pmu uprobes are broken

[lmb]

@ti-mo @lmb FYI pmu uprobes are broken; they were working before doing some refactoring. I suppose it's the offset not being passed correctly to perf_event_open:

$ sudo bpftool perf
pid 1056229  fd 3: prog_id 712  uretprobe  filename /bin/bash  offset 0

I am continuing to debug.

PS. currently I believe there aren't "e2e" tests, are these planned?

Originally posted by @mmat11 in #280 (comment)

[ti-mo]

@ungureanuvladvictor pointed me to the fact that the uprobe example works on Ubuntu's build of Bash (including 21.04), and I found that Ubuntu ships with a statically-linked version of readline. It looks like they vendor a version of readline in the Bash tree, but it can optionally be linked against libreadline.so:

~ objdump -T /bin/bash
00000000000ad900 g    DF .text  0000000000000088  Base        readline

On my system (Arch), readline is provided by libreadline.so:

λ  ~  objdump -T /bin/bash | grep readline
0000000000000000      DF *UND*	0000000000000000              readline
λ  ~  ldd /bin/bash
	libreadline.so.8 => /usr/lib/libreadline.so.8 (0x00007ffad7983000)

The 0 offset as parsed by debug/elf is correct, this symbol is to be provided by a library and still needs to be relocated. This means we'll either need to find or implement a dynamic linker in Go, and allow the caller to pass in their own offsets for now. Will close this with a PR to disable creating Uprobes from symbols from SHN_UNDEF with offset 0.