userspace: get rid of app section placeholders

We used to leave byte-long placeholder symbols to ensure
that empty application memory sections did not cause
build errors that were very difficult to understand.

Now we use some relatively portable inline assembly to
generate a symbol, but don't take up any extra space.

The malloc and libc partitions are now only instantiated
if there is some data to put in them.

Fixes: #13923

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/app_memory/app_memdomain.h

@@ -67,6 +67,37 @@ struct z_app_region {
 #define Z_APP_BSS_START(id) data_smem_##id##_bss_start
 #define Z_APP_BSS_SIZE(id) data_smem_##id##_bss_size
 
+/* If a partition is declared with K_APPMEM_PARTITION, but never has any
+ * data assigned to its contents, then no symbols with its prefix will end
+ * up in the symbol table. This prevents gen_app_partitions.py from detecting
+ * that the partition exists, and the linker symbols which specify partition
+ * bounds will not be generated, resulting in build errors.
+ *
+ * What this inline assembly code does is define a symbol with no data.
+ * This should work for all arches that produce ELF binaries, see
+ * https://sourceware.org/binutils/docs/as/Section.html
+ *
+ * We don't know what active flags/type of the pushed section were, so we are
+ * specific: "aw" indicates section is allocatable and writable,
+ * and "@progbits" indicates the section has data.
+ */
+#ifdef CONFIG_ARM
+/* ARM has a quirk in that '@' denotes a comment, so we have to send
+ * %progbits to the assembler instead.
+ */
+#define Z_PROGBITS_SYM	"\%"
+#else
+#define Z_PROGBITS_SYM "@"
+#endif
+
+#define Z_APPMEM_PLACEHOLDER(name) \
+	__asm__ ( \
+		".pushsection " STRINGIFY(K_APP_DMEM_SECTION(name)) \
+			",\"aw\"," Z_PROGBITS_SYM "progbits\n\t" \
+		".global " STRINGIFY(name) "_placeholder\n\t" \
+		STRINGIFY(name) "_placeholder:\n\t" \
+		".popsection\n\t")
+
 /**
  * @brief Define an application memory partition with linker support
  *
@@ -94,8 +125,7 @@ struct z_app_region {
 		.bss_start = &Z_APP_BSS_START(name), \
 		.bss_size = (size_t) &Z_APP_BSS_SIZE(name) \
 	}; \
-	K_APP_BMEM(name) char name##_placeholder;
-
+	Z_APPMEM_PLACEHOLDER(name);
 #else
 
 #define K_APP_BMEM(ptn)

include/misc/libc-hooks.h

@@ -37,12 +37,23 @@ __syscall size_t _zephyr_fwrite(const void *_MLIBC_RESTRICT ptr, size_t size,
 #endif /* CONFIG_NEWLIB_LIBC */
 
 #ifdef CONFIG_USERSPACE
+#if defined(CONFIG_NEWLIB_LIBC) || (CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE > 0)
+#define Z_MALLOC_PARTITION_EXISTS 1
+
 /* Memory partition containing the libc malloc arena */
 extern struct k_mem_partition z_malloc_partition;
+#endif
+
+#if defined(CONFIG_NEWLIB_LIBC) || defined(CONFIG_STACK_CANARIES)
+/* Minimal libc has no globals. We do put the stack canary global in the
+ * libc partition since it is not worth placing in a partition of its own.
+ */
+#define Z_LIBC_PARTITION_EXISTS 1
 
 /* C library globals, except the malloc arena */
 extern struct k_mem_partition z_libc_partition;
 #endif
+#endif /* CONFIG_USERSPACE */
 
 #include <syscalls/libc-hooks.h>
 

kernel/userspace.c

@@ -18,8 +18,11 @@
 #include <init.h>
 #include <stdbool.h>
 #include <app_memory/app_memdomain.h>
+#include <misc/libc-hooks.h>
 
+#ifdef Z_LIBC_PARTITION_EXISTS
 K_APPMEM_PARTITION_DEFINE(z_libc_partition);
+#endif
 
 #define LOG_LEVEL CONFIG_KERNEL_LOG_LEVEL
 #include <logging/log.h>

lib/libc/minimal/source/stdlib/malloc.c

@@ -16,12 +16,9 @@
 #include <logging/log.h>
 LOG_MODULE_DECLARE(os);
 
-#ifdef CONFIG_USERSPACE
-K_APPMEM_PARTITION_DEFINE(z_malloc_partition);
-#endif
-
 #if (CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE > 0)
 #ifdef CONFIG_USERSPACE
+K_APPMEM_PARTITION_DEFINE(z_malloc_partition);
 #define POOL_SECTION K_APP_DMEM_SECTION(z_malloc_partition)
 #else
 #define POOL_SECTION .data

subsys/testsuite/ztest/src/ztest.c

@@ -301,11 +301,15 @@ void main(void)
 {
 #ifdef CONFIG_USERSPACE
 	struct k_mem_partition *parts[] = {
-		&ztest_mem_partition,
+#ifdef Z_LIBC_PARTITION_EXISTS
 		/* C library globals, stack canary storage, etc */
 		&z_libc_partition,
+#endif
+#ifdef Z_MALLOC_PARTITION_EXISTS
 		/* Required for access to malloc arena */
-		&z_malloc_partition
+		&z_malloc_partition,
+#endif
+		&ztest_mem_partition
 	};
 
 	/* Ztests just have one memory domain with one partition.