[otbn] Document and tighten up objcopy command for embedded objects
Remove the executable flag from any code sections: these aren't
executable on Ibex.
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/util/otbn_build.py b/util/otbn_build.py
index 93f8f2f..51256c8 100755
--- a/util/otbn_build.py
+++ b/util/otbn_build.py
@@ -170,15 +170,27 @@
out_elf = out_dir / (app_name + '.elf')
call_otbn_ld(obj_files, out_elf, linker_script = args.linker_script)
+ # Use objcopy to create an ELF that can be linked into a RISC-V binary
+ # (to run on Ibex). This should set flags for all sections to look like
+ # rodata (since they're not executable on Ibex, nor does it make sense
+ # for Ibex code to manipulate OTBN data sections "in place"). We name
+ # them with a .otbn prefix, so end up with e.g. .rodata.otbn.text and
+ # .rodata.otbn.data.
+ #
+ # Symbols that are exposed by the binary (including those giving the
+ # start and end of imem and dmem) will be relocated as part of the
+ # link, so they'll give addresses in the Ibex address space. So that
+ # the RISC-V binary can link multiple OTBN applications, we give them
+ # an application-specific prefix. (Note: This prefix is used in
+ # sw/device/lib/runtime/otbn.h: so needs to be kept in sync with that).
+ sym_pfx = '_otbn_app_{}_'.format(app_name)
out_embedded_obj = out_dir / (app_name + '.rv32embed.o')
- args = [
- '-O',
- 'elf32-littleriscv',
- '--prefix-symbols',
- '_otbn_app_' + app_name + '_',
- out_elf,
- out_embedded_obj,
- ]
+ args = (['-O', 'elf32-littleriscv',
+ '--prefix-sections=.rodata.otbn',
+ '--set-section-flags=*=alloc,load,readonly',
+ '--prefix-symbols', sym_pfx] +
+ [out_elf,
+ out_embedded_obj])
call_rv32_objcopy(args)
except subprocess.CalledProcessError as e:
