Added support for symbol relocation for RISC-V architecture

[adjordjevic-TT]

Added support for symbol relocation for RISC-V architecture.

Added ENUM_RELOC_TYPE_RISCV to enums.py

Added _RELOCATION_RECIPES_RISCV, two more relocation functions (_reloc_calc_value_plus_sym_6 and _reloc_calc_value_minus_sym_addend_6) and case when e-machine is RISC-V to relocation.py.

[adjordjevic-TT]

@eliben Can you take a look at this?

[eliben]

@sevaa any concerns?

[sevaa]

Looks in line with how relocation is handled for other architectures. Is there any way to test this?

[adjordjevic-TT]

Would it be enough to just provide arbitrary elf that uses RISCV relocations and just to add it to testfiles or those elfs have some specific content? @sevaa

[sevaa]

An erroneously written piece of relocation logic most likely will not crash; it will produce wrong bits in the loaded section. How does one test that?

[adjordjevic-TT]

Ideally, we can create elf that uses RISCV relocations with some functions/variables and if we are able to find those symbols relocations are working properly, but I don't think it is practical to test every sing one relocation type. Everything that is implemented is taken from here: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#reloc-table

[sevaa]

If @eliben says it's OK not to test this exhaustively, I will be OK with that :)

[eliben]

@adjordjevic-TT can you say what you're using these RISC-V relocations for? (in other words, why this PR?)

@sevaa several of the readelf comparison tests rely on correct implementation of relocations. Wouldn't a file compiled for RISC-V with relocations inside therefore exercise this logic?

[adjordjevic-TT]

We are using it for finding variable's address or value and for implementing backtracing (callstack) for kernels that are running on our hardware that is based on RISCV arch.

[eliben]

@adjordjevic-TT could you add a simple unit test? Look at the other unit tests in the test/ directory for inspiration (the file itself can be placed in testfiles_for_unittests, and please add a .c file alongside it with build instructions to reproduce the binary)

[sevaa]

The only sections that are parsed, dumped, and compared by the readelf test are the DWARF ones and notes; in executable/SO binaries, relocations against those are rare (none in the corpus, last time I checked). Were we to dump and compare the disassembly, though, or the initialized global data section, that would be the test.

[eliben]

The only sections that are parsed, dumped, and compared by the readelf test are the DWARF ones and notes; in executable/SO binaries, relocations against those are rare (none in the corpus, last time I checked). Were we to dump and compare the disassembly, though, or the initialized global data section, that would be the test.

Let's continue this discussion in #601