Re: [PATCH v2 14/15] RISC-V: KVM: add support for FWFT SBI extension

From: "Clément Léger" <cleger@rivosinc.com>
To: Deepak Gupta <debug@rivosinc.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Anup Patel <anup@brainfault.org>,
	Atish Patra <atishp@atishpatra.org>,
	Shuah Khan <shuah@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, kvm@vger.kernel.org,
	kvm-riscv@lists.infradead.org, linux-kselftest@vger.kernel.org,
	Samuel Holland <samuel.holland@sifive.com>
Subject: Re: [PATCH v2 14/15] RISC-V: KVM: add support for FWFT SBI extension
Date: Tue, 11 Feb 2025 11:31:28 +0100	[thread overview]
Message-ID: <a8af5d3a-e739-4f65-9e2d-d92978be9a3d@rivosinc.com> (raw)
In-Reply-To: <Z6rjkk5JlMlqbl2j@debug.ba.rivosinc.com>

On 11/02/2025 06:43, Deepak Gupta wrote:
>> +static int kvm_sbi_fwft_get(struct kvm_vcpu *vcpu, unsigned long
>> feature,
>> +                unsigned long *value)
>> +{
>> +    int ret;
>> +    struct kvm_sbi_fwft_config *conf;
>> +
>> +    ret = kvm_fwft_get_feature(vcpu, feature, &conf);
>> +    if (ret)
>> +        return ret;
>> +
>> +    return conf->feature->get(vcpu, conf, value);
>> +}
>> +
>> +static int kvm_sbi_ext_fwft_handler(struct kvm_vcpu *vcpu, struct
>> kvm_run *run,
>> +                    struct kvm_vcpu_sbi_return *retdata)
>> +{
>> +    int ret = 0;
>> +    struct kvm_cpu_context *cp = &vcpu->arch.guest_context;
>> +    unsigned long funcid = cp->a6;
>> +
>> +    switch (funcid) {
>> +    case SBI_EXT_FWFT_SET:
>> +        ret = kvm_sbi_fwft_set(vcpu, cp->a0, cp->a1, cp->a2);
>> +        break;
>> +    case SBI_EXT_FWFT_GET:
>> +        ret = kvm_sbi_fwft_get(vcpu, cp->a0, &retdata->out_val);
>> +        break;
>> +    default:
>> +        ret = SBI_ERR_NOT_SUPPORTED;
>> +        break;
>> +    }
>> +
>> +    retdata->err_val = ret;
>> +
>> +    return 0;
>> +}
>> +
>> +static int kvm_sbi_ext_fwft_init(struct kvm_vcpu *vcpu)
>> +{
>> +    struct kvm_sbi_fwft *fwft = vcpu_to_fwft(vcpu);
>> +    const struct kvm_sbi_fwft_feature *feature;
>> +    struct kvm_sbi_fwft_config *conf;
>> +    int i;
>> +
>> +    fwft->configs = kcalloc(ARRAY_SIZE(features), sizeof(struct
>> kvm_sbi_fwft_config),
>> +                GFP_KERNEL);
> nit:
> 
> I understand that in next patch you grow the static array`features`. But
> in this patch
> `ARRAY_SIZE(features)` evaluates to 0, thus kcalloc will be returning a
> pointer
> to some slab block (IIRC, kcalloc will not return NULL if size
> eventually evals to 0)
> 
> This probably won't result in some bad stuff. But still there is a
> pointer in
> fwft->configs which is pointing to some random stuff if `features` turns
> out to be
> empty.
> 
> Let me know if I got that right or missing something.

So I actually searched into the kmalloc code to see what hapopens with a
zero size allocation and it actually return ZERO_SIZE_PTR:

/*
 * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
 *
 * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault.
 *
 * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL
can.
 * Both make kfree a no-op.
 */

Which seems like it's not really random and will fault if accessed. I
think that's enough for that commit (which will be bisectable if needed
then).

Clément

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