Re: arch/s390/kvm/gaccess.c:859 guest_range_to_gpas() error: uninitialized symbol 'prot'.

[Janosch Frank <frankja@linux.ibm.com>]

On 8/17/22 16:38, Claudio Imbrenda wrote:
> On Wed, 17 Aug 2022 12:30:11 +0200
> Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:
> 
>> On 8/15/22 09:23, Dan Carpenter wrote:
>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>>> head:   69dac8e431af26173ca0a1ebc87054e01c585bcc
>>> commit: 7faa543df19bf62d4583a64d3902705747f2ad29 KVM: s390: gaccess: Refactor access address range check
>>> config: s390-randconfig-m031-20220812 (https://download.01.org/0day-ci/archive/20220814/202208140141.4uzWszG4-lkp@intel.com/config)
>>> compiler: s390-linux-gcc (GCC) 12.1.0
>>>
>>> If you fix the issue, kindly add following tag where applicable
>>> Reported-by: kernel test robot <lkp@intel.com>
>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>>>
>>> smatch warnings:
>>> arch/s390/kvm/gaccess.c:859 guest_range_to_gpas() error: uninitialized symbol 'prot'.
>>>
>>> vim +/prot +859 arch/s390/kvm/gaccess.c
>>>
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  831  static int guest_range_to_gpas(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  832  			       unsigned long *gpas, unsigned long len,
>>> 92c9632119b67f David Hildenbrand        2015-11-16  833  			       const union asce asce, enum gacc_mode mode)
>>> 2293897805c2fe Heiko Carstens           2014-01-01  834  {
>>> 2293897805c2fe Heiko Carstens           2014-01-01  835  	psw_t *psw = &vcpu->arch.sie_block->gpsw;
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  836  	unsigned int offset = offset_in_page(ga);
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  837  	unsigned int fragment_len;
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  838  	int lap_enabled, rc = 0;
>>> 6ae1574c2a24ee Christian Borntraeger    2017-06-07  839  	enum prot_type prot;
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  840  	unsigned long gpa;
>>> 2293897805c2fe Heiko Carstens           2014-01-01  841
>>> 75a1812230ad7a Alexander Yarygin        2015-01-22  842  	lap_enabled = low_address_protection_enabled(vcpu, asce);
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  843  	while (min(PAGE_SIZE - offset, len) > 0) {
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  844  		fragment_len = min(PAGE_SIZE - offset, len);
>>> 2293897805c2fe Heiko Carstens           2014-01-01  845  		ga = kvm_s390_logical_to_effective(vcpu, ga);
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  846  		if (mode == GACC_STORE && lap_enabled && is_low_address(ga))
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  847  			return trans_exc(vcpu, PGM_PROTECTION, ga, ar, mode,
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  848  					 PROT_TYPE_LA);
>>> a752598254016d Heiko Carstens           2017-06-03  849  		if (psw_bits(*psw).dat) {
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  850  			rc = guest_translate(vcpu, ga, &gpa, asce, mode, &prot);
>>> 2293897805c2fe Heiko Carstens           2014-01-01  851  			if (rc < 0)
>>> 2293897805c2fe Heiko Carstens           2014-01-01  852  				return rc;
>>> 2293897805c2fe Heiko Carstens           2014-01-01  853  		} else {
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  854  			gpa = kvm_s390_real_to_abs(vcpu, ga);
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  855  			if (kvm_is_error_gpa(vcpu->kvm, gpa))
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  856  				rc = PGM_ADDRESSING;
>>>                                                                                          ^^^^^^^^^^^^^^^^^^^^
>>>
>>> "rc" set but "prot" not initialized.
>>
>> prot is only used in case of PGM_PROTECTION.
>> Because of that, we could initialize prot to an arbitrary value, but that seems misleading and a bit ugly to me.
>> Alternatively, we could introduce a PROT_NONE.
>> Or we do nothing, since there is no actual problem.
>> @Janosch, @Claudio, what do you think?
> 
> I agree that this is not a bug.
> 
> It does look ugly, though, and all reasonable solutions are also ugly
> (each for a different reason).
> 
> another solution is to set prot to an arbitrary value only in the if
> case marked above. that way prot is not arbitrarily initialized, and
> there is no need to add a new PROT_NONE (which then would need to be
> checked for in trans_exc)
> 
> I do not have a strong opinion, though

We either have a prot value or terminate is set, right?

So I'd opt to add a PROT_NONE (with a comment that it's only used for 
init). If the statement above is true we can then also add a WARN if no 
valid PROT_* is set in case if terminate isn't true and a PGM_PROTECTION 
has been specified.


> 
>>
>>>
>>> 2293897805c2fe Heiko Carstens           2014-01-01  857  		}
>>> cde0dcfb5df1db David Hildenbrand        2016-05-31  858  		if (rc)
>>> 6ae1574c2a24ee Christian Borntraeger    2017-06-07 @859  			return trans_exc(vcpu, rc, ga, ar, mode, prot);
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  860  		if (gpas)
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  861  			*gpas++ = gpa;
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  862  		offset = 0;
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  863  		ga += fragment_len;
>>> 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  864  		len -= fragment_len;
>>> 2293897805c2fe Heiko Carstens           2014-01-01  865  	}
>>> 2293897805c2fe Heiko Carstens           2014-01-01  866  	return 0;
>>> 2293897805c2fe Heiko Carstens           2014-01-01  867  }
>>>    
>>
>