[PATCH 1/6] fbdev: Return number of bytes read or written

[Thomas Zimmermann]

Always return the number of bytes read or written within the
framebuffer. Only return an errno code if framebuffer memory
was not touched. This is the semantics required by POSIX and
makes fb_read() and fb_write() compatible with IGT tests. [1]

This bug has been fixed for fb_write() long ago by
commit 6a2a88668e90 ("[PATCH] fbdev: Fix return error of
fb_write"). The code in fb_read() and the corresponding fb_sys_()
helpers was forgotten.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
Link: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/fbdev.c # 1
---
 drivers/video/fbdev/core/fb_sys_fops.c | 24 ++++++++++++++----------
 drivers/video/fbdev/core/fbmem.c       |  2 +-
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/video/fbdev/core/fb_sys_fops.c b/drivers/video/fbdev/core/fb_sys_fops.c
index ff275d7f3eaf..cefb77b9546d 100644
--- a/drivers/video/fbdev/core/fb_sys_fops.c
+++ b/drivers/video/fbdev/core/fb_sys_fops.c
@@ -19,7 +19,8 @@ ssize_t fb_sys_read(struct fb_info *info, char __user *buf, size_t count,
 	unsigned long p = *ppos;
 	void *src;
 	int err = 0;
-	unsigned long total_size;
+	unsigned long total_size, c;
+	ssize_t ret;
 
 	if (info->state != FBINFO_STATE_RUNNING)
 		return -EPERM;
@@ -43,13 +44,14 @@ ssize_t fb_sys_read(struct fb_info *info, char __user *buf, size_t count,
 	if (info->fbops->fb_sync)
 		info->fbops->fb_sync(info);
 
-	if (copy_to_user(buf, src, count))
+	c = copy_to_user(buf, src, count);
+	if (c)
 		err = -EFAULT;
+	ret = count - c;
 
-	if  (!err)
-		*ppos += count;
+	*ppos += ret;
 
-	return (err) ? err : count;
+	return ret ? ret : err;
 }
 EXPORT_SYMBOL_GPL(fb_sys_read);
 
@@ -59,7 +61,8 @@ ssize_t fb_sys_write(struct fb_info *info, const char __user *buf,
 	unsigned long p = *ppos;
 	void *dst;
 	int err = 0;
-	unsigned long total_size;
+	unsigned long total_size, c;
+	size_t ret;
 
 	if (info->state != FBINFO_STATE_RUNNING)
 		return -EPERM;
@@ -89,13 +92,14 @@ ssize_t fb_sys_write(struct fb_info *info, const char __user *buf,
 	if (info->fbops->fb_sync)
 		info->fbops->fb_sync(info);
 
-	if (copy_from_user(dst, buf, count))
+	c = copy_from_user(dst, buf, count);
+	if (c)
 		err = -EFAULT;
+	ret = count - c;
 
-	if  (!err)
-		*ppos += count;
+	*ppos += ret;
 
-	return (err) ? err : count;
+	return ret ? ret : err;
 }
 EXPORT_SYMBOL_GPL(fb_sys_write);
 
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 3fd95a79e4c3..abc92d79a295 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -820,7 +820,7 @@ fb_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 
 	kfree(buffer);
 
-	return (err) ? err : cnt;
+	return cnt ? cnt : err;
 }
 
 static ssize_t
-- 
2.40.0