[PATCH] fix off-by-one mistakes in attribute access syntax checking (PR 93640)

[Martin Sebor]

PR 93641 points out a number of off-by-one mistakes in calls to strncmp
in GCC where the bound is (unintentionally) less than the length of
the string literal argument.

PR 93640 is a separate report of another such mistake in the code that
validates the mode argument in attribute access.  As a result, the code
accepts even some invalid modes.  The attached patch fixes these.

It's not technically a regression so I was going to wait until stage
1 to submit a fix but seeing that Jakub already committed fixes for
the former I figure I should also handle this one for GCC 10.

Tested on x86_64-linux.

Martin

PR c/93640 - The write_only and read_write attributes can be mistyped due to invalid strncmp size argument

gcc/c-family/ChangeLog:

	PR c/93640
	* c-attribs.c (handle_access_attribute): Correct off-by-one mistakes.

gcc/testsuite/ChangeLog:

	PR c/93640
	* gcc.dg/attr-access.c: New test.

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index 7ec6fc848ac..2ea5fd5ff46 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -3999,8 +3999,8 @@ handle_access_attribute (tree *node, tree name, tree args,
     }
 
   const bool read_only = strncmp (ps, "read_only", 9) == 0;
-  const bool write_only = strncmp (ps, "write_only", 9) == 0;
-  if (!read_only && !write_only && strncmp (ps, "read_write", 9))
+  const bool write_only = strncmp (ps, "write_only", 10) == 0;
+  if (!read_only && !write_only && strncmp (ps, "read_write", 10))
     {
       error ("attribute %qE invalid mode %qs; expected one of "
 	     "%qs, %qs, or %qs", name, access_str,
diff --git a/gcc/testsuite/gcc.dg/attr-access.c b/gcc/testsuite/gcc.dg/attr-access.c
new file mode 100644
index 00000000000..f859c461b96
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/attr-access.c
@@ -0,0 +1,21 @@
+/* PR c/93640 - The write_only and read_write attributes can be mistyped
+   due to invalid strncmp size argument
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+__attribute__ ((access (read_onl))) int f0 (char*);         // { dg-error "attribute 'access' invalid mode 'read_onl'" }
+__attribute__ ((access (write_onl))) int f1 (char*);        // { dg-error "attribute 'access' invalid mode 'write_onl'" }
+__attribute__ ((access (read_writ))) int f2 (char*);        // { dg-error "attribute 'access' invalid mode 'read_writ'" }
+
+__attribute__ ((access (read_onlX))) int f3 (char*);        // { dg-error "attribute 'access' invalid mode 'read_onlX'" }
+__attribute__ ((access (write_onlX))) int f4 (char*);       // { dg-error "attribute 'access' invalid mode 'write_onlX'" }
+__attribute__ ((access (read_writX))) int f5 (char*);       // { dg-error "attribute 'access' invalid mode 'read_writX'" }
+
+
+__attribute__ ((access (read_onl, 1))) int f7 (char*);      // { dg-error "attribute 'access' invalid mode 'read_onl'" }
+__attribute__ ((access (write_onl, 1))) int f8 (char*);     // { dg-error "attribute 'access' invalid mode 'write_onl'" }
+__attribute__ ((access (read_writ, 1))) int f9 (char*);     // { dg-error "attribute 'access' invalid mode 'read_writ'" }
+
+__attribute__ ((access (read_onlX, 1))) int f10 (char*);    // { dg-error "attribute 'access' invalid mode 'read_onlX'" }
+__attribute__ ((access (write_onlX, 1))) int f11 (char*);   // { dg-error "attribute 'access' invalid mode 'write_onlX'" }
+__attribute__ ((access (read_writX, 1))) int f12 (char*);   // { dg-error "attribute 'access' invalid mode 'read_writX'" }