[PATCH v6] diff-delta: use safe integer arithmetic

From: Srujith A Poojary <poojarysrujith@gmail.com>

In create_delta_index(), buffer sizes for allocating the delta index are calculated using arithmetic operations on entries and hsize:

  memsize = sizeof(*hash) * hsize + sizeof(*entry) * entries

To prevent potential size_t wrap-around when computing allocation sizes on 32-bit systems, replace manual arithmetic with overflow-checked helpers (st_mult, st_add, st_add3).

For the unpacked index allocation:
- sizeof(struct unpacked_index_entry) is 12 bytes on 32-bit systems,
  which may be padded to 16 bytes depending on compiler alignment rules.
- sizeof(struct index_entry *) is 4 bytes.
- entries is capped by (0xffffffu / RABIN_WINDOW) where RABIN_WINDOW is 16,
  limiting entries to 1,048,575.
- hsize is the smallest power of 2 greater than or equal to entries,
  capped at 1,048,576.

While entries is bounded in current code, using st_mult() and st_add() guarantees that any unexpected growth in struct sizes or bounds changes will safely die() on size_t overflow instead of wrapping around.

For the packed index allocation:
- (hsize + 1) is passed to st_mult(), which is evaluated as an argument
  inside st_add3(), and is safe from overflow because hsize is capped at
  1,048,576 early in the function.

Signed-off-by: Srujith A Poojary <poojarysrujith@gmail.com>
---
    [Outreachy PATCH] diff-delta: use safe integer arithmetic

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2276%2Fpoojarysrujith%2Fc-refactor-target-name-v6
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2276/poojarysrujith/c-refactor-target-name-v6
Pull-Request: https://github.com/gitgitgadget/git/pull/2276

Range-diff vs v5:

 1:  23fcfd3227 ! 1:  b91a1ed54b diff-delta: use safe integer arithmetic
     @@ Commit message
          will safely die() on size_t overflow instead of wrapping around.
      
          For the packed index allocation:
     -    - Passing (hsize + 1) as an argument to st_add3() is safe from overflow
     -      because hsize is capped at 1,048,576 early in the function.
     +    - (hsize + 1) is passed to st_mult(), which is evaluated as an argument
     +      inside st_add3(), and is safe from overflow because hsize is capped at
     +      1,048,576 early in the function.
      
          Signed-off-by: Srujith A Poojary <poojarysrujith@gmail.com>
      

 diff-delta.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)