Re: [PATCH] KVM: selftests: Fix the never-true negative UFFD delay check

[Gautam Menghani <gautam@linux.ibm.com>]

On Mon, Sep 07, 2026 at 04:52:46AM +0000, Chaithanya Lagisetty wrote:
> demand_paging_test parses the -d option with strtoul() and then asserts
> that the result is not negative:
> 
> 	p.uffd_delay = strtoul(optarg, NULL, 0);
> 	TEST_ASSERT(p.uffd_delay >= 0, "A negative UFFD delay is not supported.");
> 
> p.uffd_delay is a useconds_t, which is an unsigned type, so the comparison
> is always true and the assertion can never fire. GCC points this out with
> -Wtype-limits, which is enabled by -Wextra.
> 
> As a result, "-d -1" is accepted and converted to a very large unsigned
> delay, causing each demand paging fault to sleep for an unexpectedly long
> time in usleep() instead of rejecting the argument up front. The return
> value of strtoul() is not validated either, so a non-numeric argument
> such as "-d abc" is silently treated as a zero delay.
> 
> Use atoi_non_negative() instead. It rejects negative values, unparsable
> input, and trailing garbage. It is already used a few lines below for -v,
> and hexadecimal input keeps working because atoi_paranoid() also passes a
> base of 0 to strtol().
> 
> Fixes: 0119cb365c93 ("KVM: selftests: Add configurable demand paging delay")
> Signed-off-by: Chaithanya Lagisetty <nagachaithanya9911@gmail.com>
> ---
>  tools/testing/selftests/kvm/demand_paging_test.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
> index f8b3d0b68830..619a0b2be45c 100644
> --- a/tools/testing/selftests/kvm/demand_paging_test.c
> +++ b/tools/testing/selftests/kvm/demand_paging_test.c
> @@ -297,8 +297,7 @@ int main(int argc, char *argv[])
>  			p.single_uffd = true;
>  			break;
>  		case 'd':
> -			p.uffd_delay = strtoul(optarg, NULL, 0);
> -			TEST_ASSERT(p.uffd_delay >= 0, "A negative UFFD delay is not supported.");
> +			p.uffd_delay = atoi_non_negative("UFFD delay", optarg);
>  			break;
>  		case 'b':
>  			guest_percpu_mem_size = parse_size(optarg);
> -- 
> 2.43.0
> 

With this patch applied, the test fails as intended:

# ./demand_paging_test -d -1
Random seed: 0x236c0ed9

==== Test Assertion Failure ====
  include/test_util.h:236: num >= 0
  pid=3674565 tid=3674565 errno=0 - Success
     1  0x00000000004010d0: atoi_non_negative at test_util.h:236
     2   (inlined by) main at demand_paging_test.c:300
     3  0x00007f907d9f05b4: ?? ??:0
     4  0x00007f907d9f0667: ?? ??:0
     5  0x0000000000401374: _start at ??:?
  UFFD delay must be non-negative, got '-1'


Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>