Re: [Tarantool-patches] [PATCH 2/6] sql: fix possible null dereference in sql_expr_coll()

[Nikita Pettik <korablev@tarantool.org>]

On 06 Dec 00:59, Vladislav Shpilevoy wrote:
> On 05/12/2019 12:40, Nikita Pettik wrote:
> > On 28 Nov 23:42, Vladislav Shpilevoy wrote:
> >> Thanks for the patch!
> >>
> >> Is it possible to test this?
> > 
> > As a pure test case - I've failed to come up with it.
> > But the next patch (which adds collation to metadata) definitely fails
> > without this fix (when TRIM() or REPLACE() are called without args).
> 
> Why does not it fail right after finding that there are no
> arguments? TRIM looks strange, but REPLACE expects 3 arguments.
> 
> > I can dive into details, but I guess it's not so important here (since
> > this is obviously buggy place). 
> 
> Is it? As I pointed above, maybe the bug is really in another
> place? I see, that REPLACE checks argument count. But somewhy
> too late:
> 
>     tarantool> box.execute('SELECT REPLACE()')
>     ---
>     - null
>     - 'Wrong number of arguments is passed to REPLACE(): expected 3, got 0'
>     ...
> 
> > If you want further investigation, let me
> > know and I will do it.

Sorry, in fact I mean SUBSTR() function. It can take 1 or 2 arguments.
But the check of arguments count is delayed till function execution (see
substrFunc in sql/func.c).
To generate collation for metadata we should call sql_expr_coll()
(without such necessity we avoid call of this function). And it leads
to null-dereference.

Finally, I've come up with example which results in null-dereference on
master branch as well (added it to the current patch):

box.execute("CREATE TABLE t (id INT PRIMARY KEY, a TEXT, b TEXT COLLATE \"binary\", c TEXT COLLATE \"unicode_ci\");")
box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = substr();")

Here we have to call sql_expr_coll() with expression representing
substr() in order to verify that collations of LHS and RHS are
compatible. So we eventually we get in the same situation.
 
> Yeah. I think we need to understand the bug before fixing
> it.

New patch:


    sql: fix possible null dereference in sql_expr_coll()
    
    Some built-in functions can accept different number of arguments.
    Check of argument count for such functions takes place right before
    its execution. So it is possible that expression-list representing
    arguments for built-in function is NULL. On the other hand, in
    sql_expr_coll() (which returns collation of expression) it is assumed
    that if function features SQL_FUNC_DERIVEDCOLL flag (it implies that
    resulting collation depends on collation of arguments) then it has at
    least one argument. The last assumption is wrong considering for example
    SUBSTR() function: it may have 1 or 2 arguments, so check of argument
    count doesn't occur during compilation. Hence, if it is required to
    calculate collation for SUBSTR() function and there's no arguments,
    Tarantool crashes due to null-dereference.
    This patch fixes this bug with one additional check in sql_expr_coll().

diff --git a/src/box/sql/expr.c b/src/box/sql/expr.c
index 648b7170e..0bdcfe576 100644
--- a/src/box/sql/expr.c
+++ b/src/box/sql/expr.c
@@ -332,7 +332,8 @@ sql_expr_coll(Parse *parse, Expr *p, bool *is_explicit_coll, uint32_t *coll_id,
                                sql_func_by_signature(p->u.zToken, arg_count);
                        if (func == NULL)
                                break;
-                       if (sql_func_flag_is_set(func, SQL_FUNC_DERIVEDCOLL)) {
+                       if (sql_func_flag_is_set(func, SQL_FUNC_DERIVEDCOLL) &&
+                           arg_count > 0) {
                                /*
                                 * Now we use quite straightforward
                                 * approach assuming that resulting
diff --git a/test/sql/collation.result b/test/sql/collation.result
index 11962ef47..fbc7ce9aa 100644
--- a/test/sql/collation.result
+++ b/test/sql/collation.result
@@ -292,6 +292,15 @@ box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = c COLLATE \"unicode\";
 - null
 - Illegal mix of collations
 ...
+-- Make sure that using function featuring variable arguemnts
+-- length  and resulting collation which depends on arguments
+-- is processed correctly.
+--
+box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = substr();")
+---
+- null
+- 'Wrong number of arguments is passed to SUBSTR(): expected 1 or 2, got 0'
+...
 -- Compound queries perform implicit comparisons between values.
 -- Hence, rules for collations compatibilities are the same.
 --
diff --git a/test/sql/collation.test.lua b/test/sql/collation.test.lua
index 1be28b3ff..a013253cd 100644
--- a/test/sql/collation.test.lua
+++ b/test/sql/collation.test.lua
@@ -80,6 +80,11 @@ box.execute("SELECT * FROM t WHERE b = c;")
 box.execute("SELECT * FROM t WHERE b COLLATE \"binary\" = c;")
 box.execute("SELECT * FROM t WHERE a = c;")
 box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = c COLLATE \"unicode\";")
+-- Make sure that using function featuring variable arguemnts
+-- length  and resulting collation which depends on arguments
+-- is processed correctly.
+--
+box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = substr();")