linux-ext4 - Re: [RFCv1 02/72] gen_bitmaps: Fix ext2fs_compare_generic

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Date:   Tue, 22 Nov 2022 22:04:58 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        Li Xi <lixi@....com>, Wang Shilong <wangshilong1991@...il.com>
Subject: Re: [RFCv1 02/72] gen_bitmaps: Fix ext2fs_compare_generic_bmap/bitmap
 logic

On Nov 7, 2022, at 5:20 AM, Ritesh Harjani (IBM) <ritesh.list@...il.com> wrote:
> 
> Currently this function was not correctly comparing against the right
> length of the bitmap. Also when we compare bitarray v/s rbtree bitmap
> the value returned by ext2fs_test_generic_bmap() could be different in
> these two implementations. Hence only check against boolean value.
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> ---
> lib/ext2fs/gen_bitmap.c   |  9 ++++++---
> lib/ext2fs/gen_bitmap64.c | 10 +++++++---
> 2 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/ext2fs/gen_bitmap.c b/lib/ext2fs/gen_bitmap.c
> index 1536d4b3..f7764fca 100644
> --- a/lib/ext2fs/gen_bitmap.c
> +++ b/lib/ext2fs/gen_bitmap.c
> @@ -385,10 +385,13 @@ errcode_t ext2fs_compare_generic_bitmap(errcode_t magic, errcode_t neq,

        if ((bm1->start != bm2->start) ||
            (bm1->end != bm2->end) ||
            (memcmp(bm1->bitmap, bm2->bitmap,
> 		    (size_t) (bm1->end - bm1->start)/8)))
> 		return neq;
> 
> -	for (i = bm1->end - ((bm1->end - bm1->start) % 8); i <= bm1->end; i++)

On first review it appears that this is only comparing at most the last 7 bits
before "end", which appears to be wrong.  However, if you include the context
earlier in the function (which I've added above), it is using memcmp() to compare
the majority of the bitmaps in byte-wise order, so this is only needs to compare
the remaining bits, and doing the full bitwise scan is much less efficient.

I was going to say that the "start" value needs to be added to the bitmap offset
for the comparison, but I don't think that is correct either.  Looking at the
code (and thinking about it some more), it doesn't make sense to allocate space
for bits before "start", so this must be a virtual offset, and the *actual* bits
are always starting at "bitmap", so the above code is correct, AFAICS.

In summary, I don't think this patch will introduce a visible defect, but it
makes the comparisons orders of magnitude less efficient when comparing each
bit individually instead of using memcmp() that is likely hardware accelerated.

It may well be that the calling convention of this code is such that it is
always called with end=byte-aligned value so this code is never used, but that
doesn't mean it shouldn't be coded properly, in case that isn't true in the future.

> -		if (ext2fs_fast_test_block_bitmap(gen_bm1, i) !=
> -		    ext2fs_fast_test_block_bitmap(gen_bm2, i))
> +	for (i = bm1->start; i <= bm1->end; i++) {
> +		int ret1, ret2;
> +		ret1 = !!ext2fs_fast_test_block_bitmap(gen_bm1, i);
> +		ret2 = !!ext2fs_fast_test_block_bitmap(gen_bm2, i);

Strictly speaking, the !! here is not needed, and ! is enough for comparing
the inequality of the two values.  However, this is only used for 0-7 bits
and isn't performance critical since the memcmp() does the heavy lifting.

> +		if (ret1 != ret2)
> 			return neq;
> +	}
> 
> 	return 0;
> }
> diff --git a/lib/ext2fs/gen_bitmap64.c b/lib/ext2fs/gen_bitmap64.c
> index c860c10e..f7710afd 100644
> --- a/lib/ext2fs/gen_bitmap64.c
> +++ b/lib/ext2fs/gen_bitmap64.c
> @@ -629,10 +629,14 @@ errcode_t ext2fs_compare_generic_bmap(errcode_t neq,
> 	    (bm1->end != bm2->end))

Conversely, *this* version of the function is *not* doing the memcmp() of
the bulk of the bitmap contents, so it would appear to have a bug that the
patch fixes, but in a very slow manner.  It would be better to use memcmp().

> 		return neq;
> 
> -	for (i = bm1->end - ((bm1->end - bm1->start) % 8); i <= bm1->end; i++)
> -		if (ext2fs_test_generic_bmap(gen_bm1, i) !=
> -		    ext2fs_test_generic_bmap(gen_bm2, i))
> +	for (i = bm1->start; i < bm1->end; i++) {
> +		int ret1, ret2;
> +		ret1 = !!ext2fs_test_generic_bmap(gen_bm1, i);
> +		ret2 = !!ext2fs_test_generic_bmap(gen_bm2, i);

Cheers, Andreas

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