[PATCH v2 3/6] shallow.c: make paint_alloc slightly more robust

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

paint_alloc() allocates a big block of memory and splits it into
smaller, fixed size, chunks of memory whenever it's called. Each chunk
contains enough bits to present all "new refs" [1] in a fetch from a
shallow repository.

We do not check if the new "big block" is smaller than the requested
memory chunk though. If it happens, we'll happily pass back a memory
region smaller than expected. Which will lead to problems eventually.

A normal fetch may add/update a dozen new refs. Let's stay on the
"reasonably extreme" side and say we need 16k refs (or bits from
paint_alloc's perspective). Each chunk of memory would be 2k, much
smaller than the memory pool (512k).

So, normally, the under-allocation situation should never happen. A bad
guy, however, could make a fetch that adds more than 4m new/updated refs
to this code which results in a memory chunk larger than pool size.
Check this case and abort.

Noticed-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>

[1] Details are in commit message of 58babff (shallow.c: the 8 steps to
    select new commits for .git/shallow - 2013-12-05), step 6.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 shallow.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/shallow.c b/shallow.c
index 2512ed3..75e1702 100644
--- a/shallow.c
+++ b/shallow.c
@@ -447,6 +447,9 @@ static uint32_t *paint_alloc(struct paint_info *info)
 	unsigned size = nr * sizeof(uint32_t);
 	void *p;
 	if (!info->pool_count || info->free + size > info->end) {
+		if (size > POOL_SIZE)
+			die("BUG: pool size too small for %d in paint_alloc()",
+			    size);
 		info->pool_count++;
 		REALLOC_ARRAY(info->pools, info->pool_count);
 		info->free = xmalloc(POOL_SIZE);
-- 
2.8.2.524.g6ff3d78