linux-kernel - Re: [PATCH v4 06/13] KVM: Move WARN on invalid memslot index to update

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Date:   Wed, 18 Aug 2021 16:35:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 06/13] KVM: Move WARN on invalid memslot index to
 update_memslots()

On 13.08.21 21:33, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> 
> Since kvm_memslot_move_forward() can theoretically return a negative
> memslot index even when kvm_memslot_move_backward() returned a positive one
> (and so did not WARN) let's just move the warning to the common code.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> ---
>   virt/kvm/kvm_main.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 03ef42d2e421..7000efff1425 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1293,8 +1293,7 @@ static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
>   	struct kvm_memory_slot *mslots = slots->memslots;
>   	int i;
>   
> -	if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
> -	    WARN_ON_ONCE(!slots->used_slots))
> +	if (slots->id_to_index[memslot->id] == -1 || !slots->used_slots)
>   		return -1;
>   
>   	/*
> @@ -1398,6 +1397,9 @@ static void update_memslots(struct kvm_memslots *slots,
>   			i = kvm_memslot_move_backward(slots, memslot);
>   		i = kvm_memslot_move_forward(slots, memslot, i);
>   
> +		if (WARN_ON_ONCE(i < 0))
> +			return;
> +
>   		/*
>   		 * Copy the memslot to its new position in memslots and update
>   		 * its index accordingly.
> 


Note that WARN_ON_* is frowned upon, because it can result in crashes 
with panic_on_warn enabled, which is what some distributions do enable.

We tend to work around that by using pr_warn()/pr_warn_once(), avoiding 
eventually crashing the system when there is a way to continue.

-- 
Thanks,

David / dhildenb