LoongArch: ftrace: Add direct call support

Select the HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide the
register_ftrace_direct[_multi] interfaces allowing users to register
the customed trampoline (direct_caller) as the mcount for one or more
target functions. And modify_ftrace_direct[_multi] are also provided
for modifying direct_caller.

There are a few cases to distinguish:
- If a direct call ops is the only one tracing a function AND the direct
  called trampoline is within the reach of a 'bl' instruction
  -> the ftrace patchsite jumps to the trampoline
- Else
  -> the ftrace patchsite jumps to the ftrace_regs_caller trampoline points
     to ftrace_list_ops so it iterates over all registered ftrace ops,
     including the direct call ops and calls its call_direct_funcs handler
     which stores the direct called trampoline's address in the ftrace_regs
     and the ftrace_regs_caller trampoline will return to that address
     instead of returning to the traced function

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -93,6 +93,7 @@ config LOONGARCH
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_DYNAMIC_FTRACE_WITH_ARGS
+	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_EBPF_JIT
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !ARCH_STRICT_ALIGN

arch/loongarch/include/asm/ftrace.h

@@ -82,6 +82,18 @@ ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs, unsigned long ip)
 #define ftrace_graph_func ftrace_graph_func
 void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
 		       struct ftrace_ops *op, struct ftrace_regs *fregs);
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+static inline void
+__arch_ftrace_set_direct_caller(struct pt_regs *regs, unsigned long addr)
+{
+	regs->regs[13] = addr;	/* t1 */
+}
+
+#define arch_ftrace_set_direct_caller(fregs, addr) \
+	__arch_ftrace_set_direct_caller(&(fregs)->regs, addr)
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+
 #endif
 
 #endif /* __ASSEMBLY__ */

arch/loongarch/kernel/ftrace_dyn.c

@@ -65,6 +65,14 @@ static bool ftrace_find_callable_addr(struct dyn_ftrace *rec, struct module *mod
 	unsigned long pc = rec->ip + LOONGARCH_INSN_SIZE;
 	struct plt_entry *plt;
 
+	/*
+	 * If a custom trampoline is unreachable, rely on the ftrace_regs_caller
+	 * trampoline which knows how to indirectly reach that trampoline through
+	 * ops->direct_call.
+	 */
+	if (*addr != FTRACE_ADDR && *addr != FTRACE_REGS_ADDR && !reachable_by_bl(*addr, pc))
+		*addr = FTRACE_REGS_ADDR;
+
 	/*
 	 * When the target is within range of the 'bl' instruction, use 'addr'
 	 * as-is and branch to that directly.

arch/loongarch/kernel/mcount_dyn.S

@@ -42,7 +42,6 @@
 	.if \allregs
 	PTR_S		tp, sp, PT_R2
 	PTR_S		t0, sp, PT_R12
-	PTR_S		t1, sp, PT_R13
 	PTR_S		t2, sp, PT_R14
 	PTR_S		t3, sp, PT_R15
 	PTR_S		t4, sp, PT_R16
@@ -64,6 +63,8 @@
 	PTR_S		zero, sp, PT_R0
 	.endif
 	PTR_S		ra, sp, PT_ERA /* Save trace function ra at PT_ERA */
+	move		t1, zero
+	PTR_S		t1, sp, PT_R13
 	PTR_ADDI	t8, sp, PT_SIZE
 	PTR_S		t8, sp, PT_R3
 	.endm
@@ -104,8 +105,12 @@ ftrace_common_return:
 	PTR_L		a7, sp, PT_R11
 	PTR_L		fp, sp, PT_R22
 	PTR_L		t0, sp, PT_ERA
+	PTR_L		t1, sp, PT_R13
 	PTR_ADDI	sp, sp, PT_SIZE
+	bnez		t1, .Ldirect
 	jr		t0
+.Ldirect:
+	jr		t1
 SYM_CODE_END(ftrace_common)
 
 SYM_CODE_START(ftrace_caller)
@@ -147,3 +152,9 @@ SYM_CODE_START(return_to_handler)
 	jr		ra
 SYM_CODE_END(return_to_handler)
 #endif
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+SYM_CODE_START(ftrace_stub_direct_tramp)
+	jr		t0
+SYM_CODE_END(ftrace_stub_direct_tramp)
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */