Re: [PATCH v2 7/7] KVM: selftests: Avoid infinite loop if ucall_alloc() fails

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, Dec 09, 2022 at 09:03:45PM +0000, Sean Christopherson wrote:

[...]

> > -	GUEST_ASSERT(0);
> > +out:
> > +	/*
> > +	 * If the guest cannot grab a ucall structure from the pool then the
> > +	 * only option to get out to userspace is a bare ucall. This is probably
> > +	 * a good time to mention that guest assertions depend on ucalls with
> > +	 * arguments too.
> > +	 */
> > +	GUEST_UCALL_NONE();
> 
> UCALL_NONE isn't much better than infinite stack recursion, e.g. a test might end
> up passing by dumb luck, or go in the wrong direction because it sometimes handles
> UCALL_NONE.

Oh, I was just seeking an end to my misery. Yeah, we can use a magic
value to signal this instead.

> How about this?

LGTM.

--
Thanks,
Oliver

> From: Sean Christopherson <seanjc@google.com>
> Date: Fri, 9 Dec 2022 12:55:44 -0800
> Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
>  failure
> 
> Use a magic value to signal a ucall_alloc() failure instead of simply
> doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
> failure puts the guest into an infinite loop.
> 
> Use -1 as the magic value, as a real ucall struct should never wrap.
> 
> Reported-by: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..2f0e2ea941cc 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -4,6 +4,8 @@
>  #include "linux/bitmap.h"
>  #include "linux/atomic.h"
>  
> +#define GUEST_UCALL_FAILED -1
> +
>  struct ucall_header {
>  	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
>  	struct ucall ucalls[KVM_MAX_VCPUS];
> @@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
>  	struct ucall *uc;
>  	int i;
>  
> -	GUEST_ASSERT(ucall_pool);
> +	if (!ucall_pool)
> +		goto ucall_failed;
>  
>  	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>  		if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
>  		}
>  	}
>  
> -	GUEST_ASSERT(0);
> +ucall_failed:
> +	/*
> +	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
> +	 * magic value to signal to get_ucall() that things went sideways.
> +	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
> +	 */
> +	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
>  	return NULL;
>  }
>  
> @@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
>  
>  	addr = ucall_arch_get_ucall(vcpu);
>  	if (addr) {
> +		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
> +			    "Guest failed to allocate ucall struct");
> +
>  		memcpy(uc, addr, sizeof(*uc));
>  		vcpu_run_complete_io(vcpu);
>  	} else {
> 
> base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
> -- 
> 

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