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 #ifdef LTTNG_CONFIG_PPC64_ELF_ABI_V1
  24 #if (LTTNG_LINUX_VERSION_CODE >= LTTNG_KERNEL_VERSION(5,18,0))
  25 #include <asm/elf.h>
  26
  27 #define LTTNG_FUNC_DESC_TYPE struct func_desc
  28 #define LTTNG_FUNC_DESC_ADDR_NAME addr
  29
  30 #else
  31
  32 #include <asm/types.h>
  33
  34 #define LTTNG_FUNC_DESC_TYPE func_descr_t
  35 #define LTTNG_FUNC_DESC_ADDR_NAME entry
  36 #endif
  37
  38 static
  39 LTTNG_FUNC_DESC_TYPE kallsyms_lookup_name_func_desc;
  40 #endif
  41
  42 static
  43 unsigned long (*kallsyms_lookup_name_sym)(const char *name);
  44
  45 static
  46 int dummy_kprobe_handler(struct kprobe *p, struct pt_regs *regs)
  47 {
  48         return 0;
  49 }
  50
  51 static
  52 unsigned long do_get_kallsyms(void)
  53 {
  54         struct kprobe probe;
  55         int ret;
  56         unsigned long addr;
  57
  58         memset(&probe, 0, sizeof(probe));
  59         probe.pre_handler = dummy_kprobe_handler;
  60         probe.symbol_name = "kallsyms_lookup_name";
  61 #ifdef LTTNG_CONFIG_PPC64_ELF_ABI_V2
  62         /*
  63          * With powerpc64 ABIv2, we need the global entry point of
  64          * kallsyms_lookup_name to call it later, while kprobe_register would
  65          * automatically adjust the global entry point to the local entry point,
  66          * when a kprobe was registered at a function entry. So we add 4 bytes
  67          * which is the length of one instruction to kallsyms_lookup_name to
  68          * avoid the adjustment.
  69          */
  70         probe.offset = 4;
  71 #endif
  72         ret = register_kprobe(&probe);
  73         if (ret)
  74                 return 0;
  75 #ifdef LTTNG_CONFIG_PPC64_ELF_ABI_V2
  76         /* Substract 4 bytes to get what we originally want */
  77         addr = (unsigned long)(((char *)probe.addr) - 4);
  78 #elif defined(LTTNG_CONFIG_PPC64_ELF_ABI_V1)
  79         /*
  80          * Build a function descriptor from the address of
  81          * 'kallsyms_lookup_name' returned by kprobe and the toc of
  82          * 'sprint_symbol' which is in the same compile unit and exported. I
  83          * hate this on so many levels but it works.
  84          */
  85         kallsyms_lookup_name_func_desc.LTTNG_FUNC_DESC_ADDR_NAME = (unsigned long) probe.addr;
  86         kallsyms_lookup_name_func_desc.toc = ((LTTNG_FUNC_DESC_TYPE *) &sprint_symbol)->toc;
  87         addr = (unsigned long) &kallsyms_lookup_name_func_desc;
  88 #else
  89         addr = (unsigned long)probe.addr;
  90 #endif
  91 #ifdef CONFIG_ARM
  92 #ifdef CONFIG_THUMB2_KERNEL
  93         if (addr)
  94                 addr |= 1; /* set bit 0 in address for thumb mode */
  95 #endif
  96 #endif
  97         unregister_kprobe(&probe);
  98         return addr;
  99 }
 100
 101 unsigned long wrapper_kallsyms_lookup_name(const char *name)
 102 {
 103         if (!kallsyms_lookup_name_sym) {
 104                 kallsyms_lookup_name_sym = (void *)do_get_kallsyms();
 105         }
 106         if (kallsyms_lookup_name_sym)
 107                 return kallsyms_lookup_name_sym(name);
 108         else {
 109                 printk_once(KERN_WARNING "LTTng: requires kallsyms_lookup_name\n");
 110                 return 0;
 111         }
 112 }
 113 EXPORT_SYMBOL_GPL(wrapper_kallsyms_lookup_name);
 114
 115 #endif