1 /* 2 * Dynamic function tracing support. 3 * 4 * Copyright (C) 2008 Abhishek Sagar <sagar.abhishek@gmail.com> 5 * 6 * For licencing details, see COPYING. 7 * 8 * Defines low-level handling of mcount calls when the kernel 9 * is compiled with the -pg flag. When using dynamic ftrace, the 10 * mcount call-sites get patched lazily with NOP till they are 11 * enabled. All code mutation routines here take effect atomically. 12 */ 13 14 #include <linux/ftrace.h> 15 16 #include <asm/cacheflush.h> 17 #include <asm/ftrace.h> 18 19 #define PC_OFFSET 8 20 #define BL_OPCODE 0xeb000000 21 #define BL_OFFSET_MASK 0x00ffffff 22 23 static unsigned long bl_insn; 24 static const unsigned long NOP = 0xe1a00000; /* mov r0, r0 */ 25 26 unsigned char *ftrace_nop_replace(void) 27 { 28 return (char *)&NOP; 29 } 30 31 /* construct a branch (BL) instruction to addr */ 32 unsigned char *ftrace_call_replace(unsigned long pc, unsigned long addr) 33 { 34 long offset; 35 36 offset = (long)addr - (long)(pc + PC_OFFSET); 37 if (unlikely(offset < -33554432 || offset > 33554428)) { 38 /* Can't generate branches that far (from ARM ARM). Ftrace 39 * doesn't generate branches outside of kernel text. 40 */ 41 WARN_ON_ONCE(1); 42 return NULL; 43 } 44 offset = (offset >> 2) & BL_OFFSET_MASK; 45 bl_insn = BL_OPCODE | offset; 46 return (unsigned char *)&bl_insn; 47 } 48 49 int ftrace_modify_code(unsigned long pc, unsigned char *old_code, 50 unsigned char *new_code) 51 { 52 unsigned long err = 0, replaced = 0, old, new; 53 54 old = *(unsigned long *)old_code; 55 new = *(unsigned long *)new_code; 56 57 __asm__ __volatile__ ( 58 "1: ldr %1, [%2] \n" 59 " cmp %1, %4 \n" 60 "2: streq %3, [%2] \n" 61 " cmpne %1, %3 \n" 62 " movne %0, #2 \n" 63 "3:\n" 64 65 ".section .fixup, \"ax\"\n" 66 "4: mov %0, #1 \n" 67 " b 3b \n" 68 ".previous\n" 69 70 ".section __ex_table, \"a\"\n" 71 " .long 1b, 4b \n" 72 " .long 2b, 4b \n" 73 ".previous\n" 74 75 : "=r"(err), "=r"(replaced) 76 : "r"(pc), "r"(new), "r"(old), "0"(err), "1"(replaced) 77 : "memory"); 78 79 if (!err && (replaced == old)) 80 flush_icache_range(pc, pc + MCOUNT_INSN_SIZE); 81 82 return err; 83 } 84 85 int ftrace_update_ftrace_func(ftrace_func_t func) 86 { 87 int ret; 88 unsigned long pc, old; 89 unsigned char *new; 90 91 pc = (unsigned long)&ftrace_call; 92 memcpy(&old, &ftrace_call, MCOUNT_INSN_SIZE); 93 new = ftrace_call_replace(pc, (unsigned long)func); 94 ret = ftrace_modify_code(pc, (unsigned char *)&old, new); 95 return ret; 96 } 97 98 /* run from ftrace_init with irqs disabled */ 99 int __init ftrace_dyn_arch_init(void *data) 100 { 101 ftrace_mcount_set(data); 102 return 0; 103 } 104