linux-next: build warning after merge of the tip tree

linux-next: build warning after merge of the tip tree

[Stephen Rothwell]

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Hi all,

After merging the tip tree, today's linux-next build (arm
multi_v7_defconfig) produced this warning:

kernel/futex.c: In function 'do_futex':
kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
 1676 |   return oldval == cmparg;
      |          ~~~~~~~^~~~~~~~~
kernel/futex.c:1652:6: note: 'oldval' was declared here
 1652 |  int oldval, ret;
      |      ^~~~~~

Introduced by commit

  a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling conventions change")

but I don't see how it makes this difference :-(

-- 
Cheers,
Stephen Rothwell

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Re: linux-next: build warning after merge of the tip tree

[Stephen Rothwell]

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Hi all,

On Mon, 30 Mar 2020 13:47:46 +1100 Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>
> Hi all,
> 
> After merging the tip tree, today's linux-next build (arm
> multi_v7_defconfig) produced this warning:
> 
> kernel/futex.c: In function 'do_futex':
> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
>  1676 |   return oldval == cmparg;
>       |          ~~~~~~~^~~~~~~~~
> kernel/futex.c:1652:6: note: 'oldval' was declared here
>  1652 |  int oldval, ret;
>       |      ^~~~~~
> 
> Introduced by commit
> 
>   a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling conventions change")
> 
> but I don't see how it makes this difference :-(

I now get this warning from building Linus' tree :-(

-- 
Cheers,
Stephen Rothwell

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Re: linux-next: build warning after merge of the tip tree

[Thomas Gleixner]

Stephen Rothwell <sfr@canb.auug.org.au> writes:
> Hi all,
>
> On Mon, 30 Mar 2020 13:47:46 +1100 Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>>
>> Hi all,
>> 
>> After merging the tip tree, today's linux-next build (arm
>> multi_v7_defconfig) produced this warning:
>> 
>> kernel/futex.c: In function 'do_futex':
>> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
>>  1676 |   return oldval == cmparg;
>>       |          ~~~~~~~^~~~~~~~~
>> kernel/futex.c:1652:6: note: 'oldval' was declared here
>>  1652 |  int oldval, ret;
>>       |      ^~~~~~
>> 
>> Introduced by commit
>> 
>>   a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling
>>   conventions change")

Huch?
 
>> but I don't see how it makes this difference :-(

Me neither. Which compiler version?

I'm using arm-linux-gnueabi-gcc (Debian 8.3.0-2) 8.3.0 which does not
show that oddity.

Thanks,

        tglx

Re: linux-next: build warning after merge of the tip tree

[Stephen Rothwell]

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Hi Thomas,

On Wed, 01 Apr 2020 12:25:25 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Stephen Rothwell <sfr@canb.auug.org.au> writes:
> >
> > On Mon, 30 Mar 2020 13:47:46 +1100 Stephen Rothwell <sfr@canb.auug.org.au> wrote:  
> >>
> >> After merging the tip tree, today's linux-next build (arm
> >> multi_v7_defconfig) produced this warning:
> >> 
> >> kernel/futex.c: In function 'do_futex':
> >> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
> >>  1676 |   return oldval == cmparg;
> >>       |          ~~~~~~~^~~~~~~~~
> >> kernel/futex.c:1652:6: note: 'oldval' was declared here
> >>  1652 |  int oldval, ret;
> >>       |      ^~~~~~
> >> 
> >> Introduced by commit
> >> 
> >>   a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling
> >>   conventions change")  
> 
> Huch?
>  
> >> but I don't arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130see how it makes this difference :-(  
> 
> Me neither. Which compiler version?

arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130

> I'm using arm-linux-gnueabi-gcc (Debian 8.3.0-2) 8.3.0 which does not
> show that oddity.

I assume it is because of the change to arch_futex_atomic_op_inuser()
for arm and the compiler is not clever enough to work out that the early
return from arch_futex_atomic_op_inuser() means that oldval is not
referenced in its caller.

