Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow protection

[Josh Poimboeuf]

On Tue, Jul 18, 2017 at 05:03:34PM -0700, Kees Cook wrote:
> +/*
> + * Body of refcount error handling: in .text.unlikely, saved into CX the
> + * address of the refcount that has entered a bad state, and trigger an
> + * exception. Fixup address is back in regular execution flow in .text.
> + */
> +#define _REFCOUNT_EXCEPTION				\
> +	".pushsection .text.unlikely\n"			\
> +	"111:\tlea %[counter], %%" _ASM_CX "\n"		\
> +	"112:\t" ASM_UD0 "\n"				\
> +	".popsection\n"					\
> +	"113:\n"					\
> +	_ASM_EXTABLE_REFCOUNT(112b, 113b)

This confuses the freshly merged objtool 2.0, which is now too smart for
its own good.  It's reporting some errors like:

  >> kernel/sched/autogroup.o: warning: objtool: sched_autogroup_exit()+0x48: return with modified stack frame
  >> kernel/sched/autogroup.o: warning: objtool: .text.unlikely+0x27: stack state mismatch: reg1[3]=-2-40 reg2[3]=-2-24
  >> kernel/sched/autogroup.o: warning: objtool: sched_autogroup_exit()+0x14: stack state mismatch: reg1[3]=-2-40 reg2[3]=-2-24

Because the UD instructions are used for both WARN and BUG, objtool
doesn't know whether control flow continues past the instruction.  So in
cases like this, it needs an "unreachable" annotation.

Here's a patch to fix it, feel free to squash it into yours:


diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
index 13b91e850a02..e7587db3487c 100644
--- a/arch/x86/include/asm/refcount.h
+++ b/arch/x86/include/asm/refcount.h
@@ -15,6 +15,7 @@
 	".pushsection .text.unlikely\n"			\
 	"111:\tlea %[counter], %%" _ASM_CX "\n"		\
 	"112:\t" ASM_UD0 "\n"				\
+	ASM_UNREACHABLE					\
 	".popsection\n"					\
 	"113:\n"					\
 	_ASM_EXTABLE_REFCOUNT(112b, 113b)
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index cd4bbe8242bd..85e0b8f42ca0 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -202,15 +202,25 @@
 #endif
 
 #ifdef CONFIG_STACK_VALIDATION
+
 #define annotate_unreachable() ({					\
 	asm("%c0:\t\n"							\
-	    ".pushsection .discard.unreachable\t\n"			\
-	    ".long %c0b - .\t\n"					\
-	    ".popsection\t\n" : : "i" (__LINE__));			\
+	    ".pushsection .discard.unreachable\n\t"			\
+	    ".long %c0b - .\n\t"					\
+	    ".popsection\n\t" : : "i" (__LINE__));			\
 })
+
+#define ASM_UNREACHABLE							\
+	"999: .pushsection .discard.unreachable\n\t"			\
+	".long 999b - .\n\t"						\
+	".popsection\n\t"
+
 #else
+
 #define annotate_unreachable()
-#endif
+#define ASM_UNREACHABLE
+
+#endif /* CONFIG_STACK_VALIDATION */
 
 /*
  * Mark a position in code as unreachable.  This can be used to