better validation on delta base object offsets

In one case, it was possible to have a bad offset equal to 0 effectively
pointing a delta onto itself and crashing git after too many recursions.
In the other cases, a negative offset could result due to off_t being
signed.  Catch those.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1038,10 +1038,10 @@ static void check_object(struct object_entry *entry)
 				c = buf[used_0++];
 				ofs = (ofs << 7) + (c & 127);
 			}
-			if (ofs >= entry->in_pack_offset)
+			ofs = entry->in_pack_offset - ofs;
+			if (ofs <= 0 || ofs >= entry->in_pack_offset)
 				die("delta base offset out of bound for %s",
 				    sha1_to_hex(entry->idx.sha1));
-			ofs = entry->in_pack_offset - ofs;
 			if (reuse_delta && !entry->preferred_base) {
 				struct revindex_entry *revidx;
 				revidx = find_pack_revindex(p, ofs);

builtin-unpack-objects.c

@@ -370,6 +370,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 			base_offset = (base_offset << 7) + (c & 127);
 		}
 		base_offset = obj_list[nr].offset - base_offset;
+		if (base_offset <= 0 || base_offset >= obj_list[nr].offset)
+			die("offset value out of bound for delta base object");
 
 		delta_data = get_data(delta_size);
 		if (dry_run || !delta_data) {

index-pack.c

@@ -334,7 +334,7 @@ static void *unpack_raw_entry(struct object_entry *obj, union delta_base *delta_
 			base_offset = (base_offset << 7) + (c & 127);
 		}
 		delta_base->offset = obj->idx.offset - base_offset;
-		if (delta_base->offset >= obj->idx.offset)
+		if (delta_base->offset <= 0 || delta_base->offset >= obj->idx.offset)
 			bad_object(obj->idx.offset, "delta base offset is out of bound");
 		break;
 	case OBJ_COMMIT:

sha1_file.c

@@ -1355,7 +1355,7 @@ static off_t get_delta_base(struct packed_git *p,
 			base_offset = (base_offset << 7) + (c & 127);
 		}
 		base_offset = delta_obj_offset - base_offset;
-		if (base_offset >= delta_obj_offset)
+		if (base_offset <= 0 || base_offset >= delta_obj_offset)
 			return 0;  /* out of bound */
 		*curpos += used;
 	} else if (type == OBJ_REF_DELTA) {