Date: Wed, 21 Jun 2023 14:19:56 -0400
From: Tom Talpey <tom@...pey.com>
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Subject: Re: [PATCH 01/79] fs: add ctime accessors infrastructure

On 6/21/2023 2:01 PM, Jeff Layton wrote:
> On Wed, 2023-06-21 at 13:29 -0400, Tom Talpey wrote:
>> On 6/21/2023 10:45 AM, Jeff Layton wrote:
>>> struct timespec64 has unused bits in the tv_nsec field that can be used
>>> for other purposes. In future patches, we're going to change how the
>>> inode->i_ctime is accessed in certain inodes in order to make use of
>>> them. In order to do that safely though, we'll need to eradicate raw
>>> accesses of the inode->i_ctime field from the kernel.
>>>
>>> Add new accessor functions for the ctime that we can use to replace them.
>>>
>>> Signed-off-by: Jeff Layton <jlayton@...nel.org>
>>> ---
>>>    fs/inode.c         | 16 ++++++++++++++
>>>    include/linux/fs.h | 53 +++++++++++++++++++++++++++++++++++++++++++++-
>>>    2 files changed, 68 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/inode.c b/fs/inode.c
>>> index d37fad91c8da..c005e7328fbb 100644
>>> --- a/fs/inode.c
>>> +++ b/fs/inode.c
>>> @@ -2499,6 +2499,22 @@ struct timespec64 current_time(struct inode *inode)
>>>    }
>>>    EXPORT_SYMBOL(current_time);
>>>    
>>> +/**
>>> + * inode_ctime_set_current - set the ctime to current_time
>>> + * @inode: inode
>>> + *
>>> + * Set the inode->i_ctime to the current value for the inode. Returns
>>> + * the current value that was assigned to i_ctime.
>>> + */
>>> +struct timespec64 inode_ctime_set_current(struct inode *inode)
>>> +{
>>> +	struct timespec64 now = current_time(inode);
>>> +
>>> +	inode_set_ctime(inode, now);
>>> +	return now;
>>> +}
>>> +EXPORT_SYMBOL(inode_ctime_set_current);
>>> +
>>>    /**
>>>     * in_group_or_capable - check whether caller is CAP_FSETID privileged
>>>     * @idmap:	idmap of the mount @inode was found from
>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>> index 6867512907d6..9afb30606373 100644
>>> --- a/include/linux/fs.h
>>> +++ b/include/linux/fs.h
>>> @@ -1474,7 +1474,58 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
>>>    	       kgid_has_mapping(fs_userns, kgid);
>>>    }
>>>    
>>> -extern struct timespec64 current_time(struct inode *inode);
>>> +struct timespec64 current_time(struct inode *inode);
>>> +struct timespec64 inode_ctime_set_current(struct inode *inode);
>>> +
>>> +/**
>>> + * inode_ctime_peek - fetch the current ctime from the inode
>>> + * @inode: inode from which to fetch ctime
>>> + *
>>> + * Grab the current ctime from the inode and return it.
>>> + */
>>> +static inline struct timespec64 inode_ctime_peek(const struct inode *inode)
>>> +{
>>> +	return inode->i_ctime;
>>> +}
>>> +
>>> +/**
>>> + * inode_ctime_set - set the ctime in the inode to the given value
>>> + * @inode: inode in which to set the ctime
>>> + * @ts: timespec value to set the ctime
>>> + *
>>> + * Set the ctime in @inode to @ts.
>>> + */
>>> +static inline struct timespec64 inode_ctime_set(struct inode *inode, struct timespec64 ts)
>>> +{
>>> +	inode->i_ctime = ts;
>>> +	return ts;
>>> +}
>>> +
>>> +/**
>>> + * inode_ctime_set_sec - set only the tv_sec field in the inode ctime
>>
>> I'm curious about why you choose to split the tv_sec and tv_nsec
>> set_ functions. Do any callers not set them both? Wouldn't a
>> single call enable a more atomic behavior someday?
>>
>>     inode_ctime_set_sec_nsec(struct inode *, time64_t, time64_t)
>>
>> (or simply initialize a timespec64 and use inode_ctime_spec() )
>>
> 
> Yes, quite a few places set the fields individually. For example, when
> loading a value from disk that doesn't have sufficient granularity to
> set the nsecs field to anything but 0.

Well, they still need to set the tv_nsec so they could just pass 0.
But ok.

> Could I have done it by declaring a local timespec64 variable and just
> use the inode_ctime_set function in these places? Absolutely.
> 
> That's a bit more difficult to handle with coccinelle though. If someone
> wants to suggest a way to do that without having to change all of these
> call sites manually, then I'm open to redoing the set.
> 
> That might be better left for a later cleanup though.

Acked-by: Tom Talpey <tom@...pey.com>

>>> + * @inode: inode in which to set the ctime
>>> + * @sec:  value to set the tv_sec field
>>> + *
>>> + * Set the sec field in the ctime. Returns @sec.
>>> + */
>>> +static inline time64_t inode_ctime_set_sec(struct inode *inode, time64_t sec)
>>> +{
>>> +	inode->i_ctime.tv_sec = sec;
>>> +	return sec;
>>> +}
>>> +
>>> +/**
>>> + * inode_ctime_set_nsec - set only the tv_nsec field in the inode ctime
>>> + * @inode: inode in which to set the ctime
>>> + * @nsec:  value to set the tv_nsec field
>>> + *
>>> + * Set the nsec field in the ctime. Returns @nsec.
>>> + */
>>> +static inline long inode_ctime_set_nsec(struct inode *inode, long nsec)
>>> +{
>>> +	inode->i_ctime.tv_nsec = nsec;
>>> +	return nsec;
>>> +}
>>>    
>>>    /*
>>>     * Snapshotting support.
> 

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