Updating a Release System with ELF Programs – No Symbol Tables
You can easily extend the firmware in your released, embedded system using ELF program provide via a file system. For example, an SD card or, perhaps, downloaded into on-board SPI FLASH. In order to support such post-release update, your released firmware would have to support execution of fully linked, relocatable ELF programs loaded into RAM (see, for example, apps/examples/elf).
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Alan Carvalho de Assis has also made a video based on this example in the YouTube NuttX Channel.
Creating the Export Package
At the time that you release the firmware, you should create and save an export package. The export packet is all that you need to create post-release, add-on modules for your embedded system. Let's illustrate this using the STM32F4-Discovery networking NSH configuration with the STM32F4DIS-BB baseboard (This demonstration assumes that you also have support for some externally modifiable media in the board configuration. The could be removable media such as SD card or a USB FLASH stick, an internal file system that is remotely accessible via USB MSC, FTP, or whatever, or a remote file system (NFS). The networking NSH configuration uses the SD card on the STM32 baseboard for this demonstration. Other NSH configurations could be used provided that you supply the necessary file system support in some fashion.)(No baseboard? You can add support file system support to the basic STM32F4-Discovery board following these instructions: USB FLASH drive or SD card)
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nuttx-export-x.x |- arch/ |- build/ |- include/ |- libs/ |- startup/ |- System.map `- .config |
The Add-On Build Directory
In order to create the add-on ELF program, you will need (1) the export package, (2) the program build Makefile, (3) a linker script used by the Makefile, and (3) a Bash script to create a linker script. That Makefile and Bash Script are discussed in the following paragraphs(NOTE that these example files implicitly assume a GNU tool chain is used and, in at least one place, and the target is a ARMv7-M platform. A non-GNU tool chain would probably require a significantly different Makefile and linker script. There as at least one ARMv7-M specific change that would have to be made for other platforms in the script that creates the linker script, mkdefines.sh.)
Hello Example
To keep things manageable, let's use a concrete example Suppose the ELF program that we wish to add to the release code is the since source file hello.c:
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Let's say that we have a a directory called addon and contains the hello.c source file, a Makefile that will create the the ELF program, and a Bash script called mkdefines.sh that will create the a linker script.
Building the ELF Program =
The first step in creating the ELF program is the unzip the Export Package. We start with out addon directory containing the following:
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Then we have a new directory called nuttx-export-7.25 that contains all of the content from the released NuttX code that we need to build the ELF program.
The Makefile
The ELF program is created simply as:
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clean:
rm -f $(BIN)
rm -f $(REL)
rm -f defines.ld
rm -f System.map
rm -f *.o
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The Linker Script
Two linker scripts are used. One, I'll call it the main linker script, is just a normal file. The other, defined.ld is a created on-the-fly as described in the following paragraph.
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| Code Block |
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.stab 0 : { *(.stab) }
.stabstr 0 : { *(.stabstr) }
.stab.excl 0 : { *(.stab.excl) }
.stab.exclstr 0 : { *(.stab.exclstr) }
.stab.index 0 : { *(.stab.index) }
.stab.indexstr 0 : { *(.stab.indexstr) }
.comment 0 : { *(.comment) }
.debug_abbrev 0 : { *(.debug_abbrev) }
.debug_info 0 : { *(.debug_info) }
.debug_line 0 : { *(.debug_line) }
.debug_pubnames 0 : { *(.debug_pubnames) }
.debug_aranges 0 : { *(.debug_aranges) }
}
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Creating the defined.ld Linker Script
The additional linker script defines.ld was created through a three step process: First, the Makefile generates a partial linked ELF object, hello.r. The partially linked object is the hello ELF program but with undefined symbols. The Makefile then invokes the mkdefines.sh script which generates the defined.ld linker script that provides values for all of the undefined symbols. Finally, the Makefile then produces the fully linked, relocatable hello ELF object using the defines.ld linker script..
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printf = 0x0800aefc | 0x00000001 ; |
Replacing an NSH Built-In Function
Files can be executed by NSH from the command line by simply typing the name of the ELF program. This requires (1) that the feature be enabled with CONFIG_NSH_FILE_APP=y and (2) that support for the PATH variable is enabled (CONFIG_BINFMT_EXEPATH=y and CONFIG_PATH_INITIAL set to the mount point of the file system that may contain ELF programs.
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nsh> mount -t vfat /dev/mmcsd0 /bin nsh> hello Hello from Add-On Program! nsh> |
Version Dependency
NOTE that this technique generates ELF programs using fixed addresses from the System.map map file of a versioned release. The generated ELF programs can only be used with that specific firmware version. A crash will most likely result if used with the different firmware version because the addresses from the System.map will not match the addresses in a different version of the firmware.
The alternative approach using Symbol Tables is more or less version independent.
Tightly Coupled Memories
Most MCUs based on ARMv7-M family processors support some kind of Tightly Coupled Memory (TCM). These TCMs have somewhat different properties for specialized operations. The STM32 F4 supports similar Core Coupled Memory (CCM). It is important to not that you cannot execute programs from CCM!
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