On 21.11.2017 20:10, Christian Borntraeger wrote:
> Old kernels did not check for zero in the irq_state.flags field and old
> QEMUs did not zero the flag/reserved fields when calling
> KVM_S390_*_IRQ_STATE. Let's add comments to prevent future uses of
> these fields.
>
> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
> Documentation/virtual/kvm/api.txt | 9 +++++++--
> arch/s390/kvm/kvm-s390.c | 2 ++
> include/uapi/linux/kvm.h | 4 ++--
> 3 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index f670e4b..e84c1c3 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2901,14 +2901,19 @@ userspace buffer and its length:
>
> struct kvm_s390_irq_state {
> __u64 buf;
> - __u32 flags;
> + __u32 flags; /* will be unused for compatibility reasons */
> __u32 len;
> - __u32 reserved[4];
> + __u32 reserved[4]; /* will be unused for compatibility reasons */
> };
>
> Userspace passes in the above struct and for each pending interrupt a
> struct kvm_s390_irq is copied to the provided buffer.
>
> +The structure contains a flags and a reserved field for future extensions. As
> +the kernel never checked for flags == 0 and QEMU never pre-zeroed flags and
> +reserved, these fields can not be used in the future without breaking
> +compatibility.
> +
> If -ENOBUFS is returned the buffer provided was too small and userspace
> may retry with a bigger buffer.
>
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 6a5e02f..2a9cc0f 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3834,6 +3834,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> r = -EINVAL;
> break;
> }
> + /* do not use irq_state.flags, it will break old QEMUs */
> r = kvm_s390_set_irq_state(vcpu,
> (void __user *) irq_state.buf,
> irq_state.len);
> @@ -3849,6 +3850,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> r = -EINVAL;
> break;
> }
> + /* do not use irq_state.flags, it will break old QEMUs */
> r = kvm_s390_get_irq_state(vcpu,
> (__u8 __user *) irq_state.buf,
> irq_state.len);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 282d7613..6f91b41 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -630,9 +630,9 @@ struct kvm_s390_irq {
>
> struct kvm_s390_irq_state {
> __u64 buf;
> - __u32 flags;
> + __u32 flags; /* will be unused for compatibility reasons */
> __u32 len;
> - __u32 reserved[4];
> + __u32 reserved[4]; /* will be unused for compatibility reasons */
> };
>
> /* for KVM_SET_GUEST_DEBUG */
>
Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
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