build: (Custom) App version info is now on a dedicated section, independent of the rodata alignment

It is now possible to have any alignment restriction on rodata in the user
applicaiton. It will not affect the first section which must be aligned
on a 16-byte bound.

Closes https://github.com/espressif/esp-idf/issues/6719
Closes https://github.com/espressif/esp-idf/issues/6976

components/esp32/ld/esp32.ld

@@ -134,3 +134,12 @@ REGION_ALIAS("rtc_data_location", rtc_data_seg );
 #else
   REGION_ALIAS("default_rodata_seg", dram0_0_seg);
 #endif // CONFIG_APP_BUILD_USE_FLASH_SECTIONS
+
+/**
+ *  If rodata default segment is placed in `drom0_0_seg`, then flash's first rodata section must
+ *  also be first in the segment.
+ */
+#ifdef CONFIG_APP_BUILD_USE_FLASH_SECTIONS
+  ASSERT(_rodata_start == ORIGIN(default_rodata_seg),
+         ".flash.appdesc section must be placed at the beginning of the rodata segment.")
+#endif

components/esp32/ld/esp32.project.ld.in

@@ -250,13 +250,21 @@ SECTIONS
           "DRAM segment data does not fit.")
 
   /* When modifying the alignment, update tls_section_alignment in pxPortInitialiseStack */
-  .flash.rodata : ALIGN(0x10)
+  .flash.appdesc : ALIGN(0x10)
   {
     _rodata_start = ABSOLUTE(.);
 
     *(.rodata_desc .rodata_desc.*)               /* Should be the first.  App version info.        DO NOT PUT ANYTHING BEFORE IT! */
     *(.rodata_custom_desc .rodata_custom_desc.*) /* Should be the second. Custom app version info. DO NOT PUT ANYTHING BEFORE IT! */
 
+    /* Create an empty gap within this section. Thanks to this, the end of this
+     * section will match .flah.rodata's begin address. Thus, both sections
+     * will be merged when creating the final bin image. */
+    . = ALIGN(ALIGNOF(.flash.rodata));
+  } >default_rodata_seg
+
+  .flash.rodata : ALIGN(0x10)
+  {
     mapping[flash_rodata]
 
     *(.irom1.text) /* catch stray ICACHE_RODATA_ATTR */

components/esp32s2/ld/esp32s2.ld

@@ -120,3 +120,13 @@ REGION_ALIAS("rtc_data_location", rtc_data_seg );
 #else
   REGION_ALIAS("default_rodata_seg", dram0_0_seg);
 #endif // CONFIG_APP_BUILD_USE_FLASH_SECTIONS
+
+
+/**
+ *  If rodata default segment is placed in `drom0_0_seg`, then flash's first rodata section must
+ *  also be first in the segment.
+ */
+#ifdef CONFIG_APP_BUILD_USE_FLASH_SECTIONS
+  ASSERT(_rodata_reserved_start == ORIGIN(default_rodata_seg),
+         ".flash.appdesc section must be placed at the beginning of the rodata segment.")
+#endif

components/esp32s2/ld/esp32s2.project.ld.in

@@ -233,7 +233,7 @@ SECTIONS
   } > dram0_0_seg
 
   /* When modifying the alignment, update tls_section_alignment in pxPortInitialiseStack */
-  .flash.rodata : ALIGN(0x10)
+  .flash.appdesc : ALIGN(0x10)
   {
     _rodata_reserved_start = ABSOLUTE(.);
     _rodata_start = ABSOLUTE(.);
@@ -241,6 +241,14 @@ SECTIONS
     *(.rodata_desc .rodata_desc.*)               /* Should be the first.  App version info.        DO NOT PUT ANYTHING BEFORE IT! */
     *(.rodata_custom_desc .rodata_custom_desc.*) /* Should be the second. Custom app version info. DO NOT PUT ANYTHING BEFORE IT! */
 
+    /* Create an empty gap within this section. Thanks to this, the end of this
+     * section will match .flah.rodata's begin address. Thus, both sections
+     * will be merged when creating the final bin image. */
+    . = ALIGN(ALIGNOF(.flash.rodata));
+  } >default_rodata_seg
+
+  .flash.rodata : ALIGN(0x10)
+  {
     mapping[flash_rodata]
 
