arm64: ftrace: Add dynamic ftrace support

This patch allows "dynamic ftrace" if CONFIG_DYNAMIC_FTRACE is enabled.
Here we can turn on and off tracing dynamically per-function base.

On arm64, this is done by patching single branch instruction to _mcount()
inserted by gcc -pg option. The branch is replaced to NOP initially at
kernel start up, and later on, NOP to branch to ftrace_caller() when
enabled or branch to NOP when disabled.
Please note that ftrace_caller() is a counterpart of _mcount() in case of
'static' ftrace.

More details on architecture specific requirements are described in
Documentation/trace/ftrace-design.txt.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

4 files changed, 171 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 8a25a4ad3159..4e689ad5f27c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -36,6 +36,7 @@ config ARM64
         select HAVE_DMA_API_DEBUG
         select HAVE_DMA_ATTRS
         select HAVE_DMA_CONTIGUOUS
+        select HAVE_DYNAMIC_FTRACE
         select HAVE_EFFICIENT_UNALIGNED_ACCESS
         select HAVE_FTRACE_MCOUNT_RECORD
         select HAVE_FUNCTION_TRACER
diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 58ea5951b198..ed5c448ece99 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -18,6 +18,21 @@
 
 #ifndef __ASSEMBLY__
 extern void _mcount(unsigned long);
+
+struct dyn_arch_ftrace {
+        /* No extra data needed for arm64 */
+};
+
+extern unsigned long ftrace_graph_call;
+
+static inline unsigned long ftrace_call_adjust(unsigned long addr)
+{
+        /*
+         * addr is the address of the mcount call instruction.
+         * recordmcount does the necessary offset calculation.
+         */
+        return addr;
+}
 #endif /* __ASSEMBLY__ */
 
 #endif /* __ASM_FTRACE_H */
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index b2d8c4559cd8..b051871f2965 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -86,6 +86,7 @@
         add     \reg, \reg, #8
         .endm
 
+#ifndef CONFIG_DYNAMIC_FTRACE
 /*
  * void _mcount(unsigned long return_address)
  * @return_address: return address to instrumented function
@@ -134,6 +135,48 @@ skip_ftrace_call:
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 ENDPROC(_mcount)
 
+#else /* CONFIG_DYNAMIC_FTRACE */
+/*
+ * _mcount() is used to build the kernel with -pg option, but all the branch
+ * instructions to _mcount() are replaced to NOP initially at kernel start up,
+ * and later on, NOP to branch to ftrace_caller() when enabled or branch to
+ * NOP when disabled per-function base.
+ */
+ENTRY(_mcount)
+        ret
+ENDPROC(_mcount)
+
+/*
+ * void ftrace_caller(unsigned long return_address)
+ * @return_address: return address to instrumented function
+ *
+ * This function is a counterpart of _mcount() in 'static' ftrace, and
+ * makes calls to:
+ *     - tracer function to probe instrumented function's entry,
+ *     - ftrace_graph_caller to set up an exit hook
+ */
+ENTRY(ftrace_caller)
+        mcount_enter
+
+        mcount_get_pc0  x0              //     function's pc
+        mcount_get_lr   x1              //     function's lr
+
+        .global ftrace_call
+ftrace_call:                            // tracer(pc, lr);
+        nop                             // This will be replaced with "bl xxx"
+                                        // where xxx can be any kind of tracer.
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+        .global ftrace_graph_call
+ftrace_graph_call:                      // ftrace_graph_caller();
+        nop                             // If enabled, this will be replaced
+                                        // "b ftrace_graph_caller"
+#endif
+
+        mcount_exit
+ENDPROC(ftrace_caller)
+#endif /* CONFIG_DYNAMIC_FTRACE */
+
 ENTRY(ftrace_stub)
         ret
 ENDPROC(ftrace_stub)
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index a559ab86999b..7924d73b6476 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -17,6 +17,87 @@
 #include <asm/ftrace.h>
 #include <asm/insn.h>
 
+#ifdef CONFIG_DYNAMIC_FTRACE
+/*
+ * Replace a single instruction, which may be a branch or NOP.
+ * If @validate == true, a replaced instruction is checked against 'old'.
+ */
+static int ftrace_modify_code(unsigned long pc, u32 old, u32 new,
+                              bool validate)
+{
+        u32 replaced;
+
+        /*
+         * Note:
+         * Due to modules and __init, code can disappear and change,
+         * we need to protect against faulting as well as code changing.
+         * We do this by aarch64_insn_*() which use the probe_kernel_*().
+         *
+         * No lock is held here because all the modifications are run
+         * through stop_machine().
+         */
+        if (validate) {
+                if (aarch64_insn_read((void *)pc, &replaced))
+                        return -EFAULT;
+
+                if (replaced != old)
+                        return -EINVAL;
+        }
+        if (aarch64_insn_patch_text_nosync((void *)pc, new))
+                return -EPERM;
+
+        return 0;
+}
+
+/*
+ * Replace tracer function in ftrace_caller()
+ */
+int ftrace_update_ftrace_func(ftrace_func_t func)
+{
+        unsigned long pc;
+        u32 new;
+
+        pc = (unsigned long)&ftrace_call;
+        new = aarch64_insn_gen_branch_imm(pc, (unsigned long)func, true);
+
+        return ftrace_modify_code(pc, 0, new, false);
+}
+
+/*
+ * Turn on the call to ftrace_caller() in instrumented function
+ */
+int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+        unsigned long pc = rec->ip;
+        u32 old, new;
+
+        old = aarch64_insn_gen_nop();
+        new = aarch64_insn_gen_branch_imm(pc, addr, true);
+
+        return ftrace_modify_code(pc, old, new, true);
+}
+
+/*
+ * Turn off the call to ftrace_caller() in instrumented function
+ */
+int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
+                    unsigned long addr)
+{
+        unsigned long pc = rec->ip;
+        u32 old, new;
+
+        old = aarch64_insn_gen_branch_imm(pc, addr, true);
+        new = aarch64_insn_gen_nop();
+
+        return ftrace_modify_code(pc, old, new, true);
+}
+
+int __init ftrace_dyn_arch_init(void)
+{
+        return 0;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 /*
  * function_graph tracer expects ftrace_return_to_handler() to be called
@@ -61,4 +142,35 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
                 return;
         }
 }
+
+#ifdef CONFIG_DYNAMIC_FTRACE
+/*
+ * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
+ * depending on @enable.
+ */
+static int ftrace_modify_graph_caller(bool enable)
+{
+        unsigned long pc = (unsigned long)&ftrace_graph_call;
+        u32 branch, nop;
+
+        branch = aarch64_insn_gen_branch_imm(pc,
+                        (unsigned long)ftrace_graph_caller, false);
+        nop = aarch64_insn_gen_nop();
+
+        if (enable)
+                return ftrace_modify_code(pc, nop, branch, true);
+        else
+                return ftrace_modify_code(pc, branch, nop, true);
+}
+
+int ftrace_enable_ftrace_graph_caller(void)
+{
+        return ftrace_modify_graph_caller(true);
+}
+
+int ftrace_disable_ftrace_graph_caller(void)
+{
+        return ftrace_modify_graph_caller(false);
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */