[PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

When the function tracer starts modifying the code via breakpoints
it sets a variable (modifying_ftrace_code) to inform the breakpoint
handler to call the ftrace int3 code.

But there's no synchronization between setting this code and the
handler, thus it is possible for the handler to be called on another
CPU before it sees the variable. This will cause a kernel crash as
the int3 handler will not know what to do with it.

I originally added smp_mb()'s to force the visibility of the variable
but H. Peter Anvin suggested that I just make it atomic.

Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 arch/x86/include/asm/ftrace.h |    2 +-
 arch/x86/kernel/ftrace.c      |    6 +++---
 arch/x86/kernel/traps.c       |    3 ++-
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index 18d9005..b0767bc 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -34,7 +34,7 @@
 
 #ifndef __ASSEMBLY__
 extern void mcount(void);
-extern int modifying_ftrace_code;
+extern atomic_t modifying_ftrace_code;
 
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
 {
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 32ff365..ea9904f 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -168,7 +168,7 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
 	return ret;
 }
 
-int modifying_ftrace_code __read_mostly;
+atomic_t modifying_ftrace_code __read_mostly;
 
 /*
  * A breakpoint was added to the code address we are about to
@@ -491,11 +491,11 @@ void ftrace_replace_code(int enable)
 
 void arch_ftrace_update_code(int command)
 {
-	modifying_ftrace_code++;
+	atomic_inc(&modifying_ftrace_code);
 
 	ftrace_modify_all_code(command);
 
-	modifying_ftrace_code--;
+	atomic_dec(&modifying_ftrace_code);
 }
 
 int __init ftrace_dyn_arch_init(void *data)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index ff08457..32443ae 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -304,7 +304,8 @@ dotraplinkage void __kprobes notrace do_int3(struct pt_regs *regs, long error_co
 {
 #ifdef CONFIG_DYNAMIC_FTRACE
 	/* ftrace must be first, everything else may cause a recursive crash */
-	if (unlikely(modifying_ftrace_code) && ftrace_int3_handler(regs))
+	if (unlikely(atomic_read(&modifying_ftrace_code)) &&
+	    ftrace_int3_handler(regs))
 		return;
 #endif
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
-- 
1.7.10

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