linux-ext4 - Re: [PATCH] ext4: Fix loff_t overflow in ext4_max_bitmap

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Date:   Sat, 19 Jun 2021 07:11:44 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: [PATCH] ext4: Fix loff_t overflow in ext4_max_bitmap_size()

On 21/06/07 12:00PM, Jan Kara wrote:
> On Sat 05-06-21 10:39:32, Ritesh Harjani wrote:
> > We should use unsigned long long rather than loff_t to avoid
> > overflow in ext4_max_bitmap_size() for comparison before returning.
> > w/o this patch sbi->s_bitmap_maxbytes was becoming a negative
> > value due to overflow of upper_limit (with has_huge_files as true)
> >
> > Below is a quick test to trigger it on a 64KB pagesize system.
> >
> > sudo mkfs.ext4 -b 65536 -O ^has_extents,^64bit /dev/loop2
> > sudo mount /dev/loop2 /mnt
> > sudo echo "hello" > /mnt/hello 	-> This will error out with
> > 				"echo: write error: File too large"
> >
> > Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
>
> OK, this works (although it's really tight ;). Won't it be somewhat safer
> if we compared upper_limit and res before shifting both by blocksize_bits
> to the left? Basically we need to shift only for comparison with
> MAX_LFS_FILESIZE which is in bytes... But either way feel free to add:

Yes, at 1st I did think that, but since for comparing "res" against
MAX_LFS_FILESIZE we will be anyway doing the bit shifting and since this logic
too was (just) fitting into the limits so I thought of keeping it the same.
But, if absolutely required, I can make those changes.

>
> Reviewed-by: Jan Kara <jack@...e.cz>


Thanks for the review :)

-ritesh

>
> 								Honza
>
> > ---
> >  fs/ext4/super.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 7dc94f3e18e6..bedb66386966 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -3189,17 +3189,17 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
> >   */
> >  static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
> >  {
> > -	loff_t res = EXT4_NDIR_BLOCKS;
> > +	unsigned long long upper_limit, res = EXT4_NDIR_BLOCKS;
> >  	int meta_blocks;
> > -	loff_t upper_limit;
> > -	/* This is calculated to be the largest file size for a dense, block
> > +
> > +	/*
> > +	 * This is calculated to be the largest file size for a dense, block
> >  	 * mapped file such that the file's total number of 512-byte sectors,
> >  	 * including data and all indirect blocks, does not exceed (2^48 - 1).
> >  	 *
> >  	 * __u32 i_blocks_lo and _u16 i_blocks_high represent the total
> >  	 * number of 512-byte sectors of the file.
> >  	 */
> > -
> >  	if (!has_huge_files) {
> >  		/*
> >  		 * !has_huge_files or implies that the inode i_block field
> > @@ -3242,7 +3242,7 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
> >  	if (res > MAX_LFS_FILESIZE)
> >  		res = MAX_LFS_FILESIZE;
> >
> > -	return res;
> > +	return (loff_t)res;
> >  }
> >
> >  static ext4_fsblk_t descriptor_loc(struct super_block *sb,
> > --
> > 2.31.1
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR