closes bpo-34400: Fix undefined behavior in parsetok(). (GH-4439)

miss-islington · ZackerySpytz · web-flow · commit 981aa46dce92 · 2018-08-15T03:08:29.000-04:00
Avoid undefined pointer arithmetic with NULL. (cherry picked from commit 7c4ab2a) Co-authored-by: Zackery Spytz <zspytz@gmail.com>
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-08-14-03-52-43.bpo-34400.AJD0bz.rst b/Misc/NEWS.d/next/Core and Builtins/2018-08-14-03-52-43.bpo-34400.AJD0bz.rst
@@ -0,0 +1 @@
+Fix undefined behavior in parsetok.c.  Patch by Zackery Spytz.
diff --git a/Parser/parsetok.c b/Parser/parsetok.c
@@ -225,7 +225,7 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
         }
         else
             started = 1;
-        len = b - a; /* XXX this may compute NULL - NULL */
+        len = (a != NULL && b != NULL) ? b - a : 0;
         str = (char *) PyObject_MALLOC(len + 1);
         if (str == NULL) {
             err_ret->error = E_NOMEM;

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+Fix undefined behavior in parsetok.c. Patch by Zackery Spytz.`
Original file line number	Diff line number	Diff line change
`@@ -225,7 +225,7 @@ parsetok(struct tok_state tok, grammar g, int start, perrdetail *err_ret,`
`225`	`225`	`}`
`226`	`226`	`else`
`227`	`227`	`started = 1;`
`228`		`- len = b - a; /* XXX this may compute NULL - NULL */`
	`228`	`+ len = (a != NULL && b != NULL) ? b - a : 0;`
`229`	`229`	`str = (char *) PyObject_MALLOC(len + 1);`
`230`	`230`	`if (str == NULL) {`
`231`	`231`	`err_ret->error = E_NOMEM;`