linux-kernel - Re: [PATCH 1/6] ext4: sysfs: print ext4_super

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Message-Id: <4D58091B-6028-4E24-9EDD-0B4D53314FB8@dilger.ca>
Date:   Thu, 21 Jun 2018 11:49:08 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        y2038@...ts.linaro.org,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        stable@...r.kernel.org, Tyson Nottingham <tgnottingham@...il.com>,
        Riccardo Schirone <sirmy15@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] ext4: sysfs: print ext4_super_block fields as
 little-endian

On Jun 20, 2018, at 9:32 AM, Arnd Bergmann <arnd@...db.de> wrote:
> 
> While working on extended rand for last_error/first_error timestamps,
> I noticed that the endianess is wrong, we access the little-endian
> fields in struct ext4_super_block as native-endian when we print them.
> 
> This adds a special case in ext4_attr_show() and ext4_attr_store()
> to byteswap the superblock fields if needed.
> 
> In older kernels, this code was part of super.c, it got moved to sysfs.c
> in linux-4.4.
> 
> Cc: stable@...r.kernel.org
> Fixes: 52c198c6820f ("ext4: add sysfs entry showing whether the fs contains errors")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

I was wondering why this didn't just use le32_to_cpu() all the time,
but I see that these functions are being used for both ext4_super_block
(on-disk) fields, as well as ext4_sb_info (in-memory) fields.  A bit
ugly, but I don't think there is a better solution.

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

> ---
> fs/ext4/sysfs.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index f34da0bb8f17..b970a200f20c 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -274,8 +274,12 @@ static ssize_t ext4_attr_show(struct kobject *kobj,
> 	case attr_pointer_ui:
> 		if (!ptr)
> 			return 0;
> -		return snprintf(buf, PAGE_SIZE, "%u\n",
> -				*((unsigned int *) ptr));
> +		if (a->attr_ptr == ptr_ext4_super_block_offset)
> +			return snprintf(buf, PAGE_SIZE, "%u\n",
> +					le32_to_cpup(ptr));
> +		else
> +			return snprintf(buf, PAGE_SIZE, "%u\n",
> +					*((unsigned int *) ptr));
> 	case attr_pointer_atomic:
> 		if (!ptr)
> 			return 0;
> @@ -308,7 +312,10 @@ static ssize_t ext4_attr_store(struct kobject *kobj,
> 		ret = kstrtoul(skip_spaces(buf), 0, &t);
> 		if (ret)
> 			return ret;
> -		*((unsigned int *) ptr) = t;
> +		if (a->attr_ptr == ptr_ext4_super_block_offset)
> +			*((__le32 *) ptr) = cpu_to_le32(t);
> +		else
> +			*((unsigned int *) ptr) = t;
> 		return len;
> 	case attr_inode_readahead:
> 		return inode_readahead_blks_store(sbi, buf, len);
> --
> 2.9.0
> 


Cheers, Andreas






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