  27 int dummy_kprobe_handler(struct kprobe *p, struct pt_regs *regs)
  28 {
  29         return 0;
  30 }
  31
  32 static
  33 unsigned long do_get_kallsyms(void)
  34 {
  35         struct kprobe probe;
  36         int ret;
  37         unsigned long addr;
  38
  39         memset(&probe, 0, sizeof(probe));
  40         probe.pre_handler = dummy_kprobe_handler;
  41         probe.symbol_name = "kallsyms_lookup_name";
  42 #ifdef PPC64_ELF_ABI_v2
  43         /*
  44          * With powerpc64 ABIv2, we need the global entry point of
  45          * kallsyms_lookup_name to call it later, while kprobe_register would
  46          * automatically adjust the global entry point to the local entry point,
  47          * when a kprobe was registered at a function entry. So we add 4 bytes
  48          * which is the length of one instruction to kallsyms_lookup_name to
  49          * avoid the adjustment.
  50          */
  51         probe.offset = 4;
  52 #endif
  53         ret = register_kprobe(&probe);
  54         if (ret)
  55                 return 0;
  56 #ifdef PPC64_ELF_ABI_v2
  57         /* Substract 4 bytes to get what we originally want */
  58         addr = (unsigned long)(((char *)probe.addr) - 4);
  59 #else
  60         addr = (unsigned long)probe.addr;
  61 #endif
  62 #ifdef CONFIG_ARM
  63 #ifdef CONFIG_THUMB2_KERNEL
  64         if (addr)
  65                 addr |= 1; /* set bit 0 in address for thumb mode */
  66 #endif
  67 #endif
  68         unregister_kprobe(&probe);
  69         return addr;
  70 }
  71
  72 unsigned long wrapper_kallsyms_lookup_name(const char *name)
  73 {
  74         if (!kallsyms_lookup_name_sym) {
  75                 kallsyms_lookup_name_sym = (void *)do_get_kallsyms();
  76         }
  77         if (kallsyms_lookup_name_sym)
  78                 return kallsyms_lookup_name_sym(name);
  79         else {
  80                 printk_once(KERN_WARNING "LTTng: requires kallsyms_lookup_name\n");
  81                 return 0;
  82         }
  83 }
  84 EXPORT_SYMBOL_GPL(wrapper_kallsyms_lookup_name);
  85
  86 #endif