-- 
Cheers,
Stephen Rothwell

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Re: linux-next: build warning after merge of the tip tree

[Thomas Gleixner]

Stephen,

Stephen Rothwell <sfr@canb.auug.org.au> writes:
> On Wed, 01 Apr 2020 12:25:25 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:
>> Me neither. Which compiler version?
>
> arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130
>
>> I'm using arm-linux-gnueabi-gcc (Debian 8.3.0-2) 8.3.0 which does not
>> show that oddity.
>
> I assume it is because of the change to arch_futex_atomic_op_inuser()
> for arm and the compiler is not clever enough to work out that the early
> return from arch_futex_atomic_op_inuser() means that oldval is not
> referenced in its caller.

Actually no. It's the ASM part which causes this. With the following
hack applied it compiles:

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index e133da303a98..2c6b40f71009 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -132,7 +132,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 static inline int
 arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 {
-	int oldval = 0, ret, tmp;
+	int oldval = 0, ret;
 
 	if (!access_ok(uaddr, sizeof(u32)))
 		return -EFAULT;
@@ -142,6 +142,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 #endif
 
 	switch (op) {
+#if 0
 	case FUTEX_OP_SET:
 		__futex_atomic_op("mov	%0, %4", ret, oldval, tmp, uaddr, oparg);
 		break;
@@ -157,6 +158,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 	case FUTEX_OP_XOR:
 		__futex_atomic_op("eor	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
 		break;
+#endif
 	default:
 		ret = -ENOSYS;
 	}

but with this is emits the warning:

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index e133da303a98..5191d7b61b83 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -145,6 +145,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 	case FUTEX_OP_SET:
 		__futex_atomic_op("mov	%0, %4", ret, oldval, tmp, uaddr, oparg);
 		break;
+#if 0
 	case FUTEX_OP_ADD:
 		__futex_atomic_op("add	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
 		break;
@@ -157,6 +158,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 	case FUTEX_OP_XOR:
 		__futex_atomic_op("eor	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
 		break;
+#endif
 	default:
 		ret = -ENOSYS;
 	}

and the below proves it:

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index e133da303a98..a9151884bc85 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -165,8 +165,13 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 	preempt_enable();
 #endif
 
-	if (!ret)
-		*oval = oldval;
+	/*
+	 * Store unconditionally. If ret != 0 the extra store is the least
+	 * of the worries but GCC cannot figure out that __futex_atomic_op()
+	 * is either setting ret to -EFAULT or storing the old value in
+	 * oldval which results in a uninitialized warning at the call site.
+	 */
+	*oval = oldval;
 
 	return ret;
 }

I think that's the right thing to do anyway. The conditional is pointless.

Thanks,

        tglx

Re: linux-next: build warning after merge of the tip tree

[Stephen Rothwell]

