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From:   Youling Tang <tangyouling@loongson.cn>
To:     Huacai Chen <chenhuacai@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Cc:     WANG Xuerui <kernel@xen0n.name>, linux-kernel@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org, loongarch@lists.linux.dev,
        loongson-kernel@lists.loongnix.cn
Subject: [PATCH 2/5] LoongArch: ftrace: Implement ftrace_find_callable_addr() to simplify code
Date:   Wed, 26 Apr 2023 09:49:11 +0800
Message-Id: <1682473754-25077-3-git-send-email-tangyouling@loongson.cn>
In-Reply-To: <1682473754-25077-1-git-send-email-tangyouling@loongson.cn>
References: <1682473754-25077-1-git-send-email-tangyouling@loongson.cn>
Precedence: bulk

In the module processing part, the same code is reused by implementing
ftrace_find_callable_addr().

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
---
 arch/loongarch/kernel/ftrace_dyn.c | 120 +++++++++++++++--------------
 1 file changed, 61 insertions(+), 59 deletions(-)

diff --git a/arch/loongarch/kernel/ftrace_dyn.c b/arch/loongarch/kernel/ftrace_dyn.c
index c5f4b4681ddc..3cc4f8159f48 100644
--- a/arch/loongarch/kernel/ftrace_dyn.c
+++ b/arch/loongarch/kernel/ftrace_dyn.c
@@ -31,16 +31,11 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new, bool validate)
 }
 
 #ifdef CONFIG_MODULES
-static inline int __get_mod(struct module **mod, unsigned long addr)
+static bool reachable_by_bl(unsigned long addr, unsigned long pc)
 {
-	preempt_disable();
-	*mod = __module_text_address(addr);
-	preempt_enable();
+	long offset = (long)addr - (long)pc;
 
-	if (WARN_ON(!(*mod)))
-		return -EINVAL;
-
-	return 0;
+	return offset >= -SZ_128M && offset < SZ_128M;
 }
 
 static struct plt_entry *get_ftrace_plt(struct module *mod, unsigned long addr)
@@ -56,18 +51,63 @@ static struct plt_entry *get_ftrace_plt(struct module *mod, unsigned long addr)
 	return NULL;
 }
 
-static unsigned long get_plt_addr(struct module *mod, unsigned long addr)
+/*
+ * Find the address the callsite must branch to in order to reach '*addr'.
+ *
+ * Due to the limited range of 'bl' instructions, modules may be placed too far
+ * away to branch directly and must use a PLT.
+ *
+ * Returns true when '*addr' contains a reachable target address, or has been
+ * modified to contain a PLT address. Returns false otherwise.
+ */
+static bool ftrace_find_callable_addr(struct dyn_ftrace *rec, struct module *mod,
+				      unsigned long *addr)
 {
+	unsigned long pc = rec->ip + LOONGARCH_INSN_SIZE;
 	struct plt_entry *plt;
 
-	plt = get_ftrace_plt(mod, addr);
+	/*
+	 * When the target is within range of the 'bl' instruction, use 'addr'
+	 * as-is and branch to that directly.
+	 */
+	if (reachable_by_bl(*addr, pc))
+		return true;
+
+	/*
+	 * 'mod' is only set at module load time, but if we end up
+	 * dealing with an out-of-range condition, we can assume it
+	 * is due to a module being loaded far away from the kernel.
+	 *
+	 * NOTE: __module_text_address() must be called with preemption
+	 * disabled, but we can rely on ftrace_lock to ensure that 'mod'
+	 * retains its validity throughout the remainder of this code.
+	 */
+	if (!mod) {
+		preempt_disable();
+		mod = __module_text_address(pc);
+		preempt_enable();
+	}
+
+	if (WARN_ON(!mod))
+		return false;
+
+	plt = get_ftrace_plt(mod, *addr);
 	if (!plt) {
-		pr_err("ftrace: no module PLT for %ps\n", (void *)addr);
-		return -EINVAL;
+		pr_err("ftrace: no module PLT for %ps\n", (void *)*addr);
+		return false;
 	}
 
-	return (unsigned long)plt;
+	*addr = (unsigned long)plt;
+	return true;
 }
+
+#else /* !CONFIG_MODULES */
+static bool ftrace_find_callable_addr(struct dyn_ftrace *rec, struct module *mod,
+				      unsigned long *addr)
+{
+	return true;
+}
+
 #endif
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
@@ -75,26 +115,14 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr, unsigned
 {
 	u32 old, new;
 	unsigned long pc;
-	long offset __maybe_unused;
 
 	pc = rec->ip + LOONGARCH_INSN_SIZE;
 
-#ifdef CONFIG_MODULES
-	offset = (long)pc - (long)addr;
-
-	if (offset < -SZ_128M || offset >= SZ_128M) {
-		int ret;
-		struct module *mod;
-
-		ret = __get_mod(&mod, pc);
-		if (ret)
-			return ret;
-
-		addr = get_plt_addr(mod, addr);
+	if (!ftrace_find_callable_addr(rec, NULL, &addr))
+		return -EINVAL;
 
-		old_addr = get_plt_addr(mod, old_addr);
-	}
-#endif
+	if (!ftrace_find_callable_addr(rec, NULL, &old_addr))
+		return -EINVAL;
 
 	new = larch_insn_gen_bl(pc, addr);
 	old = larch_insn_gen_bl(pc, old_addr);
@@ -151,24 +179,11 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 {
 	u32 old, new;
 	unsigned long pc;
-	long offset __maybe_unused;
 
 	pc = rec->ip + LOONGARCH_INSN_SIZE;
 
-#ifdef CONFIG_MODULES
-	offset = (long)pc - (long)addr;
-
-	if (offset < -SZ_128M || offset >= SZ_128M) {
-		int ret;
-		struct module *mod;
-
-		ret = __get_mod(&mod, pc);
-		if (ret)
-			return ret;
-
-		addr = get_plt_addr(mod, addr);
-	}
-#endif
+	if (!ftrace_find_callable_addr(rec, NULL, &addr))
+		return -EINVAL;
 
 	old = larch_insn_gen_nop();
 	new = larch_insn_gen_bl(pc, addr);
@@ -180,24 +195,11 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long ad
 {
 	u32 old, new;
 	unsigned long pc;
-	long offset __maybe_unused;
 
 	pc = rec->ip + LOONGARCH_INSN_SIZE;
 
-#ifdef CONFIG_MODULES
-	offset = (long)pc - (long)addr;
-
-	if (offset < -SZ_128M || offset >= SZ_128M) {
-		int ret;
-		struct module *mod;
-
-		ret = __get_mod(&mod, pc);
-		if (ret)
-			return ret;
-
-		addr = get_plt_addr(mod, addr);
-	}
-#endif
+	if (!ftrace_find_callable_addr(rec, NULL, &addr))
+		return -EINVAL;
 
 	new = larch_insn_gen_nop();
 	old = larch_insn_gen_bl(pc, addr);
-- 
2.37.1