     *(.irom1.text) /* catch stray ICACHE_RODATA_ATTR */

tools/test_apps/build_system/ldalign_test/CMakeLists.txt

@@ -0,0 +1,18 @@
+# The following lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.5)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(ldalign_test)
+
+idf_build_get_property(python PYTHON)
+idf_build_get_property(elf EXECUTABLE)
+
+set(check_alignment  "${CMAKE_CURRENT_LIST_DIR}/check_alignment.py")
+set(readelf "${CMAKE_TOOLCHAIN_PREFIX}readelf")
+
+add_custom_command(
+    TARGET ${elf}
+    POST_BUILD
+    COMMAND ${python} ${check_alignment} ${readelf} $<TARGET_FILE:${elf}>
+)

tools/test_apps/build_system/ldalign_test/README.txt

@@ -0,0 +1,7 @@
+Runs a build test to check alignment and position of `.flash.appdesc` and
+`.flash.rodata` sections. Indeed, `.flash.appdesc` shall ALWAYS be aligned on
+a 16-byte bounds, whereas `.flash.rodata` can have any alignment. In any case,
+the end address of first one shall match the start address of the second one.
+This will let both of them be merged when generating the final bin image.
+The Python script that performs the checks, `check_alignment.py`, automatically
+runs after the app is built.

tools/test_apps/build_system/ldalign_test/check_alignment.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python
+#
+# Copyright 2020 Espressif Systems (Shanghai) PTE LTD
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import argparse
+import re
+import subprocess
+from typing import Tuple
+
+argparser = argparse.ArgumentParser()
+
+argparser.add_argument('readelf')
+argparser.add_argument('elf')
+
+args = argparser.parse_args()
+
+# Get the content of the readelf command
+contents = subprocess.check_output([args.readelf, '-S', args.elf]).decode()
+
+
+# Define a class for readelf parsing error
+class ParsingError(Exception):
+    pass
+
+
+# Look for the start address and size of any section
+def find_partition_info(sectionname):  # type: (str) -> Tuple[int, int, int]
+    match = re.search(sectionname + r'\s+PROGBITS\s+([a-f0-9]+) [a-f0-9]+ ([a-f0-9]+) \d+\s+[A-Z]+  0   0 (\d+)',
+                      contents)
+    if not match:
+        raise ParsingError('ELF header parsing error')
+    # Return the address of the section, the size and the alignment
+    address = match.group(1)
+    size = match.group(2)
+    alignment = match.group(3)
+    return (int(address, 16), int(size, 16), int(alignment, 10))
+
+
+# Get address and size for .flash.appdesc section
+app_address, app_size, app_align = find_partition_info('.flash.appdesc')
+
+# Same goes for .flash.rodata section
+rodata_address, _, rodata_align = find_partition_info('.flash.rodata')
+
+# Assert than everything is as expected:
+# appdesc is aligned on 16
+# rodata is aligned on 64
+# appdesc ends where rodata starts
+assert app_align == 16, '.flash.appdesc section should have been aligned on 16!'
+assert rodata_align == 64, '.flash.rodata section should have been aligned on 64!'
+assert app_address + app_size == rodata_address, ".flash.appdesc's end address and .flash.rodata's begin start must have no gap in between!"

tools/test_apps/build_system/ldalign_test/main/CMakeLists.txt

@@ -0,0 +1,2 @@
+idf_component_register(SRCS "test_main.c"
+                       INCLUDE_DIRS ".")

tools/test_apps/build_system/ldalign_test/main/test_main.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+const static uint32_t __attribute__ ((aligned (64))) testTab[] =
+  {
+   0xff445566, 0x44556677, 0x33221100,
+   0x88997755, 0x99887755, 0x88997755,
+   0x99546327, 0x7946fa9e, 0xa6b5f8ee,
+   0x12345678
+  };
+
+void app_main(void)
+{
+  /* Do something with the array, in order to avoid it being discarded. */
+  for (uint32_t i = 0; i < 10; i++)
+    printf ("%x\n", testTab[i]);
+}