How to tell compiler to pad a specific amount of bytes on every C function?

Question

I'm trying to practice some live instrumentation and I saw there was a linker option -call-nop=prefix-nop, but it has some restriction as it only works with GOT function (I don't know how to force compiler to generate GOT function, and not sure if it's good idea for performance reason.) Also, -call-nop=* cannot pad more than 1 byte.

Ideally, I'd like to see a compiler option to pad any specific amount of bytes, and compiler will still perform all the normal function alignment.

Once I have this pad area, I can at run time to reuse these padding area to store some values or redirect the control flow.

P.S. I believe Linux kernel use similar trick to dynamically enable some software tracepoint.

Answer 1

-pg is intended for profile-guided optimization. The correct option for this is -fpatchable-function-entry

• -fpatchable-function-entry=N[,M]

  Generate N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to 0 so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

It'll insert N single-byte 0x90 NOPs and doesn't make use of multi-byte NOPs thus performance isn't as good as it should, but you probably don't care about that in this case so the option should work fine

Answer 2

I achieved this goal by implement my own mcount function in an assembly file and compile the code with -pg.