ftrace/x86/extable: Add is_ftrace_trampoline() function

Stack traces that happen from function tracing check if the address
on the stack is a __kernel_text_address(). That is, is the address
kernel code. This calls core_kernel_text() which returns true
if the address is part of the builtin kernel code. It also calls
is_module_text_address() which returns true if the address belongs
to module code.

But what is missing is ftrace dynamically allocated trampolines.
These trampolines are allocated for individual ftrace_ops that
call the ftrace_ops callback functions directly. But if they do a
stack trace, the code checking the stack wont detect them as they
are neither core kernel code nor module address space.

Adding another field to ftrace_ops that also stores the size of
the trampoline assigned to it we can create a new function called
is_ftrace_trampoline() that returns true if the address is a
dynamically allocate ftrace trampoline. Note, it ignores trampolines
that are not dynamically allocated as they will return true with
the core_kernel_text() function.

Link: http://lkml.kernel.org/r/20141119034829.497125839@goodmis.org

Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

4 files changed, 59 insertions, 3 deletions

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 1aea94d336c7..60881d919432 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -712,7 +712,8 @@ union ftrace_op_code_union {
         } __attribute__((packed));
 };
 
-static unsigned long create_trampoline(struct ftrace_ops *ops)
+static unsigned long
+create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
 {
         unsigned const char *jmp;
         unsigned long start_offset;
@@ -749,6 +750,8 @@ static unsigned long create_trampoline(struct ftrace_ops *ops)
         if (!trampoline)
                 return 0;
 
+        *tramp_size = size + MCOUNT_INSN_SIZE + sizeof(void *);
+
         /* Copy ftrace_caller onto the trampoline memory */
         ret = probe_kernel_read(trampoline, (void *)start_offset, size);
         if (WARN_ON(ret < 0)) {
@@ -819,6 +822,7 @@ void arch_ftrace_update_trampoline(struct ftrace_ops *ops)
         unsigned char *new;
         unsigned long offset;
         unsigned long ip;
+        unsigned int size;
         int ret;
 
         if (ops->trampoline) {
@@ -829,9 +833,10 @@ void arch_ftrace_update_trampoline(struct ftrace_ops *ops)
                 if (!(ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP))
                         return;
         } else {
-                ops->trampoline = create_trampoline(ops);
+                ops->trampoline = create_trampoline(ops, &size);
                 if (!ops->trampoline)
                         return;
+                ops->trampoline_size = size;
         }
 
         offset = calc_trampoline_call_offset(ops->flags & FTRACE_OPS_FL_SAVE_REGS);
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 619e37cc17fd..7b2616fa2472 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -150,6 +150,7 @@ struct ftrace_ops {
         struct ftrace_ops_hash          *func_hash;
         struct ftrace_ops_hash          old_hash;
         unsigned long                   trampoline;
+        unsigned long                   trampoline_size;
 #endif
 };
 
@@ -297,6 +298,8 @@ extern int ftrace_text_reserved(const void *start, const void *end);
 
 extern int ftrace_nr_registered_ops(void);
 
+bool is_ftrace_trampoline(unsigned long addr);
+
 /*
  * The dyn_ftrace record's flags field is split into two parts.
  * the first part which is '0-FTRACE_REF_MAX' is a counter of
@@ -596,6 +599,11 @@ static inline ssize_t ftrace_notrace_write(struct file *file, const char __user
                              size_t cnt, loff_t *ppos) { return -ENODEV; }
 static inline int
 ftrace_regex_release(struct inode *inode, struct file *file) { return -ENODEV; }
+
+static inline bool is_ftrace_trampoline(unsigned long addr)
+{
+        return false;
+}
 #endif /* CONFIG_DYNAMIC_FTRACE */
 
 /* totally disable ftrace - can not re-enable after this */
diff --git a/kernel/extable.c b/kernel/extable.c
index d8a6446adbcb..c98f926277a8 100644
--- a/kernel/extable.c
+++ b/kernel/extable.c
@@ -18,6 +18,7 @@
 #include <linux/ftrace.h>
 #include <linux/memory.h>
 #include <linux/module.h>
+#include <linux/ftrace.h>
 #include <linux/mutex.h>
 #include <linux/init.h>
 
@@ -102,6 +103,8 @@ int __kernel_text_address(unsigned long addr)
                 return 1;
         if (is_module_text_address(addr))
                 return 1;
+        if (is_ftrace_trampoline(addr))
+                return 1;
         /*
          * There might be init symbols in saved stacktraces.
          * Give those symbols a chance to be printed in
@@ -119,7 +122,9 @@ int kernel_text_address(unsigned long addr)
 {
         if (core_kernel_text(addr))
                 return 1;
-        return is_module_text_address(addr);
+        if (is_module_text_address(addr))
+                return 1;
+        return is_ftrace_trampoline(addr);
 }
 
 /*
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 6233f9102179..fa0f36bb32e9 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1117,6 +1117,43 @@ static struct ftrace_ops global_ops = {
                                           FTRACE_OPS_FL_INITIALIZED,
 };
 
+/*
+ * This is used by __kernel_text_address() to return true if the
+ * the address is on a dynamically allocated trampoline that would
+ * not return true for either core_kernel_text() or
+ * is_module_text_address().
+ */
+bool is_ftrace_trampoline(unsigned long addr)
+{
+        struct ftrace_ops *op;
+        bool ret = false;
+
+        /*
+         * Some of the ops may be dynamically allocated,
+         * they are freed after a synchronize_sched().
+         */
+        preempt_disable_notrace();
+
+        do_for_each_ftrace_op(op, ftrace_ops_list) {
+                /*
+                 * This is to check for dynamically allocated trampolines.
+                 * Trampolines that are in kernel text will have
+                 * core_kernel_text() return true.
+                 */
+                if (op->trampoline && op->trampoline_size)
+                        if (addr >= op->trampoline &&
+                            addr < op->trampoline + op->trampoline_size) {
+                                ret = true;
+                                goto out;
+                        }
+        } while_for_each_ftrace_op(op);
+
+ out:
+        preempt_enable_notrace();
+
+        return ret;
+}
+
 struct ftrace_page {
         struct ftrace_page      *next;
         struct dyn_ftrace       *records;
@@ -5373,6 +5410,7 @@ static struct ftrace_ops graph_ops = {
                                    FTRACE_OPS_FL_STUB,
 #ifdef FTRACE_GRAPH_TRAMP_ADDR
         .trampoline             = FTRACE_GRAPH_TRAMP_ADDR,
+        /* trampoline_size is only needed for dynamically allocated tramps */
 #endif
         ASSIGN_OPS_HASH(graph_ops, &global_ops.local_hash)
 };