riscv: ftrace: prepare ftrace for atomic code patching

We use an AUIPC+JALR pair to jump into a ftrace trampoline. Since
instruction fetch can break down to 4 byte at a time, it is impossible
to update two instructions without a race. In order to mitigate it, we
initialize the patchable entry to AUIPC + NOP4. Then, the run-time code
patching can change NOP4 to JALR to eable/disable ftrcae from a
function. This limits the reach of each ftrace entry to +-2KB displacing
from ftrace_caller.

Starting from the trampoline, we add a level of indirection for it to
reach ftrace caller target. Now, it loads the target address from a
memory location, then perform the jump. This enable the kernel to update
the target atomically.

The new don't-stop-the-world text patching on change only one RISC-V
instruction:

  |  -8: &ftrace_ops of the associated tracer function.
  | <ftrace enable>:
  |   0: auipc  t0, hi(ftrace_caller)
  |   4: jalr   t0, lo(ftrace_caller)
  |
  |  -8: &ftrace_nop_ops
  | <ftrace disable>:
  |   0: auipc  t0, hi(ftrace_caller)
  |   4: nop

This means that f+0x0 is fixed, and should not be claimed by ftrace,
e.g. kprobe should be able to put a probe in f+0x0. Thus, we adjust the
offset and MCOUNT_INSN_SIZE accordingly.

[ alex: Fix build errors with !CONFIG_DYNAMIC_FTRACE ]

Co-developed-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20250407180838.42877-5-andybnac@gmail.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

arch/riscv/include/asm/ftrace.h

@@ -20,10 +20,9 @@ extern void *return_address(unsigned int level);
 #define ftrace_return_address(n) return_address(n)
 
 void _mcount(void);
-static inline unsigned long ftrace_call_adjust(unsigned long addr)
-{
-	return addr;
-}
+unsigned long ftrace_call_adjust(unsigned long addr);
+unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip);
+#define ftrace_get_symaddr(fentry_ip) arch_ftrace_get_symaddr(fentry_ip)
 
 /*
  * Let's do like x86/arm64 and ignore the compat syscalls.
@@ -57,12 +56,21 @@ struct dyn_arch_ftrace {
  * 2) jalr: setting low-12 offset to ra, jump to ra, and set ra to
  *          return address (original pc + 4)
  *
+ * The first 2 instructions for each tracable function is compiled to 2 nop
+ * instructions. Then, the kernel initializes the first instruction to auipc at
+ * boot time (<ftrace disable>). The second instruction is patched to jalr to
+ * start the trace.
+ *
+ *<Image>:
+ * 0: nop
+ * 4: nop
+ *
  *<ftrace enable>:
- * 0: auipc  t0/ra, 0x?
- * 4: jalr   t0/ra, ?(t0/ra)
+ * 0: auipc  t0, 0x?
+ * 4: jalr   t0, ?(t0)
  *
  *<ftrace disable>:
- * 0: nop
+ * 0: auipc  t0, 0x?
  * 4: nop
  *
  * Dynamic ftrace generates probes to call sites, so we must deal with
@@ -75,10 +83,9 @@ struct dyn_arch_ftrace {
 #define AUIPC_OFFSET_MASK	(0xfffff000)
 #define AUIPC_PAD		(0x00001000)
 #define JALR_SHIFT		20
-#define JALR_RA			(0x000080e7)
-#define AUIPC_RA		(0x00000097)
 #define JALR_T0			(0x000282e7)
 #define AUIPC_T0		(0x00000297)
+#define JALR_RANGE		(JALR_SIGN_MASK - 1)
 
 #define to_jalr_t0(offset)						\
 	(((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_T0)
@@ -96,26 +103,14 @@ do {									\
 	call[1] = to_jalr_t0(offset);					\
 } while (0)
 
-#define to_jalr_ra(offset)						\
-	(((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_RA)
-
-#define to_auipc_ra(offset)						\
-	((offset & JALR_SIGN_MASK) ?					\
-	(((offset & AUIPC_OFFSET_MASK) + AUIPC_PAD) | AUIPC_RA) :	\
-	((offset & AUIPC_OFFSET_MASK) | AUIPC_RA))
-
-#define make_call_ra(caller, callee, call)				\
-do {									\
-	unsigned int offset =						\
-		(unsigned long) (callee) - (unsigned long) (caller);	\
-	call[0] = to_auipc_ra(offset);					\
-	call[1] = to_jalr_ra(offset);					\
-} while (0)
-
 /*
- * Let auipc+jalr be the basic *mcount unit*, so we make it 8 bytes here.
+ * Only the jalr insn in the auipc+jalr is patched, so we make it 4
+ * bytes here.
  */
-#define MCOUNT_INSN_SIZE 8
+#define MCOUNT_INSN_SIZE	4
+#define MCOUNT_AUIPC_SIZE	4
+#define MCOUNT_JALR_SIZE	4
+#define MCOUNT_NOP4_SIZE	4
 
 #ifndef __ASSEMBLY__
 struct dyn_ftrace;

arch/riscv/kernel/ftrace.c

@@ -8,11 +8,22 @@
 #include <linux/ftrace.h>
 #include <linux/uaccess.h>
 #include <linux/memory.h>
+#include <linux/irqflags.h>
 #include <linux/stop_machine.h>
 #include <asm/cacheflush.h>
 #include <asm/text-patching.h>
 
 #ifdef CONFIG_DYNAMIC_FTRACE
+unsigned long ftrace_call_adjust(unsigned long addr)
+{
+	return addr + MCOUNT_AUIPC_SIZE;
+}
+
+unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
+{
+	return fentry_ip - MCOUNT_AUIPC_SIZE;
+}
+
 void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
 {
 	mutex_lock(&text_mutex);
@@ -32,51 +43,32 @@ void ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
 	mutex_unlock(&text_mutex);
 }
 
-static int ftrace_check_current_call(unsigned long hook_pos,
-				     unsigned int *expected)
+static int __ftrace_modify_call(unsigned long source, unsigned long target, bool validate)
 {
+	unsigned int call[2], offset;
 	unsigned int replaced[2];
-	unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
 
-	/* we expect nops at the hook position */
-	if (!expected)
-		expected = nops;
+	offset = target - source;
+	call[1] = to_jalr_t0(offset);
 
-	/*
-	 * Read the text we want to modify;
-	 * return must be -EFAULT on read error
-	 */
-	if (copy_from_kernel_nofault(replaced, (void *)hook_pos,
-			MCOUNT_INSN_SIZE))
-		return -EFAULT;
+	if (validate) {
+		call[0] = to_auipc_t0(offset);
+		/*
+		 * Read the text we want to modify;
+		 * return must be -EFAULT on read error
+		 */
+		if (copy_from_kernel_nofault(replaced, (void *)source, 2 * MCOUNT_INSN_SIZE))
+			return -EFAULT;
 
-	/*
-	 * Make sure it is what we expect it to be;
-	 * return must be -EINVAL on failed comparison
-	 */
-	if (memcmp(expected, replaced, sizeof(replaced))) {
-		pr_err("%p: expected (%08x %08x) but got (%08x %08x)\n",
-		       (void *)hook_pos, expected[0], expected[1], replaced[0],
-		       replaced[1]);
-		return -EINVAL;
+		if (replaced[0] != call[0]) {
+			pr_err("%p: expected (%08x) but got (%08x)\n",
+			       (void *)source, call[0], replaced[0]);
+			return -EINVAL;
+		}
 	}
 
-	return 0;
-}
-
-static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
-				bool enable, bool ra)
-{
-	unsigned int call[2];
-	unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
-
-	if (ra)
-		make_call_ra(hook_pos, target, call);
-	else
-		make_call_t0(hook_pos, target, call);
-
-	/* Replace the auipc-jalr pair at once. Return -EPERM on write error. */
-	if (patch_insn_write((void *)hook_pos, enable ? call : nops, MCOUNT_INSN_SIZE))
+	/* Replace the jalr at once. Return -EPERM on write error. */
+	if (patch_insn_write((void *)(source + MCOUNT_AUIPC_SIZE), call + 1, MCOUNT_JALR_SIZE))
 		return -EPERM;
 
 	return 0;
@@ -84,22 +76,21 @@ static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
 
 int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 {
-	unsigned int call[2];
+	unsigned long distance, orig_addr, pc = rec->ip - MCOUNT_AUIPC_SIZE;
 
-	make_call_t0(rec->ip, addr, call);
+	orig_addr = (unsigned long)&ftrace_caller;
+	distance = addr > orig_addr ? addr - orig_addr : orig_addr - addr;
+	if (distance > JALR_RANGE)
+		return -EINVAL;
 
-	if (patch_insn_write((void *)rec->ip, call, MCOUNT_INSN_SIZE))
-		return -EPERM;
-
-	return 0;
+	return __ftrace_modify_call(pc, addr, false);
 }
 
