linux-ext4 - Re: [PATCH 2/4] ext4: Initialize timestamps limits

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Message-Id: <802B4CD4-953D-4E0F-9B22-1350BF91EDD3@dilger.ca>
Date:   Tue, 16 Apr 2019 12:17:23 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Deepa Dinamani <deepa.kernel@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Theodore Ts'o <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/4] ext4: Initialize timestamps limits

On Apr 16, 2019, at 8:34 AM, Deepa Dinamani <deepa.kernel@...il.com> wrote:
> 
> ext4 has different overflow limits for max filesystem
> timestamps based on the extra bytes available.
> 
> The timestamp limits are calculated according to the
> encoding table in
> a4dad1ae24f85i(ext4: Fix handling of extended tv_sec):
> 
> * extra  msb of                         adjust for signed
> * epoch  32-bit                         32-bit tv_sec to
> * bits   time    decoded 64-bit tv_sec  64-bit tv_sec      valid time range
> * 0 0    1    -0x80000000..-0x00000001  0x000000000   1901-12-13..1969-12-31
> * 0 0    0    0x000000000..0x07fffffff  0x000000000   1970-01-01..2038-01-19
> * 0 1    1    0x080000000..0x0ffffffff  0x100000000   2038-01-19..2106-02-07
> * 0 1    0    0x100000000..0x17fffffff  0x100000000   2106-02-07..2174-02-25
> * 1 0    1    0x180000000..0x1ffffffff  0x200000000   2174-02-25..2242-03-16
> * 1 0    0    0x200000000..0x27fffffff  0x200000000   2242-03-16..2310-04-04
> * 1 1    1    0x280000000..0x2ffffffff  0x300000000   2310-04-04..2378-04-22
> * 1 1    0    0x300000000..0x37fffffff  0x300000000   2378-04-22..2446-05-10
> 
> Note that the time limits are not correct for deletion times.
> 
> Signed-off-by: Deepa Dinamani <deepa.kernel@...il.com>
> Cc: "Theodore Ts'o" <tytso@....edu>
> Cc: Andreas Dilger <adilger.kernel@...ger.ca>
> Cc: linux-ext4@...r.kernel.org

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> #define EXT4_GOOD_OLD_INODE_SIZE 128
> 
> +#define EXT4_EXTRA_TIMESTAMP_MAX	(((s64)1 << 34) - 1  + S32_MIN)
> +#define EXT4_NON_EXTRA_TIMESTAMP_MAX	S32_MAX
> +#define EXT4_TIMESTAMP_MIN		S32_MIN
> +
> /*
>  * Feature set definitions
>  */
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index f5b828bf1299..3a63a23219a8 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3930,8 +3930,20 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> 			       sbi->s_inode_size);
> 			goto failed_mount;
> 		}
> -		if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE)
> -			sb->s_time_gran = 1 << (EXT4_EPOCH_BITS - 2);
> +		/* i_atime_extra is the last extra field available for [acm]times in
> +		 * struct ext4_inode. Checking for that field should suffice to ensure
> +		 * we have extra spaces for all three.

There was never a time when only i_ctime_extra or i_mtime_extra was present in the
inode, so this should be totally safe.  The code may check for them independently
for correctness reasons (e.g. in case of some corruption of the on-disk inode or
whatever), but for a global state like this it is totally safe to check all three
of the fields based on the presence of i_atime_extra.

> +		 */
> +		if (sbi->s_inode_size >= offsetof(struct ext4_inode, i_atime_extra) +
> +			sizeof(((struct ext4_inode *)0)->i_atime_extra)) {
> +			sb->s_time_gran = 1;
> +			sb->s_time_max = EXT4_EXTRA_TIMESTAMP_MAX;
> +		} else {
> +			sb->s_time_gran = 0;
> +			sb->s_time_max = EXT4_NON_EXTRA_TIMESTAMP_MAX;
> +		}
> +
> +		sb->s_time_min = EXT4_TIMESTAMP_MIN;
> 	}
> 
> 	sbi->s_desc_size = le16_to_cpu(es->s_desc_size);
> --
> 2.17.1
> 


Cheers, Andreas






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