Re: [PATCH v2] fbdev: fix incorrect address computation in deferred IO

[Thomas Zimmermann]

Hi

Am 23.04.24 um 13:50 schrieb Nam Cao:
> With deferred IO enabled, a page fault happens when data is written to the
> framebuffer device. Then driver determines which page is being updated by
> calculating the offset of the written virtual address within the virtual
> memory area, and uses this offset to get the updated page within the
> internal buffer. This page is later copied to hardware (thus the name
> "deferred IO").
>
> This offset calculation is only correct if the virtual memory area is
> mapped to the beginning of the internal buffer. Otherwise this is wrong.
> For example, if users do:
>      mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0xff000);
>
> Then the virtual memory area will mapped at offset 0xff000 within the
> internal buffer. This offset 0xff000 is not accounted for, and wrong page
> is updated.
>
> Correct the calculation by using vmf->pgoff instead. With this change, the
> variable "offset" will no longer hold the exact offset value, but it is
> rounded down to multiples of PAGE_SIZE. But this is still correct, because
> this variable is only used to calculate the page offset.
>
> Reported-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-fbdev/271372d6-e665-4e7f-b088-dee5f4ab341a@xxxxxxxxxx
> Fixes: 56c134f7f1b5 ("fbdev: Track deferred-I/O pages in pageref struct")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>

Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Thank you so much. I'll take care of merging the patch later this week.

Best regards
Thomas

> ---
> v2:
>    - simplify the patch by using vfg->pgoff
>    - remove tested-by tag, as the patch is now different
>
>   drivers/video/fbdev/core/fb_defio.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> index 1ae1d35a5942..b9607d5a370d 100644
> --- a/drivers/video/fbdev/core/fb_defio.c
> +++ b/drivers/video/fbdev/core/fb_defio.c
> @@ -196,7 +196,7 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long
>    */
>   static vm_fault_t fb_deferred_io_page_mkwrite(struct fb_info *info, struct vm_fault *vmf)
>   {
> -	unsigned long offset = vmf->address - vmf->vma->vm_start;
> +	unsigned long offset = vmf->pgoff << PAGE_SHIFT;
>   	struct page *page = vmf->page;
>   
>   	file_update_time(vmf->vma->vm_file);

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)