Trying to build the EtherCAT master driver (realtime I/O driver) on a RT Linux (SL6.3) I keep bumping into a missing symbol issue when building the EtherCAT kernel modules:

# make modules
make -C "/usr/src/kernels/3.8.13-rt27.40.el6rt.x86_64" M="/root/etherlabmaster-code-08aa7305b9baba37bdd9eb4d8c2a8762aa56a7e2" modules
make[1]: Entering directory `/usr/src/kernels/3.8.13-rt27.40.el6rt.x86_64'
  Building modules, stage 2.
  MODPOST 4 modules
WARNING: "__fentry__" [/root/etherlabmaster-code-08aa7305b9baba37bdd9eb4d8c2a8762aa56a7e2/master/ec_master.ko] undefined!
WARNING: "__fentry__" [/root/etherlabmaster-code-08aa7305b9baba37bdd9eb4d8c2a8762aa56a7e2/examples/mini/ec_mini.ko] undefined!
WARNING: "__fentry__" [/root/etherlabmaster-code-08aa7305b9baba37bdd9eb4d8c2a8762aa56a7e2/devices/ec_generic.ko] undefined!
WARNING: "__fentry__" [/root/etherlabmaster-code-08aa7305b9baba37bdd9eb4d8c2a8762aa56a7e2/devices/e1000e/ec_e1000e.ko] undefined!
make[1]: Leaving directory `/usr/src/kernels/3.8.13-rt27.40.el6rt.x86_64'

# grep -Hin "fentry" /boot/config-`uname -r`

# strings /boot/System.map-3.8.13-rt27.40.el6rt.x86_64 | grep -i "fentry"
-- nothing --

I'm using GCC v 4.9.1, as you can see, the kernel config file does contain an entry related to 'fentry' and is enabled.

Weird thing is that I would expect the System.map to contain the __fentry__ symbol declared in there, but it isn't!?

I've tried building the EtherCAT driver against the kernel 3.10-rt & 3.14-rt, same issue, it's complaining about about the __fentry__ symbol missing.

If I go ahead and launch the EtherCAT service, I get the following:

# service ethercat start
Starting EtherCAT master 1.5.2 FATAL: Error inserting ec_master (/lib/modules/3.8.13-rt27.40.el6rt.x86_64/ethercat/master/ec_master.ko): Unknown symbol in module, or unknown parameter (see dmesg)

# dmesg | tail
ec_master: Unknown symbol __fentry__ (err 0)
  • be careful with using strings on binaries ... if the binary is a file type that it recognises (e.g. ELF) then it will only look in certain sections (to quote from my man page only those sequences that are in loadable, initialized data sections) ... use strings --all file to make sure you get them all, on any type of file (I use strings - file as that is what I grew up with :-) Commented Feb 4, 2016 at 0:26
  • @MurrayJensen, thanks for the info, using strings --all still results with the same output.
    – fduff
    Commented Feb 4, 2016 at 6:23
  • 1
    look in arch/x86/kernel/entry_64.S for some clues ... __fentry__ is defined in that file, but only if CC_USING_FENTRY is defined ... this in turn is only defined if CONFIG_FUNCTION_TRACER and CONFIG_HAVE_FENTRY are defined. I'm guessing that somehow you've got a mismatch between the kernel config that is running and the config you are using to build the modules ... Commented Feb 4, 2016 at 12:06

1 Answer 1


I got in touch with the CERN IT department regarding their pre-built kernel package and to confirm my suspicions, they've built the kernel-rt package (rpm) based on GCC 4.4.7-x.

As I find out, in GCC versions prior to 4.6, it uses the __mcount__ symbol for function tracing when required. This has been replaced with __fentry__ in GCC 4.6 and above.

Resolution: In order to fix the missing symbol I had, I had to recompile the Linux kernel from source using a more recent version of the compiler (4.9.1 in this case) and then I got to build and link the kernel module no problem using the same compiler.

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