Re: [PATCH RFC 09/17] KVM: selftests: Test rejection of swap tmpfs mounts

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] kselftests/kvm: incorrect fd validation in create_tmpfs_pool_fd()
--

commit 757011eabe5ad7927d1894ae72e08f8c628d45bc
Author: Ackerley Tng <ackerleytng@google.com>

KVM: selftests: Test rejection of swap tmpfs mounts

This commit adds a test to ensure that guest_memfd creation fails with
-EINVAL when passed a tmpfs resource pool mount descriptor that has swap
enabled.

> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index a7a1b5470cb2c..5d017164d7b45 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
[ ... ]
> @@ -477,6 +478,54 @@ static void test_resource_fd_tmpfs_file(struct kvm_vm *vm)
[ ... ]
> +static int create_tmpfs_pool_fd(const char *huge, bool noswap, size_t size)
> +{
> +	char size_str[32];
> +	int fs_fd, mnt_fd;
[ ... ]
> +	TEST_ASSERT(!syscall(__NR_fsconfig, fs_fd, FSCONFIG_CMD_CREATE,
> +			     NULL, NULL, 0), "fsconfig create failed");
> +
> +	mnt_fd = syscall(__NR_fsmount, fs_fd, FSMOUNT_CLOEXEC, 0);
> +	TEST_ASSERT(mnt_fd > 0, "fsmount failed");

[Severity: Low]
Does this code incorrectly reject valid file descriptors?

The fsmount syscall allocates and returns a new file descriptor. In POSIX
systems, 0 is a valid file descriptor and will be returned if it was
previously closed. The assertion incorrectly treats fd 0 as a failure.

While selftests typically run with standard file descriptors (0, 1, 2)
already open, making it very unlikely to trigger in practice, would it be
technically more correct to check for mnt_fd >= 0?

> +
> +	close(fs_fd);
> +	return mnt_fd;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260925-gmem-tmpfs-backend-v1-0-d36159822d18@google.com?part=9