Re: [PATCH v3 2/7] selinux: use u32 as bit type in ebitmap code

[Paul Moore <paul@paul-moore.com>]

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On Aug  7, 2023 =?UTF-8?q?Christian=20G=C3=B6ttsche?= <cgzones@googlemail.com> wrote:
> 
> The extensible bitmap supports bit positions up to U32_MAX due to the
> type of the member highbit being u32.  Use u32 consistently as the type
> for bit positions to announce to callers what range of values is
> supported.
> 
> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> ---
> v3:
>   - revert type change of unrelated iter variable
>   - use U32_MAX instead of (u32)-1
> v2: avoid declarations in init-clauses of for loops
> ---
>  security/selinux/ss/ebitmap.c | 29 +++++++++++++++--------------
>  security/selinux/ss/ebitmap.h | 32 ++++++++++++++++----------------
>  2 files changed, 31 insertions(+), 30 deletions(-)

...

> diff --git a/security/selinux/ss/ebitmap.c b/security/selinux/ss/ebitmap.c
> index 77875ad355f7..a313e633aa8e 100644
> --- a/security/selinux/ss/ebitmap.c
> +++ b/security/selinux/ss/ebitmap.c
> @@ -471,18 +472,18 @@ int ebitmap_read(struct ebitmap *e, void *fp)
>  int ebitmap_write(const struct ebitmap *e, void *fp)
>  {
>  	struct ebitmap_node *n;
> -	u32 count;
> +	u32 bit, count, last_bit, last_startbit;
>  	__le32 buf[3];
>  	u64 map;
> -	int bit, last_bit, last_startbit, rc;
> +	int rc;
>  
>  	buf[0] = cpu_to_le32(BITS_PER_U64);
>  
>  	count = 0;
>  	last_bit = 0;
> -	last_startbit = -1;
> +	last_startbit = U32_MAX;
>  	ebitmap_for_each_positive_bit(e, n, bit) {
> -		if (rounddown(bit, (int)BITS_PER_U64) > last_startbit) {
> +		if (last_startbit == U32_MAX || rounddown(bit, BITS_PER_U64) > last_startbit) {

I'm getting worried about what might happen if the ebitmap starts to
contain bits near the end of the range, e.g. U32_MAX.  When lastbit
was signed this was a non-issue as we could set it to a negative
value (-1) and not worry about it, although the maximum value
difference between the signed and unsigned types would eventually be
a problem.

While looking closer at this loop, I'm now wondering if we shouldn't
just rewrite the logic a bit to simplify things, and possibly speed
it up a small amount.  How about something like this:

  count = 1;
  n = e->node;
  while (n->next) {
    count++;
    n = n->next;
  }
  last_startbit = n->startbit;
  last_bit = n->startbit + find_last_bit(n->maps, EBITMAP_SIZE);

You should probably verify that there isn't something stupid like an
off-by-one bug in the code above, but I think it is a lot cleaner
than what we currently have and should resolve a lot of the type/math
issues.

>  			count++;
>  			last_startbit = rounddown(bit, BITS_PER_U64);
>  		}
> @@ -496,9 +497,9 @@ int ebitmap_write(const struct ebitmap *e, void *fp)
>  		return rc;
>  
>  	map = 0;
> -	last_startbit = INT_MIN;
> +	last_startbit = U32_MAX;
>  	ebitmap_for_each_positive_bit(e, n, bit) {
> -		if (rounddown(bit, (int)BITS_PER_U64) > last_startbit) {
> +		if (last_startbit == U32_MAX || rounddown(bit, BITS_PER_U64) > last_startbit) {
>  			__le64 buf64[1];

Similar to the above, I think we can probably rewrite this to simply
walk the ebitmap nodes and write them out.  Using
ebitmap_for_each_positive_bit() seems overly complicated to me,
although I may be missing something important and obvious ...

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