FASMARM elf relocation

Just in case someone is interested: this is a brief description of my DIY approach for linking fasmarm code with gcc code; more specifically for making a flat binary with fasmarm as main program and some support functions together with gcc-compiled object modules and static libraries with other functions.

The fasmarm code has "extrn" directives for calls to gcc C code and it has "public" directives to support calls into to it. However the "extrn" directives are wrapped into using a "LINK" macro so that the external labels get duly defined for the actual load addresses.

Code:

macro LINK reladdr, name {
  name = (0x#reladdr - 0x000100d8 ) ; relative default _start address
  extrn `name as _#name
}

The load addresses (reladdr) are picked from the "nm" output of the linked executable and the Makefile is set up to verify this prior to extracting the flat binary from the executable. All in all the Makefile includes the following generic rules (in addition to the specific dependences):

Code:

.SUFFIXES: # Disable all standard generic rules

# Generic rule to compile C into .o (ELF 32-bit LSB relocatable)
## (the same as the standard rule)
%.o: %.c
        (COMPILE.c) $(OUTPUT_OPTION) $<

# Generic rule to compile fasmarm into .elf (ELF 32-bit LSB relocatable)
## with the additional dependency on $@-externals.S
%.elf: %.S %-externals.S
        fasmarm $(FASMARGS) $< $@

# Generic rule to link .elf and .o into .bin (ELF 32-bit LSB executable)
LINKARGS = -nostartfiles -nolibc -nostdlib -nodefaultlibs \
        -pie -static -static-pie -fno-plt -Wl,--no-wchar-size-warning
%.bin: %.elf
       gcc $(CFLAGS) $(LINKARGS) $^ -o $@ $(LIBS)

# Generic rule to obtain the flat binary (after confirming %-externals.S)
## with a shell command to report entry points in $@.bin
define NMLIST
nm -t x $@.bin  2> /dev/null | sed '/ T /!d;s/^//'
endef
## and an awk script to confirm $@-externals.S with that nm output
define AWKPROG
$$1 == "LINK" { LN = LN sprintf( "\nLINK %s, %s", MAP[$$3], $$3 ); \
                if ( MAP[$$3] != $$2 ) { ER = 1 } ; next } \
$$2=="T" { MAP[$$3]=$$1 ; next } \
END { if ( ER != 1 ) exit 0; \
      print "*** $@-external.S should be:" LN | "LC_ALL=C sort"; exit 1; }
endef
##
%: %.bin
        ( $(NMLIST) ; tr , ' ' < $@-externals.S ) | awk '$(AWKPROG)'
        objcopy -O binary -j .text -j .rodata $^ $@

Note that $(LIBS) may include object module libraries as per normal; the linking resolves all their references and only the references out of the fasmarm module(s) need the special attention via the "$@-externals.S" file.

To make it all work the fasmarm main program, myprog.S, has a lead-in for an armv7 elf object module even though the end product is a flat binary. The lead-in is as follows:

Code:

format elf
processor CPU32_ALL
use32
postpone { include 'myprog-externals.inc' }
section '.text' executable
public _start
_start:

The Makefile will build and extract the flat binary if the LINK directives in $@-externals.S conform with the ELF executable. Otherwise it builds the ELF executable and then it fails before extraction, and prints how the LINK file should look.

EDIT: corrected LINK macro to use 0x000100d8 (had it as 0x000100b8 before, which was wrong).