linux-kernel - Re: [PATCH] mm/shmem: Fix invalid PTR_ERR(NULL) call in shmem_xattr_handler

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Message-ID: <16055fb0-bed3-e000-8c36-6e4886b08c9d@google.com>
Date: Tue, 28 Jan 2025 20:37:51 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: Qasim Ijaz <qasdev00@...il.com>
cc: hughd@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/shmem: Fix invalid PTR_ERR(NULL) call in
 shmem_xattr_handler_set()

On Tue, 28 Jan 2025, Qasim Ijaz wrote:

> In shmem_xattr_handler_set() if simple_xattr_set() succeeds and the pointer
> returned is not an error pointer, then old_xattr will be set to NULL in
> the body of the following if statement:
> 
>     if (!IS_ERR(old_xattr))
> 
> Later on shmem_xattr_handler_set() calls:
> 
>     return PTR_ERR(old_xattr);
> 
> The PTR_ERR macro is used to extract an error code from an error pointer
> and NULL is not an error pointer, PTR_ERR(NULL) simply results in 0.

NULL pointer returns 0 for success: yes, that's what's wanted there.

> 
> To improve correctness and readability,

Correctness? Please explain - I don't see any incorrectness.

Readability? Perhaps - I should not be the judge of that.

> refactor the error handling 
> to have an explicit default return value of 0 (success) in "ret". 
> If simple_xattr_set() returns an error pointer, store its 
> error code in "ret".
> 
> Signed-off-by: Qasim Ijaz <qasdev00@...il.com>
> ---
>  mm/shmem.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)

I prefer how it was written before.

> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 532afd8e049c..3e97c7890aac 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -4143,6 +4143,7 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
>  	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>  	struct simple_xattr *old_xattr;
>  	size_t ispace = 0;
> +	int ret = 0;
>  
>  	name = xattr_full_name(handler, name);
>  	if (value && sbinfo->max_inodes) {
> @@ -4158,7 +4156,9 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
>  	}
>  
>  	old_xattr = simple_xattr_set(&info->xattrs, name, value, size, flags);
> -	if (!IS_ERR(old_xattr)) {
> +	if (IS_ERR(old_xattr)) {
> +		ret = PTR_ERR(old_xattr);
> +	} else {
>  		ispace = 0;
>  		if (old_xattr && sbinfo->max_inodes)
>  			ispace = simple_xattr_space(old_xattr->name,
> @@ -4168,12 +4171,13 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
>  		inode_set_ctime_current(inode);
>  		inode_inc_iversion(inode);
>  	}
> +
>  	if (ispace) {
>  		raw_spin_lock(&sbinfo->stat_lock);
>  		sbinfo->free_ispace += ispace;
>  		raw_spin_unlock(&sbinfo->stat_lock);
>  	}
> -	return PTR_ERR(old_xattr);
> +	return ret;
>  }
>  
>  static const struct xattr_handler shmem_security_xattr_handler = {
> -- 
> 2.39.5