Re: [PATCH] fat: calculate data area start without overflow

[David Laight]

On Wed, 02 Sep 2026 21:03:26 +0900
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:

> hengyul@xxxxxxxxxx writes:
> 
> > From: Hengyu Liang <hengyul@xxxxxxxxxx>
> >
> > On 32-bit architectures, sbi->fat_length, sbi->dir_start and
> > sbi->data_start are unsigned long. The number of FATs is an 8-bit BPB
> > field, while the FAT32 length is a 32-bit BPB field. Therefore, the
> > calculation
> >
> >     sbi->fat_start + sbi->fats * sbi->fat_length
> >
> > can wrap before data_start is checked against total_sectors.  
> 
> [...]
> > -	sbi->dir_start = sbi->fat_start + sbi->fats * sbi->fat_length;
> >  	sbi->dir_entries = bpb.fat_dir_entries;
> >  	if (sbi->dir_entries & (sbi->dir_per_block - 1)) {
> >  		if (!silent)
> > @@ -1763,19 +1764,23 @@ int fat_fill_super(struct super_block *sb, struct fs_context *fc,
> >  
> >  	rootdir_sectors = sbi->dir_entries
> >  		* sizeof(struct msdos_dir_entry) / sb->s_blocksize;
> > -	sbi->data_start = sbi->dir_start + rootdir_sectors;
> > +	dir_start = sbi->fat_start +
> > +		mul_u32_u32(sbi->fats, sbi->fat_length);
> > +	data_start = dir_start + rootdir_sectors;  
> 
> Maybe, since u32 overflow is always bigger than total_sectors, so we
> should use check_*_overflow() instead, and detect early?

Why do the work twice?

David

> 
> Thanks.