Re: [PATCH v7 6/8] mm/util: Deduplicate code in {kstrdup,kstrndup,kmemdup_nul}

[Alejandro Colomar <alx@xxxxxxxxxx>]

Hi Yafang,

On Sat, Aug 17, 2024 at 10:56:22AM GMT, Yafang Shao wrote:
> These three functions follow the same pattern. To deduplicate the code,
> let's introduce a common helper __kmemdup_nul().
> 
> Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> Cc: Simon Horman <horms@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> ---
>  mm/util.c | 67 +++++++++++++++++++++----------------------------------
>  1 file changed, 26 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 4542d8a800d9..310c7735c617 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -45,33 +45,40 @@ void kfree_const(const void *x)
>  EXPORT_SYMBOL(kfree_const);
>  
>  /**
> - * kstrdup - allocate space for and copy an existing string
> - * @s: the string to duplicate
> + * __kmemdup_nul - Create a NUL-terminated string from @s, which might be unterminated.
> + * @s: The data to copy
> + * @len: The size of the data, including the null terminator
>   * @gfp: the GFP mask used in the kmalloc() call when allocating memory
>   *
> - * Return: newly allocated copy of @s or %NULL in case of error
> + * Return: newly allocated copy of @s with NUL-termination or %NULL in
> + * case of error
>   */
> -noinline
> -char *kstrdup(const char *s, gfp_t gfp)
> +static __always_inline char *__kmemdup_nul(const char *s, size_t len, gfp_t gfp)
>  {
> -	size_t len;
>  	char *buf;
>  
> -	if (!s)
> +	buf = kmalloc_track_caller(len, gfp);
> +	if (!buf)
>  		return NULL;
>  
> -	len = strlen(s) + 1;
> -	buf = kmalloc_track_caller(len, gfp);
> -	if (buf) {
> -		memcpy(buf, s, len);
> -		/* During memcpy(), the string might be updated to a new value,
> -		 * which could be longer than the string when strlen() is
> -		 * called. Therefore, we need to add a null termimator.
> -		 */
> -		buf[len - 1] = '\0';
> -	}
> +	memcpy(buf, s, len);
> +	/* Ensure the buf is always NUL-terminated, regardless of @s. */
> +	buf[len - 1] = '\0';
>  	return buf;
>  }
> +
> +/**
> + * kstrdup - allocate space for and copy an existing string
> + * @s: the string to duplicate
> + * @gfp: the GFP mask used in the kmalloc() call when allocating memory
> + *
> + * Return: newly allocated copy of @s or %NULL in case of error
> + */
> +noinline
> +char *kstrdup(const char *s, gfp_t gfp)
> +{
> +	return s ? __kmemdup_nul(s, strlen(s) + 1, gfp) : NULL;
> +}
>  EXPORT_SYMBOL(kstrdup);
>  
>  /**
> @@ -106,19 +113,7 @@ EXPORT_SYMBOL(kstrdup_const);
>   */
>  char *kstrndup(const char *s, size_t max, gfp_t gfp)
>  {
> -	size_t len;
> -	char *buf;
> -
> -	if (!s)
> -		return NULL;
> -
> -	len = strnlen(s, max);
> -	buf = kmalloc_track_caller(len+1, gfp);
> -	if (buf) {
> -		memcpy(buf, s, len);
> -		buf[len] = '\0';
> -	}
> -	return buf;
> +	return s ? __kmemdup_nul(s, strnlen(s, max) + 1, gfp) : NULL;
>  }
>  EXPORT_SYMBOL(kstrndup);
>  
> @@ -192,17 +187,7 @@ EXPORT_SYMBOL(kvmemdup);
>   */
>  char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
>  {
> -	char *buf;
> -
> -	if (!s)
> -		return NULL;
> -
> -	buf = kmalloc_track_caller(len + 1, gfp);
> -	if (buf) {
> -		memcpy(buf, s, len);
> -		buf[len] = '\0';
> -	}
> -	return buf;
> +	return s ? __kmemdup_nul(s, len + 1, gfp) : NULL;
>  }
>  EXPORT_SYMBOL(kmemdup_nul);

I like the idea of the patch, but it's plagued with all those +1 and -1.
I think that's due to a bad choice of value being passed by.  If you
pass the actual length of the string (as suggested in my reply to the
previous patch) you should end up with a cleaner set of APIs.

The only remaining +1 is for kmalloc_track_caller(), which I ignore what
it does.

	char *
	__kmemdup_nul(const char *s, size_t len, gfp_t gfp)
	{
		char *buf;

		buf = kmalloc_track_caller(len + 1, gfp);
		if (!buf)
			return NULL;

		strcpy(mempcpy(buf, s, len), "");
		return buf;
	}

	char *
	kstrdup(const char *s, gfp_t gfp)
	{
		return s ? __kmemdup_nul(s, strlen(s), gfp) : NULL;
	}

	char *
	kstrndup(const char *s, size_t n, gfp_t gfp)
	{
		return s ? __kmemdup_nul(s, strnlen(s, n), gfp) : NULL;
	}

	char *
	kmemdup_nul(const char *s, size_t len, gfp_t gfp)
	{
		return s ? __kmemdup_nul(s, len, gfp) : NULL;
	}

Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es/>

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