-int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
-		    unsigned long addr)
+int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long addr)
 {
-	unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
+	u32 nop4 = RISCV_INSN_NOP4;
 
-	if (patch_insn_write((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
+	if (patch_insn_write((void *)rec->ip, &nop4, MCOUNT_NOP4_SIZE))
 		return -EPERM;
 
 	return 0;
@@ -114,21 +105,38 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
  */
 int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
 {
-	int out;
+	unsigned long pc = rec->ip - MCOUNT_AUIPC_SIZE;
+	unsigned int nops[2], offset;
+	int ret;
+
+	offset = (unsigned long) &ftrace_caller - pc;
+	nops[0] = to_auipc_t0(offset);
+	nops[1] = RISCV_INSN_NOP4;
 
 	mutex_lock(&text_mutex);
-	out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
+	ret = patch_insn_write((void *)pc, nops, 2 * MCOUNT_INSN_SIZE);
 	mutex_unlock(&text_mutex);
 
-	return out;
+	return ret;
 }
 
+ftrace_func_t ftrace_call_dest = ftrace_stub;
 int ftrace_update_ftrace_func(ftrace_func_t func)
 {
-	int ret = __ftrace_modify_call((unsigned long)&ftrace_call,
-				       (unsigned long)func, true, true);
-
-	return ret;
+	WRITE_ONCE(ftrace_call_dest, func);
+	/*
+	 * The data fence ensure that the update to ftrace_call_dest happens
+	 * before the write to function_trace_op later in the generic ftrace.
+	 * If the sequence is not enforced, then an old ftrace_call_dest may
+	 * race loading a new function_trace_op set in ftrace_modify_all_code
+	 *
+	 * If we are in stop_machine, then we don't need to call remote fence
+	 * as there is no concurrent read-side of ftrace_call_dest.
+	 */
+	smp_wmb();
+	if (!irqs_disabled())
+		smp_call_function(ftrace_sync_ipi, NULL, 1);
+	return 0;
 }
 
 struct ftrace_modify_param {
@@ -166,23 +174,22 @@ void arch_ftrace_update_code(int command)
 
 	stop_machine(__ftrace_modify_code, &param, cpu_online_mask);
 }
-#endif
+#else /* CONFIG_DYNAMIC_FTRACE */
+unsigned long ftrace_call_adjust(unsigned long addr)
+{
+	return addr;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 		       unsigned long addr)
 {
+	unsigned long caller = rec->ip - MCOUNT_AUIPC_SIZE;
 	unsigned int call[2];
-	unsigned long caller = rec->ip;
-	int ret;
 
 	make_call_t0(caller, old_addr, call);
-	ret = ftrace_check_current_call(caller, call);
-
-	if (ret)
-		return ret;
-
-	return __ftrace_modify_call(caller, addr, true, false);
+	return __ftrace_modify_call(caller, addr, true);
 }
 #endif
 

arch/riscv/kernel/mcount-dyn.S

@@ -13,7 +13,6 @@
 
 	.text
 
-#define FENTRY_RA_OFFSET	8
 #define ABI_SIZE_ON_STACK	80
 #define ABI_A0			0
 #define ABI_A1			8
@@ -62,8 +61,7 @@
 * After the stack is established,
 *
 * 0(sp) stores the PC of the traced function which can be accessed
-* by &(fregs)->epc in tracing function. Note that the real
-* function entry address should be computed with -FENTRY_RA_OFFSET.
+* by &(fregs)->epc in tracing function.
 *
 * 8(sp) stores the function return address (i.e. parent IP) that
 * can be accessed by &(fregs)->ra in tracing function.
@@ -140,7 +138,7 @@
 	.endm
 
 	.macro PREPARE_ARGS
-	addi	a0, t0, -FENTRY_RA_OFFSET
+	addi	a0, t0, -MCOUNT_JALR_SIZE	// ip (callsite's jalr insn)
 	la	a1, function_trace_op
 	REG_L	a2, 0(a1)
 	mv	a1, ra
@@ -153,7 +151,8 @@ SYM_FUNC_START(ftrace_caller)
 	PREPARE_ARGS
 
 SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
-	call	ftrace_stub
+	REG_L	ra, ftrace_call_dest
+	jalr	ra, 0(ra)
 
 	RESTORE_ABI_REGS
 	bnez	t1, .Ldirect