Subject: Re: mmotm 2019-04-19-14-53 uploaded (objtool)

From: Peter Zijlstra <[email-masked]>
Date: Tue, 23 Apr 2019 10:24:48 +0200

On Fri, Apr 19, 2019 at 09:36:46PM -0700, Randy Dunlap wrote:
> On 4/19/19 2:53 PM, [email-masked] wrote:
> > The mm-of-the-moment snapshot 2019-04-19-14-53 has been uploaded to
> > 
> >    http://www.ozlabs.org/~akpm/mmotm/
> > 
> > mmotm-readme.txt says
> > 
> > README for mm-of-the-moment:
> > 
> > http://www.ozlabs.org/~akpm/mmotm/
> > 
> > This is a snapshot of my -mm patch queue.  Uploaded at random hopefully
> > more than once a week.
> 
> on x86_64:
> 
>   CC      lib/strncpy_from_user.o
> lib/strncpy_from_user.o: warning: objtool: strncpy_from_user()+0x315: call to __ubsan_handle_add_overflow() with UACCESS enabled
>   CC      lib/strnlen_user.o
> lib/strnlen_user.o: warning: objtool: strnlen_user()+0x337: call to __ubsan_handle_sub_overflow() with UACCESS enabled

Lemme guess, you're using GCC < 8 ? That had a bug where UBSAN
considered signed overflow UB when using -fno-strict-overflow or
-fwrapv.

Now, we could of course allow this symbol, but I found only the below
was required to make allyesconfig build without issue.

Andy, Linus?

(note: the __put_user thing is from this one:

  drivers/gpu/drm/i915/i915_gem_execbuffer.c:	if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {

 where (ptr) ends up non-trivial due to UBSAN)

---

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 22ba683afdc2..c82abd6e4ca3 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -427,10 +427,11 @@ do {									\
 ({								\
 	__label__ __pu_label;					\
 	int __pu_err = -EFAULT;					\
-	__typeof__(*(ptr)) __pu_val;				\
-	__pu_val = x;						\
+	__typeof__(*(ptr)) __pu_val = (x);			\
+	__typeof__(ptr) __pu_ptr = (ptr);			\
+	__typeof__(size) __pu_size = (size);			\
 	__uaccess_begin();					\
-	__put_user_size(__pu_val, (ptr), (size), __pu_label);	\
+	__put_user_size(__pu_val, __pu_ptr, __pu_size, __pu_label);	\
 	__pu_err = 0;						\
 __pu_label:							\
 	__uaccess_end();					\
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index 58eacd41526c..07045bc4872e 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -26,7 +26,7 @@
 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
 {
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
-	long res = 0;
+	unsigned long res = 0;
 
 	/*
 	 * Truncate 'max' to the user-specified limit, so that
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 1c1a1b0e38a5..0729378ad3e9 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -28,7 +28,7 @@
 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
 {
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
-	long align, res = 0;
+	unsigned long align, res = 0;
 	unsigned long c;
 
 	/*