Extend canned symbol table logic to work in protected build mode

libc/symtab/Kconfig

@@ -6,7 +6,8 @@
 config LIBC_SYMTAB
 	bool "Include canned symtab for applications and shell"
 	default n
-	depends on EXECFUNCS_HAVE_SYMTAB
+	depends on EXECFUNCS_HAVE_SYMTAB && !BUILD_KERNEL
+	select LIB_BOARDCTL if !BUILD_FLAT
 	---help---
 		Build and include default symbol table in the NuttX library.
 		The symbol table is selected by call canned_symtab_initialize().

libc/symtab/README.txt

@@ -1,23 +1,48 @@
 symtab
 ======
 
-This directory provide support for canned symbol table which provides
+Symbol Tables and Build Modes
+-----------------------------
+This directory provide support for a canned symbol table which provides
 all/most of system and libc services/functions to the application and NSH.
 
-The support is selected by CONFIG_LIBC_SYMTAB option and table has to be
-prepared in advance manually.
+Symbol tables have differing usefulness in different NuttX build modes:
 
-It can be prepared from NuttX top level directory by next commands
+  1. In the FLAT build (CONFIG_BUILD_FLAT), symbol tables are used to bind
+     addresses in loaded ELF or NxFLAT modules to base code that usually
+     resides in FLASH memory.  Both OS interfaces and user/application
+     libraries are made available to the loaded module via symbol tables.
+
+  2. Symbol tables may be of value in a protected build
+     (CONFIG_BUILD_PROTECTED) where the newly started user task must
+     share resources with other user code (but should use system calls to
+     interact with the OS).
+
+  3. But in the kernel build mode (CONFIG_BUILD_KERNEL), only fully linked
+     executables loadable via execl(), execv(), or posix_spawan() can used.
+     There is no use for a symbol table with the kernel build since all
+     memory resources are separate; nothing is share-able with the newly
+     started process.
+
+Creating the Canned Symbol Table
+--------------------------------
+The support is selected by CONFIG_LIBC_SYMTAB option and table has to be
+prepared in advance manually.  It can be prepared from NuttX top level
+directory by using the following commands:
 
   cat syscall/syscall.csv libc/libc.csv | sort >libc/symtab/canned_symtab.csv
   tools/mksymtab libc/symtab/canned_symtab.csv libc/symtab/canned_symtab.inc
 
-Next code selectes canned symtab from application:
+Your board-level start up code code then needs to select the canned symbol
+table by calling the OS internal function canned_symtab_initialize() in the
+board-specfic board_apps_initialize() logic:
 
   #include <nuttx/binfmt/canned_symtab.h>
   ...
     canned_symtab_initialize();
 
+Code/Text Size Implications
+---------------------------
 The option can have substantial effect on system image size, mainly
 code/text.  That is because the instructions to generate canned_symtab.inc
 above will cause EVERY interface in the NuttX RTOS and the C library to be

libc/symtab/lib_symbtab.c

@@ -49,13 +49,87 @@
 
 #include "canned_symtab.inc"
 
-/************************************************************************
+/****************************************************************************
  * Public Functions
- ************************************************************************/
+ ****************************************************************************/
 
+/****************************************************************************
+ * Name: canned_symtab_initialize
+ *
+ * Description:
+ *   Setup system provided canned symbol table.  NOTE that this a user-space
+ *   interface only.  It is not generally available to to kernel mode code
+ *   in protected or kernel builds.  That is because exec_setsymtab() and
+ *   g_symtab lie in different address spaces.
+ *
+ * Input Parameters:
+ *   None
+ *
+ * Returned Value:
+ *   None
+ *
+ ****************************************************************************/
+
+#if defined(CONFIG_BUILD_FLAT) || !defined(__KERNEL__)
 void canned_symtab_initialize(void)
 {
+#ifdef CONFIG_BUILD_FLAT
+  /* In the FLAT build, exec_symtab() can be called directly from any logic.
+   * Both the symbol table and the function exec_setsymtabe reside in the same
+   * namespace and address space.
+   */
+
   exec_setsymtab(g_symtab, NSYMBOLS);
+
+#else
+  /* In the user mode portion of a protected or kernel build, we must set
+   * the symbol table indirectly through the boardctl() call gate that will
+   * proxy the call to canned_symtab_select().  In this case
+   *
+   * - canned_symbtab_initialize() and g_symtab() lie in the user space.
+   * - boardctl(), canned_symtabl_select(), and exec_setsymtab() reside in
+   *   kernel space.
+   *
+   * Access to boardctl() is provided in user space throug a call gate.
+   */
+
+  struct symtab_desc_s symdesc;
+
+  symdesc.symtab   = g_symtab;
+  symdesc.nsymbols = NSYMBOLS;
+  (void)boardctl(BOARDIOC_SYMTAB, (uinptr_t)&symdesc);
+#endif
 }
+#endif
+
+/****************************************************************************
+ * Name: canned_symtab_select
+ *
+ * Description:
+ *   Setup system provided canned symbol table.  This function only exists
+ *   the kernel portion of a protected or kernel build.  It is called only
+ *   by boardctl().  I this case:
+ *
+ *   - canned_symbtab_initialize() and g_symtab() lie in the user space.
+ *   - boardctl(), canned_symtabl_select(), and exec_setsymtab() reside in
+ *     kernel space.
+ *
+ *   Access to boardctl() is provided in user space through a call gate.
+ *
+ * Input Parameters:
+ *   symtab - The symbol table to be used
+ *
+ * Returned Value:
+ *   Zero (OK) on success; a negated errno value on failure.
+ *
+ ****************************************************************************/
+
+#if (defined(CONFIG_BUILD_PROTECTED) || defined(CONFIG_BUILD_KERNEL)) && \
+      defined(__KERNEL__)
+int canned_symtab_select(FAR const struct symtab_desc_s *symdesc)
+{
+  exec_setsymtab(symdesc->symtab, symdesc->nsymbols);
+}
+#endif
 
 #endif /* CONFIG_LIBC_SYMTAB */