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

[Thomas Zimmermann <tzimmermann@suse.de>]

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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@intel.com>
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> 
> 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@ffwll.ch>
> 
> 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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