llext: add custom post-processing step

This commit adds a new POSTPROC_CMD argument to the add_llext_target()
macro. This argument allows the user to specify a command to run on the
generated llext file after compilation and before stripping. This is
useful for post-processing the llext file in some way, such as adjusting
the addresses of certain symbols.

The post processor will be invoked by CMake as:

  <POSTPROC_CMD> <input_file> <output_file> <POSTPROC_ARGS>

POSTPROC_ARGS can also use generator expressions that refer to the
"${target_name}_lib" target to get additional flags.

Signed-off-by: Luca Burelli <[email protected]>

cmake/modules/extensions.cmake

@@ -4802,6 +4802,14 @@ endmacro()
 # loadable extensions (llexts).
 #
 
+# Usage:
+#   add_llext_target(<target_name>
+#                    OUTPUT  <output_file>
+#                    SOURCES <source_file>
+#                    [C_FLAGS <flags> ...]
+#                    [POSTPROC_CMD <cmd> [POSTPROC_ARGS <args> ...]]
+#   )
+#
 # Add a custom target that compiles a single source file to a .llext file.
 #
 # Output and source files must be specified using the OUTPUT and SOURCES
@@ -4811,10 +4819,21 @@ endmacro()
 # in the Zephyr build, but with some important modifications. The list of
 # flags to remove and flags to append is controlled respectively by the
 # LLEXT_REMOVE_FLAGS and LLEXT_APPEND_FLAGS global variables.
-
+#
 # The C_FLAGS argument can be used to pass additional compiler flags to the
 # compilation of this particular llext.
 #
+# The POSTPROC_CMD and POSTPROC_ARGS arguments can be used to specify a command
+# to run on the generated llext file after compilation and before stripping.
+# This is useful for post-processing the llext file in some way, such as
+# adjusting the addressess of certain symbols.
+# The post processor will be invoked by CMake as:
+#
+#   <POSTPROC_CMD> <input_file> <output_file> <POSTPROC_ARGS>
+#
+# POSTPROC_ARGS can also use generator expressions that refer to the
+# "${target_name}_lib" target to get additional flags.
+#
 # Example usage:
 #   add_llext_target(hello_world
 #     OUTPUT  ${PROJECT_BINARY_DIR}/hello_world.llext
@@ -4825,8 +4844,8 @@ endmacro()
 # ${PROJECT_BINARY_DIR}/hello_world.llext, adding -Werror to the compilation.
 #
 function(add_llext_target target_name)
-  set(single_args OUTPUT)
-  set(multi_args SOURCES;C_FLAGS)
+  set(single_args OUTPUT;POSTPROC_CMD)
+  set(multi_args SOURCES;C_FLAGS;POSTPROC_ARGS)
   cmake_parse_arguments(PARSE_ARGV 1 LLEXT "${options}" "${single_args}" "${multi_args}")
 
   # Check that the llext subsystem is enabled for this build
@@ -4915,14 +4934,37 @@ function(add_llext_target target_name)
     zephyr_generated_headers
   )
 
+  if(LLEXT_POSTPROC_CMD)
+
+    # Post-process the linked llext file
+    set(postproc_target ${target_name}_pp)
+    set(postproc_file ${target_name}_pp.llext)
+    add_custom_command(
+      OUTPUT ${postproc_file}
+      COMMAND ${LLEXT_POSTPROC_CMD}
+              ${linked_file}
+              ${postproc_file}
+              ${LLEXT_POSTPROC_ARGS}
+      DEPENDS ${link_target} ${linked_file}
+      COMMAND_EXPAND_LISTS)
+    add_custom_target(${postproc_target} DEPENDS ${postproc_file})
+
+  else()
+
+    # Skip post-processing and reuse the original file
+    set(postproc_target ${link_target})
+    set(postproc_file ${linked_file})
+
+  endif()
+
   # Arch-specific conversion of the object file to an llext
   if(CONFIG_ARM)
 
     # No conversion required, simply copy the object file
     add_custom_command(
       OUTPUT ${output_file}
-      COMMAND ${CMAKE_COMMAND} -E copy ${linked_file} ${output_file}
-      DEPENDS ${link_target} ${linked_file}
+      COMMAND ${CMAKE_COMMAND} -E copy ${postproc_file} ${output_file}
+      DEPENDS ${postproc_target} ${postproc_file}
     )
 
   elseif(CONFIG_XTENSA)
@@ -4933,10 +4975,10 @@ function(add_llext_target target_name)
       COMMAND $<TARGET_PROPERTY:bintools,strip_command>
               $<TARGET_PROPERTY:bintools,strip_flag>
               $<TARGET_PROPERTY:bintools,strip_flag_remove_section>.xt.*
-              $<TARGET_PROPERTY:bintools,strip_flag_infile>${linked_file}
+              $<TARGET_PROPERTY:bintools,strip_flag_infile>${postproc_file}
               $<TARGET_PROPERTY:bintools,strip_flag_outfile>${output_file}
               $<TARGET_PROPERTY:bintools,strip_flag_final>
-      DEPENDS ${link_target} ${linked_file}
+      DEPENDS ${postproc_target} ${postproc_file}
     )
 
   else()