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Hi Thomas,

On Thu, 02 Apr 2020 00:39:55 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Stephen Rothwell <sfr@canb.auug.org.au> writes:
> > On Wed, 01 Apr 2020 12:25:25 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:  
> >> Me neither. Which compiler version?  
> >
> > arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130
> >  
> >> I'm using arm-linux-gnueabi-gcc (Debian 8.3.0-2) 8.3.0 which does not
> >> show that oddity.  
> >
> > I assume it is because of the change to arch_futex_atomic_op_inuser()
> > for arm and the compiler is not clever enough to work out that the early
> > return from arch_futex_atomic_op_inuser() means that oldval is not
> > referenced in its caller.  
> 
> Actually no. It's the ASM part which causes this. With the following
> hack applied it compiles:
> 
> diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
> index e133da303a98..2c6b40f71009 100644
> --- a/arch/arm/include/asm/futex.h
> +++ b/arch/arm/include/asm/futex.h
> @@ -132,7 +132,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
>  static inline int
>  arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  {
> -	int oldval = 0, ret, tmp;
> +	int oldval = 0, ret;
>  
>  	if (!access_ok(uaddr, sizeof(u32)))
>  		return -EFAULT;
> @@ -142,6 +142,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  #endif
>  
>  	switch (op) {
> +#if 0
>  	case FUTEX_OP_SET:
>  		__futex_atomic_op("mov	%0, %4", ret, oldval, tmp, uaddr, oparg);
>  		break;
> @@ -157,6 +158,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  	case FUTEX_OP_XOR:
>  		__futex_atomic_op("eor	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
>  		break;
> +#endif
>  	default:
>  		ret = -ENOSYS;
>  	}
> 
> but with this is emits the warning:
> 
> diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
> index e133da303a98..5191d7b61b83 100644
> --- a/arch/arm/include/asm/futex.h
> +++ b/arch/arm/include/asm/futex.h
> @@ -145,6 +145,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  	case FUTEX_OP_SET:
>  		__futex_atomic_op("mov	%0, %4", ret, oldval, tmp, uaddr, oparg);
>  		break;
> +#if 0
>  	case FUTEX_OP_ADD:
>  		__futex_atomic_op("add	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
>  		break;
> @@ -157,6 +158,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  	case FUTEX_OP_XOR:
>  		__futex_atomic_op("eor	%0, %1, %4", ret, oldval, tmp, uaddr, oparg);
>  		break;
> +#endif
>  	default:
>  		ret = -ENOSYS;
>  	}
> 
> and the below proves it:
> 
> diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
> index e133da303a98..a9151884bc85 100644
> --- a/arch/arm/include/asm/futex.h
> +++ b/arch/arm/include/asm/futex.h
> @@ -165,8 +165,13 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>  	preempt_enable();
>  #endif
>  
> -	if (!ret)
> -		*oval = oldval;
> +	/*
> +	 * Store unconditionally. If ret != 0 the extra store is the least
> +	 * of the worries but GCC cannot figure out that __futex_atomic_op()
> +	 * is either setting ret to -EFAULT or storing the old value in
> +	 * oldval which results in a uninitialized warning at the call site.
> +	 */
> +	*oval = oldval;
>  
>  	return ret;
>  }
> 
> I think that's the right thing to do anyway. The conditional is pointless.

Thanks for the analysis.

I am still getting this warning, now from Linus' tree builds.

-- 
Cheers,
Stephen Rothwell

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Re: linux-next: build warning after merge of the tip tree

[Thomas Gleixner]

Stephen,

Stephen Rothwell <sfr@canb.auug.org.au> writes:
> On Thu, 02 Apr 2020 00:39:55 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:
>> and the below proves it:
>> 
>> diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
>> index e133da303a98..a9151884bc85 100644
>> --- a/arch/arm/include/asm/futex.h
>> +++ b/arch/arm/include/asm/futex.h
>> @@ -165,8 +165,13 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
>>  	preempt_enable();
>>  #endif
>>  
>> -	if (!ret)
>> -		*oval = oldval;
>> +	/*
>> +	 * Store unconditionally. If ret != 0 the extra store is the least
>> +	 * of the worries but GCC cannot figure out that __futex_atomic_op()
>> +	 * is either setting ret to -EFAULT or storing the old value in
>> +	 * oldval which results in a uninitialized warning at the call site.
>> +	 */
>> +	*oval = oldval;
>>  
>>  	return ret;
>>  }
>> 
>> I think that's the right thing to do anyway. The conditional is pointless.
>
> Thanks for the analysis.
>
> I am still getting this warning, now from Linus' tree builds.

Yeah. Just noticed that both of us failed to CC the arm folks. :(

Let me send out a proper patch.

Thanks,

        tglx

ARM: futex: Address build warning

[Thomas Gleixner]

Stephen reported the following build warning on a ARM multi_v7_defconfig
build with GCC 9.2.1:

kernel/futex.c: In function 'do_futex':
kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
 1676 |   return oldval == cmparg;
      |          ~~~~~~~^~~~~~~~~
kernel/futex.c:1652:6: note: 'oldval' was declared here
 1652 |  int oldval, ret;
      |      ^~~~~~

introduced by commit a08971e9488d ("futex: arch_futex_atomic_op_inuser()
calling conventions change").

