Re: [PATCH v2] fbdev: Use helper to get fb_info in all file operations

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Hi

Am 04.05.22 um 11:02 schrieb Daniel Vetter:
On Tue, May 03, 2022 at 10:19:34PM +0200, Javier Martinez Canillas wrote:
A reference to the framebuffer device struct fb_info is stored in the file
private data, but this reference could no longer be valid and must not be
accessed directly. Instead, the file_fb_info() accessor function must be
used since it does sanity checking to make sure that the fb_info is valid.

This can happen for example if the registered framebuffer device is for a
driver that just uses a framebuffer provided by the system firmware. In
that case, the fbdev core would unregister the framebuffer device when a
real video driver is probed and ask to remove conflicting framebuffers.

Most fbdev file operations already use the helper to get the fb_info but
get_fb_unmapped_area() and fb_deferred_io_fsync() don't. Fix those two.

Since fb_deferred_io_fsync() is not in fbmem.o, the helper has to be
exported. Rename it and add a fb_ prefix to denote that is public now.

Reported-by: Junxiao Chang <junxiao.chang@xxxxxxxxx>
Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>

Note that fb_file_info is hilariously racy since there's nothing
preventing a concurrenct framebuffer_unregister. Or at least I'm not
seeing anything. See cf4a3ae4ef33 ("fbdev: lock_fb_info cannot fail") for
context, maybe reference that commit here in your patch.

Can we acquire registration_lock around [1]?

[1] https://elixir.bootlin.com/linux/v5.17.5/source/drivers/video/fbdev/core/fbmem.c#L756


Either way this doesn't really make anything worse, so
Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

Cheers, Daniel

---

Changes in v2:
- Fix copy & paste error passing file->private_data instead of file
   to fb_file_fb_info() function (Sam Ravnborg).

  drivers/video/fbdev/core/fb_defio.c |  5 ++++-
  drivers/video/fbdev/core/fbmem.c    | 24 +++++++++++++++---------
  include/linux/fb.h                  |  1 +
  3 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index 842c66b3e33d..ccdf903c48bd 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -68,12 +68,15 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
int fb_deferred_io_fsync(struct file *file, loff_t start, loff_t end, int datasync)
  {
-	struct fb_info *info = file->private_data;
+	struct fb_info *info = fb_file_fb_info(file);
  	struct inode *inode = file_inode(file);
  	int err = file_write_and_wait_range(file, start, end);
  	if (err)
  		return err;
+ if (!info)
+		return -ENODEV;
+
  	/* Skip if deferred io is compiled-in but disabled on this fbdev */
  	if (!info->fbdefio)
  		return 0;
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 97eb0dee411c..ba2c14a1087d 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -745,7 +745,7 @@ static const struct seq_operations __maybe_unused proc_fb_seq_ops = {
   * So look up the fb_info using the inode minor number,
   * and just verify it against the reference we have.
   */
-static struct fb_info *file_fb_info(struct file *file)
+struct fb_info *fb_file_fb_info(struct file *file)
  {
  	struct inode *inode = file_inode(file);
  	int fbidx = iminor(inode);
@@ -755,12 +755,13 @@ static struct fb_info *file_fb_info(struct file *file)
  		info = NULL;
  	return info;
  }
+EXPORT_SYMBOL(fb_file_fb_info);
static ssize_t
  fb_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
  {
  	unsigned long p = *ppos;
-	struct fb_info *info = file_fb_info(file);
+	struct fb_info *info = fb_file_fb_info(file);
  	u8 *buffer, *dst;
  	u8 __iomem *src;
  	int c, cnt = 0, err = 0;
@@ -825,7 +826,7 @@ static ssize_t
  fb_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
  {
  	unsigned long p = *ppos;
-	struct fb_info *info = file_fb_info(file);
+	struct fb_info *info = fb_file_fb_info(file);
  	u8 *buffer, *src;
  	u8 __iomem *dst;
  	int c, cnt = 0, err = 0;
@@ -1181,7 +1182,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
static long fb_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
  {
-	struct fb_info *info = file_fb_info(file);
+	struct fb_info *info = fb_file_fb_info(file);
if (!info)
  		return -ENODEV;
@@ -1293,7 +1294,7 @@ static int fb_get_fscreeninfo(struct fb_info *info, unsigned int cmd,
  static long fb_compat_ioctl(struct file *file, unsigned int cmd,
  			    unsigned long arg)
  {
-	struct fb_info *info = file_fb_info(file);
+	struct fb_info *info = fb_file_fb_info(file);
  	const struct fb_ops *fb;
  	long ret = -ENOIOCTLCMD;
@@ -1333,7 +1334,7 @@ static long fb_compat_ioctl(struct file *file, unsigned int cmd,
  static int
  fb_mmap(struct file *file, struct vm_area_struct * vma)
  {
-	struct fb_info *info = file_fb_info(file);
+	struct fb_info *info = fb_file_fb_info(file);
  	int (*fb_mmap_fn)(struct fb_info *info, struct vm_area_struct *vma);
  	unsigned long mmio_pgoff;
  	unsigned long start;
@@ -1434,7 +1435,7 @@ fb_release(struct inode *inode, struct file *file)
  __acquires(&info->lock)
  __releases(&info->lock)
  {
-	struct fb_info * const info = file_fb_info(file);
+	struct fb_info * const info = fb_file_fb_info(file);
if (!info)
  		return -ENODEV;
@@ -1453,8 +1454,13 @@ unsigned long get_fb_unmapped_area(struct file *filp,
  				   unsigned long addr, unsigned long len,
  				   unsigned long pgoff, unsigned long flags)
  {
-	struct fb_info * const info = filp->private_data;
-	unsigned long fb_size = PAGE_ALIGN(info->fix.smem_len);
+	struct fb_info * const info = fb_file_fb_info(filp);
+	unsigned long fb_size;
+
+	if (!info)
+		return -ENODEV;
+
+	fb_size = PAGE_ALIGN(info->fix.smem_len);
if (pgoff > fb_size || len > fb_size - pgoff)
  		return -EINVAL;
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 9a77ab615c36..3004b8b8c5c2 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -624,6 +624,7 @@ extern int fb_get_color_depth(struct fb_var_screeninfo *var,
  			      struct fb_fix_screeninfo *fix);
  extern int fb_get_options(const char *name, char **option);
  extern int fb_new_modelist(struct fb_info *info);
+extern struct fb_info *fb_file_fb_info(struct file *file);
extern struct fb_info *registered_fb[FB_MAX];
  extern int num_registered_fb;
--
2.35.1



--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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