KVM: selftests: Use pread() to read binary stats header

Use pread() with an explicit offset when reading the header and the header
name for a binary stats fd so that the common helper and the binary stats
test don't subtly rely on the file effectively being untouched, e.g. to
allow multiple reads of the header, name, etc.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230711230131.648752-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/include/kvm_util_base.h

@@ -362,8 +362,10 @@ static inline void read_stats_header(int stats_fd, struct kvm_stats_header *head
 {
 	ssize_t ret;
 
-	ret = read(stats_fd, header, sizeof(*header));
-	TEST_ASSERT(ret == sizeof(*header), "Read stats header");
+	ret = pread(stats_fd, header, sizeof(*header), 0);
+	TEST_ASSERT(ret == sizeof(*header),
+		    "Failed to read '%lu' header bytes, ret = '%ld'",
+		    sizeof(*header), ret);
 }
 
 struct kvm_stats_desc *read_stats_descriptors(int stats_fd,

tools/testing/selftests/kvm/kvm_binary_stats_test.c

@@ -43,8 +43,10 @@ static void stats_test(int stats_fd)
 	id = malloc(header.name_size);
 	TEST_ASSERT(id, "Allocate memory for id string");
 
-	ret = read(stats_fd, id, header.name_size);
-	TEST_ASSERT(ret == header.name_size, "Read id string");
+	ret = pread(stats_fd, id, header.name_size, sizeof(header));
+	TEST_ASSERT(ret == header.name_size,
+		    "Expected header size '%u', read '%lu' bytes",
+		    header.name_size, ret);
 
 	/* Check id string, that should start with "kvm" */
 	TEST_ASSERT(!strncmp(id, "kvm", 3) && strlen(id) < header.name_size,