While that change should not make any difference it confuses GCC which
fails to work out that oldval is not referenced when the return value is
not zero.

GCC fails to properly analyze arch_futex_atomic_op_inuser(). It's not the
early return, the issue is with the assembly macros. GCC fails to detect
that those either set 'ret' to 0 and set oldval or set 'ret' to -EFAULT
which makes oldval uninteresting. The store to the callsite supplied oldval
pointer is conditional on ret == 0.

The straight forward way to solve this is to make the store unconditional.

Aside of addressing the build warning this makes sense anyway because it
removes the conditional from the fastpath. In the error case the stored
value is uninteresting and the extra store does not matter at all.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/874ku2q18k.fsf@nanos.tec.linutronix.de
---
 arch/arm/include/asm/futex.h |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -165,8 +165,13 @@ arch_futex_atomic_op_inuser(int op, int
 	preempt_enable();
 #endif
 
-	if (!ret)
-		*oval = oldval;
+	/*
+	 * Store unconditionally. If ret != 0 the extra store is the least
+	 * of the worries but GCC cannot figure out that __futex_atomic_op()
+	 * is either setting ret to -EFAULT or storing the old value in
+	 * oldval which results in a uninitialized warning at the call site.
+	 */
+	*oval = oldval;
 
 	return ret;
 }

Re: ARM: futex: Address build warning

[Stephen Rothwell]

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Hi all,

On Tue, 14 Apr 2020 11:07:22 +0200 Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Stephen reported the following build warning on a ARM multi_v7_defconfig
> build with GCC 9.2.1:
> 
> kernel/futex.c: In function 'do_futex':
> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
>  1676 |   return oldval == cmparg;
>       |          ~~~~~~~^~~~~~~~~
> kernel/futex.c:1652:6: note: 'oldval' was declared here
>  1652 |  int oldval, ret;
>       |      ^~~~~~
> 
> introduced by commit a08971e9488d ("futex: arch_futex_atomic_op_inuser()
> calling conventions change").
> 
> While that change should not make any difference it confuses GCC which
> fails to work out that oldval is not referenced when the return value is
> not zero.
> 
> GCC fails to properly analyze arch_futex_atomic_op_inuser(). It's not the
> early return, the issue is with the assembly macros. GCC fails to detect
> that those either set 'ret' to 0 and set oldval or set 'ret' to -EFAULT
> which makes oldval uninteresting. The store to the callsite supplied oldval
> pointer is conditional on ret == 0.
> 
> The straight forward way to solve this is to make the store unconditional.
> 
> Aside of addressing the build warning this makes sense anyway because it
> removes the conditional from the fastpath. In the error case the stored
> value is uninteresting and the extra store does not matter at all.
> 
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lore.kernel.org/r/874ku2q18k.fsf@nanos.tec.linutronix.de
> ---
>  arch/arm/include/asm/futex.h |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> --- a/arch/arm/include/asm/futex.h
> +++ b/arch/arm/include/asm/futex.h
> @@ -165,8 +165,13 @@ arch_futex_atomic_op_inuser(int op, int
>  	preempt_enable();
>  #endif
>  
> -	if (!ret)
> -		*oval = oldval;
> +	/*
> +	 * Store unconditionally. If ret != 0 the extra store is the least
> +	 * of the worries but GCC cannot figure out that __futex_atomic_op()
> +	 * is either setting ret to -EFAULT or storing the old value in
> +	 * oldval which results in a uninitialized warning at the call site.
> +	 */
> +	*oval = oldval;
>  
>  	return ret;
>  }

Any response to this?  I am still getting the warning ...

The warning was introduced by commit

  a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling conventions change")

Which has been in Linus' tree since before v5.7-rc1.  Should this go in
via the tip tree, the arm tree, or will I just send ti to Linus myself?

-- 
Cheers,
Stephen Rothwell

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