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

[Alejandro Colomar]

On Sat, Aug 17, 2024 at 10:58:02AM GMT, Alejandro Colomar wrote:
> 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 at linux-foundation.org>
> > Signed-off-by: Yafang Shao <laoar.shao at gmail.com>
> > Cc: Simon Horman <horms at kernel.org>
> > Cc: Matthew Wilcox <willy at infradead.org>
> > ---
> >  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.

D'oh, of course that's the malloc.  Yes, it makes sense to have a +1
there.

> 
> 	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;

Alternatively, you can also rewrite the above two lines into one as:

		return strncat(strcpy(buf, ""), s, len);

The good thing is that you have strncat() in the kernel, AFAICS.
I reminded myself when checking the definitions that I wrote in shadow:

	#define XSTRNDUP(s)                                           \
	(                                                             \
	    STRNCAT(strcpy(XMALLOC(strnlen(s, NITEMS(s)) + 1, char), ""), s) \
	)
	#define STRNDUPA(s)                                           \
	(                                                             \
	    STRNCAT(strcpy(alloca(strnlen(s, NITEMS(s)) + 1), ""), s) \
	)


Cheers,
Alex

> 	}
> 
> 	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/>



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