MIPS: ftrace: Fix dynamic tracing of kernel modules

Dynamic tracing of kernel modules is broken on 32-bit MIPS. When modules
are loaded, the kernel crashes when dynamic tracing is enabled with:
 cd /sys/kernel/debug/tracing
 echo > set_ftrace_filter
 echo function > current_tracer

1) arch/mips/kernel/ftrace.c
When the kernel boots, or when a module is initialized, ftrace_make_nop()
modifies every _mcount call site to eliminate the ftrace overhead.
However, when ftrace is later enabled for a call site, ftrace_make_call()
does not currently restore the _mcount call correctly for module call sites.
Added ftrace_modify_code_2r() and modified ftrace_make_call() to fix this.

2) arch/mips/kernel/mcount.S
_mcount assembly routine is supposed to have the caller's _mcount call site
address in register a0. However, a0 is currently not calculated correctly for
module call sites. a0 should be (ra - 20) or (ra - 24), depending on whether
the kernel was built with KBUILD_MCOUNT_RA_ADDRESS or not.

This fix has been tested on Broadcom BMIPS5000 processor. Dynamic tracing
now works for both built-in functions and module functions.

Signed-off-by: Petri Gynther <pgynther@google.com>
Cc: linux-mips@linux-mips.org
Cc: rostedt@goodmis.org
Cc: alcooperx@gmail.com
Cc: cminyard@mvista.com
Patchwork: https://patchwork.linux-mips.org/patch/7476/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ftrace.c

@@ -63,18 +63,18 @@ static inline int in_kernel_space(unsigned long ip)
 	((unsigned int)(JAL | (((addr) >> 2) & ADDR_MASK)))
 
 static unsigned int insn_jal_ftrace_caller __read_mostly;
-static unsigned int insn_lui_v1_hi16_mcount __read_mostly;
+static unsigned int insn_la_mcount[2] __read_mostly;
 static unsigned int insn_j_ftrace_graph_caller __maybe_unused __read_mostly;
 
 static inline void ftrace_dyn_arch_init_insns(void)
 {
 	u32 *buf;
 	unsigned int v1;
 
-	/* lui v1, hi16_mcount */
+	/* la v1, _mcount */
 	v1 = 3;
-	buf = (u32 *)&insn_lui_v1_hi16_mcount;
-	UASM_i_LA_mostly(&buf, v1, MCOUNT_ADDR);
+	buf = (u32 *)&insn_la_mcount[0];
+	UASM_i_LA(&buf, v1, MCOUNT_ADDR);
 
 	/* jal (ftrace_caller + 8), jump over the first two instruction */
 	buf = (u32 *)&insn_jal_ftrace_caller;
@@ -111,14 +111,47 @@ static int ftrace_modify_code_2(unsigned long ip, unsigned int new_code1,
 				unsigned int new_code2)
 {
 	int faulted;
+	mm_segment_t old_fs;
 
 	safe_store_code(new_code1, ip, faulted);
 	if (unlikely(faulted))
 		return -EFAULT;
-	safe_store_code(new_code2, ip + 4, faulted);
+
+	ip += 4;
+	safe_store_code(new_code2, ip, faulted);
 	if (unlikely(faulted))
 		return -EFAULT;
+
+	ip -= 4;
+	old_fs = get_fs();
+	set_fs(get_ds());
 	flush_icache_range(ip, ip + 8);
+	set_fs(old_fs);
+
+	return 0;
+}
+
+static int ftrace_modify_code_2r(unsigned long ip, unsigned int new_code1,
+				 unsigned int new_code2)
+{
+	int faulted;
+	mm_segment_t old_fs;
+
+	ip += 4;
+	safe_store_code(new_code2, ip, faulted);
+	if (unlikely(faulted))
+		return -EFAULT;
+
+	ip -= 4;
+	safe_store_code(new_code1, ip, faulted);
+	if (unlikely(faulted))
+		return -EFAULT;
+
+	old_fs = get_fs();
+	set_fs(get_ds());
+	flush_icache_range(ip, ip + 8);
+	set_fs(old_fs);
+
 	return 0;
 }
 #endif
@@ -130,13 +163,14 @@ static int ftrace_modify_code_2(unsigned long ip, unsigned int new_code1,
  *
  * move at, ra
  * jal _mcount		--> nop
+ *  sub sp, sp, 8	--> nop  (CONFIG_32BIT)
  *
  * 2. For modules:
  *
  * 2.1 For KBUILD_MCOUNT_RA_ADDRESS and CONFIG_32BIT
  *
  * lui v1, hi_16bit_of_mcount	     --> b 1f (0x10000005)
- * addiu v1, v1, low_16bit_of_mcount
+ * addiu v1, v1, low_16bit_of_mcount --> nop  (CONFIG_32BIT)
  * move at, ra
  * move $12, ra_address
  * jalr v1
@@ -145,7 +179,7 @@ static int ftrace_modify_code_2(unsigned long ip, unsigned int new_code1,
  * 2.2 For the Other situations
  *
  * lui v1, hi_16bit_of_mcount	     --> b 1f (0x10000004)
- * addiu v1, v1, low_16bit_of_mcount
+ * addiu v1, v1, low_16bit_of_mcount --> nop  (CONFIG_32BIT)
  * move at, ra
  * jalr v1
  *  nop | move $12, ra_address | sub sp, sp, 8
@@ -184,10 +218,14 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	unsigned int new;
 	unsigned long ip = rec->ip;
 
-	new = in_kernel_space(ip) ? insn_jal_ftrace_caller :
-		insn_lui_v1_hi16_mcount;
+	new = in_kernel_space(ip) ? insn_jal_ftrace_caller : insn_la_mcount[0];
 
+#ifdef CONFIG_64BIT
 	return ftrace_modify_code(ip, new);
+#else
+	return ftrace_modify_code_2r(ip, new, in_kernel_space(ip) ?
+						INSN_NOP : insn_la_mcount[1]);
+#endif
 }
 
 #define FTRACE_CALL_IP ((unsigned long)(&ftrace_call))

arch/mips/kernel/mcount.S

@@ -84,6 +84,19 @@ _mcount:
 #endif
 
 	PTR_SUBU a0, ra, 8	/* arg1: self address */
+	PTR_LA   t1, _stext
+	sltu     t2, a0, t1	/* t2 = (a0 < _stext) */
+	PTR_LA   t1, _etext
+	sltu     t3, t1, a0	/* t3 = (a0 > _etext) */
+	or       t1, t2, t3
+	beqz     t1, ftrace_call
+	 nop
+#if defined(KBUILD_MCOUNT_RA_ADDRESS) && defined(CONFIG_32BIT)
+	PTR_SUBU a0, a0, 16	/* arg1: adjust to module's recorded callsite */
+#else
+	PTR_SUBU a0, a0, 12
+#endif
+
 	.globl ftrace_call
 ftrace_call:
 	nop	/* a placeholder for the call to a real tracing function */