media: vb2: frame_vector.c: don't overwrite error code

get_vaddr_frames() first calls pin_user_pages_fast() and if
that fails tries follow_pfn(). But if that also fails, then
the error code from pin_user_pages_fast() is overwritten with
the error code from follow_pfn().

Specifically if pin_user_pages_fast() returns -ENOMEM, then
follow_pfn() will overwrite that with -EINVAL, which is very
confusing.

So store the error code from pin_user_pages_fast() and return
that if follow_pfn() returns -EINVAL. -EINVAL indicates that
the page is unsuitable for follow_pfn, so pin_user_pages_fast()
was the correct call to make, and that error code should be
returned instead.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/common/videobuf2/frame_vector.c

@@ -37,6 +37,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
+	int ret_pin_user_pages_fast = 0;
 	int ret = 0;
 	int err;
 
@@ -56,6 +57,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
 		vec->is_pfns = false;
 		goto out_unlocked;
 	}
+	ret_pin_user_pages_fast = ret;
 
 	mmap_read_lock(mm);
 	vec->got_ref = false;
@@ -71,7 +73,18 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
 		while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
 			err = follow_pfn(vma, start, &nums[ret]);
 			if (err) {
-				if (ret == 0)
+				if (ret)
+					goto out;
+				// If follow_pfn() returns -EINVAL, then this
+				// is not an IO mapping or a raw PFN mapping.
+				// In that case, return the original error from
+				// pin_user_pages_fast(). Otherwise this
+				// function would return -EINVAL when
+				// pin_user_pages_fast() returned -ENOMEM,
+				// which makes debugging hard.
+				if (err == -EINVAL && ret_pin_user_pages_fast)
+					ret = ret_pin_user_pages_fast;
+				else
 					ret = err;
 				goto out;
 			}