btf_encoder: Group all function ELF syms by function name

btf_encoder collects function ELF symbols into a table, which is later
used for processing DWARF data and determining whether a function can
be added to BTF.

So far the ELF symbol name was used as a key for search in this table,
and a search by prefix match was attempted in cases when ELF symbol
name has a compiler-generated suffix.

This implementation has bugs [1][2], causing some functions to be
inappropriately excluded from (or included into) BTF.

Rework the implementation of the ELF functions table. Use a name of a
function without any suffix - symbol name before the first occurrence
of '.' - as a key. This way btf_encoder__find_function() always
returns a valid elf_function object (or NULL).

Collect an array of symbol name + address pairs from GElf_Sym for each
elf_function when building the elf_functions table.

Introduce ambiguous_addr flag to the btf_encoder_func_state. It is set
when the function is saved by examining the array of ELF symbols in
elf_function__has_ambiguous_address(). It tests whether there is only
one unique address for this function name, taking into account that
some addresses associated with it are not relevant:
  * ".cold" suffix indicates a piece of hot/cold split
  * ".part" suffix indicates a piece of partial inline

When inspecting symbol name we have to search for any occurrence of
the target suffix, as opposed to testing the entire suffix, or the end
of a string. This is because suffixes may be combined by the compiler,
for example producing ".isra0.cold", and the conclusion will be
incorrect.

In saved_functions_combine() check ambiguous_addr when deciding
whether a function should be included in BTF.

Successful CI run: https://github.com/acmel/dwarves/pull/68/checks

I manually spot checked some of the ~200 functions from vmlinux (BPF
CI-like kconfig) that are now excluded: all of those that I checked
had multiple addresses, and some where static functions from different
files with the same name.

[1] https://lore.kernel.org/bpf/[email protected]/
[2] https://lore.kernel.org/dwarves/[email protected]/

v1: https://lore.kernel.org/dwarves/[email protected]/
Signed-off-by: Ihor Solodrai <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Eduard Zingerman <[email protected]>
Cc: Menglong Dong <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/dwarves/[email protected]/
Signed-off-by: Alan Maguire <[email protected]>

Author: theihor