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

Stephen Rothwell
2020-04-13T00:01:25Z

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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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