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tock / elf2tab

Tool to create Tock Application Bundles from ELF files.
MIT License
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elf2tab

elf2tab is a tool that converts Tock userland apps from .elf files to Tock Application Bundles (TABs or .tab files). TABs are Tock apps that have been compiled for the various architectures that Tock runs on.

Usage

Usage: elf2tab [OPTIONS] <elf[,architecture]>...

Arguments:
  <elf[,architecture]>...  application file(s) to package

Options:
  -v, --verbose                                        Be verbose
      --deterministic                                  Produce a deterministic TAB file
      --disable                                        Mark the app as disabled in the TBF flags
      --app-version <APP_VERSION>                      Set the version number [default: 0]
      --minimum-ram-size <min-ram-size>                in bytes
  -o, --output-file <filename>                         output file name [default: TockApp.tab]
  -n, --package-name <pkg-name>                        package name
      --stack <stack-size>                             in bytes
      --app-heap <heap-size>                           in bytes [default: 1024]
      --kernel-heap <kernel-heap-size>                 in bytes [default: 1024]
      --protected-region-size <protected-region-size>  Size of the protected region (including headers)
      --permissions <permissions>...                   A list of driver numbers and allowed commands
      --write_id <write_id>                            A storage ID used for writing data
      --read_ids <read_ids>...                         Storage IDs that this app is allowed to read
      --access_ids <access_ids>...                     Storage IDs that this app is allowed to write
      --kernel-major <kernel-major-version>            The kernel version that the app requires
      --kernel-minor <kernel-minor-version>            The minimum kernel minor version that the app requires
      --supported-boards <supported-boards>            comma separated list of boards this app is compatible with
      --minimum-footer-size <min-footer-size>          Minimum number of bytes to reserve space for in the footer [default: 0]
      --sha256                                         Add a SHA256 hash credential to each TBF
      --sha384                                         Add a SHA384 hash credential to each TBF
      --sha512                                         Add a SHA512 hash credential to each TBF
      --rsa4096-private <rsa4096-private-key>          Add an 4096-bit RSA signature credential using this private key
  -h, --help                                           Print help
  -V, --version                                        Print version

For example, converting a "blink" app from a compiled .elf file (for a Cortex-M4 device) with this tool would look like:

$ elf2tab -o blink.tab -n blink --stack 1024 --app-heap 1024 --kernel-heap 1024 cortex-m4.elf

It also supports (and encourages!) combining .elf files for multiple architectures into a single tab:

$ elf2tab -o blink.tab -n blink --stack 1024 --app-heap 1024 --kernel-heap 1024 cortex-m0.elf cortex-m3.elf cortex-m4.elf

Compiling elf2tab

With rustup installed, simply run:

cargo build

Adding TBF Credentials

elf2tab supports adding credentials to the TBF footer of the generated TBF files. To add a hash, use one or more of these flags: --sha256, --sha384, --sha512.

elf2tab can also sign the TBF with a public/private RSA key pair. To generate compatible keys:

$ openssl genrsa -aes256 -out tockkey.private.pem 4096
$ openssl pkcs8 -topk8 -nocrypt -outform der -in tockkey.private.pem -out tockkey.private.pk8
$ openssl rsa -in tockkey.private.pem -outform der -pubout -out tockkey.public.der

Then pass the keys to elf2tab:

$ elf2tab --rsa4096-private tockkey.private.pk8 ...

Example including multiple credentials:

$ elf2tab --sha256 --sha384 --sha512 --rsa4096-private tockkey.private.pk8 ...

elf2tab Details

elf2tab tries to be as generic as possible for creating apps that can be flashed onto a Tock board. It does three main things:

  1. Extracts the various sections in each .elf file and creates a binary file per .elf from the sections.
  2. Prepends a Tock Binary Format header to each binary.
  3. Creates the TAB file by creating a tar file with each of the Tock binaries.

Creating binary files from .elf files

elf2tab tries to process .elf files in as generic of a way as possible. To create the binary file, elf2tab iterates through the sections in the .elf file in their offset order that are writeable, executable, or allocated, have nonzero length, and are of type PROGBITS. The binary data for each of these sections are concatenated into the output file.

Next, elf2tab appends to the binary all writeable or allocated sections that contain the string .rel in their name. Because of how these sections are created for PIC code by the linker, it seems these sections have to be special cased and not grouped into the first step.

Creating the TBF Header

All Tock apps must start with a Tock Binary Format header so that the kernel knows how big the app is, how much memory it requires, and other important properties. elf2tab handles creating this header automatically, and mostly just requires the --stack, --app-heap, and --kernel-heap flags so it knows the memory requirements.

However, the TBF header also contains information about "writeable flash regions", or portions of the application's address space in flash that the app intends to use to store persistent data. This information is added to the header so that the kernel and other tools know that there is persistent that should be maintained intact. To specify to elf2tab that a linker section is one of these writeable flash regions, the name of the section should include the string .wfr. Any sections in the .elf that include .wfr in their name will have their relative address offset included in the TBF header via the TbfHeaderWriteableFlashRegions TLV.

elf2tab will also automatically add a TBF "fixed addresses" TLV header if it finds that the .elf file was compiled for a fixed address in RAM or flash instead of being position independent. To detect a fixed flash address, elf2tab looks to see if the flash segment is at the dummy flash address for PIC apps or not. To detect a fixed RAM address, elf2tab looks for a _sram_origin symbol, and if it exists checks if the address matches the dummy RAM address for PIC apps or not.

elf2tab has to choose a length for the protected region after the TBF header and before the start of the actual application binary. Normally, this defaults to 0. It can be fixed for all TBFs in the TAB using the command line argument --protected-region-size (which takes as an argument entire size before the application binary, including the TBF header). However, a TAB can include both PIC apps and non-PIC apps, and setting the size for all TBFs isn't always desirable. Therefore, elf2tab further supports supplying the protected region size through a tbf_protected_region_size symbol in the input ELF files. If neither this symbol nor --protected-region-size are passed, for apps compiled for fixed addresses (as determined above) elf2tab will estimate a protected region size that tries to ensure the start of the TBF headers and the application binary are placed at useful addresses in flash. elf2tab will try to increase the size of the protected region to make the start of the TBF header at an address aligned to 256 bytes when the application binary is at its correct fixed address.

Syscall Permissions

elf2tab allows explicitly specifying the syscalls that an app is allowed to call. This is done with the --permissions flag. An example of allowing driver number 1 command 0 and command 1 looks like this:

$ elf2tab --permissions 1,0 1,1 ...

It is then up to the Tock kernel and board to apply the filters.

Storage IDs

elf2tab also allows specifying the storage IDs. These are used to access nonvolatile data from userspace. You can specify a single write_id used to store new data and multiple read_ids and access_ids used to enforce read/write permissions on existing data.

An example looks like this:

$ elf2tab  --write_id 12345678 --read_ids 1 2 --access_ids 2 3 ...

Creating the TAB file

After generating the program binary and TBF header for each .elf file specified in the command line, elf2tab will store those files along side the .elf files (using the .tbf extension), and create a TAB file containing each .tbf file. These .tab files are used by tools like Tockloader to load Tock apps on to boards.

Inspecting TABs

Tockloader can show some details of a .tab file. Simply:

$ tockloader inspect-tab <tab file name>