[PATCH v3] selinux: optimize ebitmap_and()

[PATCH v3] selinux: optimize ebitmap_and()

[Christian Göttsche]

Iterate on nodes instead of single bits to save node resolution for each
single bit.

Similar to userspace patch efcd00814879 ("libsepol: optimize
ebitmap_and").

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
v3:
  apply format style
v2:
  fix array size computation
---
 security/selinux/ss/ebitmap.c | 50 +++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 8 deletions(-)

diff --git a/security/selinux/ss/ebitmap.c b/security/selinux/ss/ebitmap.c
index 67c1a73cd5ee..47cb90106118 100644
--- a/security/selinux/ss/ebitmap.c
+++ b/security/selinux/ss/ebitmap.c
@@ -78,19 +78,53 @@ int ebitmap_cpy(struct ebitmap *dst, const struct ebitmap *src)
 int ebitmap_and(struct ebitmap *dst, const struct ebitmap *e1,
 		const struct ebitmap *e2)
 {
-	struct ebitmap_node *n;
-	int bit, rc;
+	const struct ebitmap_node *n1, *n2;
+	struct ebitmap_node *new = NULL, **prev;
 
 	ebitmap_init(dst);
 
-	ebitmap_for_each_positive_bit(e1, n, bit)
-	{
-		if (ebitmap_get_bit(e2, bit)) {
-			rc = ebitmap_set_bit(dst, bit, 1);
-			if (rc < 0)
-				return rc;
+	prev = &dst->node;
+	n1 = e1->node;
+	n2 = e2->node;
+	while (n1 && n2) {
+		if (n1->startbit == n2->startbit) {
+			unsigned long testmap[EBITMAP_UNIT_NUMS];
+			unsigned int i;
+			bool match = false;
+
+			for (i = 0; i < ARRAY_SIZE(testmap); i++) {
+				testmap[i] = n1->maps[i] & n2->maps[i];
+				if (testmap[i] != 0)
+					match = true;
+			}
+
+			if (match) {
+				new = kmem_cache_zalloc(ebitmap_node_cachep,
+							GFP_ATOMIC);
+				if (!new) {
+					ebitmap_destroy(dst);
+					return -ENOMEM;
+				}
+				new->startbit = n1->startbit;
+				memcpy(new->maps, testmap, EBITMAP_SIZE / 8);
+				new->next = NULL;
+
+				*prev = new;
+				prev = &(new->next);
+			}
+
+			n1 = n1->next;
+			n2 = n2->next;
+		} else if (n1->startbit > n2->startbit) {
+			n2 = n2->next;
+		} else {
+			n1 = n1->next;
 		}
 	}
+
+	if (new)
+		dst->highbit = new->startbit + EBITMAP_SIZE;
+
 	return 0;
 }
 
-- 
2.43.0

Re: [PATCH v3] selinux: optimize ebitmap_and()

[Paul Moore]

On Mar 15, 2024 =?UTF-8?q?Christian=20G=C3=B6ttsche?= <cgzones@googlemail.com> wrote:
> 
> Iterate on nodes instead of single bits to save node resolution for each
> single bit.
> 
> Similar to userspace patch efcd00814879 ("libsepol: optimize
> ebitmap_and").
> 
> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> ---
> v3:
>   apply format style
> v2:
>   fix array size computation
> ---
>  security/selinux/ss/ebitmap.c | 50 +++++++++++++++++++++++++++++------
>  1 file changed, 42 insertions(+), 8 deletions(-)

Some minor comments below, but do you have any performance measurements
for this change?  I realize that ebitmap_and() isn't widely used, but
it would be nice to understand the performance difference, and if there
isn't much/any difference we might want to stick with the original code
as it is much simpler.

> diff --git a/security/selinux/ss/ebitmap.c b/security/selinux/ss/ebitmap.c
> index 67c1a73cd5ee..47cb90106118 100644
> --- a/security/selinux/ss/ebitmap.c
> +++ b/security/selinux/ss/ebitmap.c
> @@ -78,19 +78,53 @@ int ebitmap_cpy(struct ebitmap *dst, const struct ebitmap *src)
>  int ebitmap_and(struct ebitmap *dst, const struct ebitmap *e1,
>  		const struct ebitmap *e2)
>  {
> -	struct ebitmap_node *n;
> -	int bit, rc;
> +	const struct ebitmap_node *n1, *n2;
> +	struct ebitmap_node *new = NULL, **prev;
>  
>  	ebitmap_init(dst);
>  
> -	ebitmap_for_each_positive_bit(e1, n, bit)
> -	{
> -		if (ebitmap_get_bit(e2, bit)) {
> -			rc = ebitmap_set_bit(dst, bit, 1);
> -			if (rc < 0)
> -				return rc;
> +	prev = &dst->node;

Later in this function you include parenthesis, that might be nice
here too:

  prev = &(dst->node);

> +	n1 = e1->node;
> +	n2 = e2->node;
> +	while (n1 && n2) {
> +		if (n1->startbit == n2->startbit) {
> +			unsigned long testmap[EBITMAP_UNIT_NUMS];

This is very bikeshed-y, but I much prefer "dstmaps" over "testmap".

> +			unsigned int i;
> +			bool match = false;
> +
> +			for (i = 0; i < ARRAY_SIZE(testmap); i++) {
> +				testmap[i] = n1->maps[i] & n2->maps[i];
> +				if (testmap[i] != 0)

If I'm going to be nitpicky, I'd probably prefer 'if (!dstmaps[i])'.

> +					match = true;
> +			}
> +
> +			if (match) {
> +				new = kmem_cache_zalloc(ebitmap_node_cachep,
> +							GFP_ATOMIC);
> +				if (!new) {
> +					ebitmap_destroy(dst);
> +					return -ENOMEM;
> +				}
> +				new->startbit = n1->startbit;
> +				memcpy(new->maps, testmap, EBITMAP_SIZE / 8);

Why not just use 'sizeof(dstmaps)'?

  memcpy(new->maps, dstmaps, sizeof(dstmaps));

> +				new->next = NULL;

You shouldn't need the line above since you're doing a _zalloc().

> +				*prev = new;
> +				prev = &(new->next);
> +			}
> +
> +			n1 = n1->next;
> +			n2 = n2->next;
> +		} else if (n1->startbit > n2->startbit) {
> +			n2 = n2->next;
> +		} else {
> +			n1 = n1->next;
>  		}
>  	}
> +
> +	if (new)
> +		dst->highbit = new->startbit + EBITMAP_SIZE;
> +
>  	return 0;
>  }
>  
> -- 
> 2.43.0

--
paul-moore.com