libdwfl: Work around ET_REL files with sh_addr fields set to non-zero

libdwfl: Work around ET_REL files with sh_addr fields set to non-zero

libdwfl tries to model the loading of modules (executables, shared
libraries, the linux kernel and/or kernel modules) in
memory. Depending on the (offline) load address it then also applies
(simple) relocations for ET_REL (object files or kernel modules). Load
addresses are normally represented through phdr segments for ET_EXEC
or ET_DYN, but for ET_REL files (which don't have phdrs) the Elf
section sh_addr fields are used.

The sh_addr fields of the ET_REL Elf images are updated in two places
__libdwfl_elf_address_range (through __libdwfl_report_elf) and
__libdwfl_relocate (through dwfl_module_getelf and
dwfl_module_getdwarf). Both rely on sh_addr being zero if no load
address has been set yet, so the address layout for each (SHF_ALLOC)
section is done only once.

Recent linux kernels use a linker script that does set the sh_addr
fields to (random, linker assigned) non-zero addresses. See commit
1ba9f8979426 ("vmlinux.lds: Unify TEXT_MAIN, DATA_MAIN, and related
macros").

The sh_addr values seems unnecessary, but because they aren't zero
anymore our layout/relocation code doesn't know it still has to figure
out a "real" load value for these sections.

Introduce __libdwfl_reset_sh_addr which resets all sh_addr fields to
zero for SHF_ALLOC sections in ET_REL files.

We don't call __libdwfl_reset_sh_addr on aux_sym files (from
.gnu_debugdata) and when constructing an Elf from a core file. In all
other cases we know (or assume) that the Elf file is being opened
through libdw_open_elf (called indirectly through __libdw_open_elf,
__libdwfl_report_offline, dwfl_module_getelf and dwfl_module_getdwarf)

This technically changes the Elf that goes through libdwfl, but we
would already update the sh_addr fields for ET_REL Elf sections in
memory anyway to represent the load address as libdwfl would see
them. So this isn't really a change in behavior (it just might update
the sh_addr field twice).

	* libdwfl/libdwflP.h (__libdwfl_reset_sh_addr): Define new
        internal function.
	* libdwfl/relocate.c (__libdwfl_reset_sh_addr): New internal
        function.
	* libdwfl/open.c (libdw_open_elf): Call __libdwfl_reset_sh_addr.
	* libdwfl/dwfl_module_getdwarf.c (open_elf_file): Add comment.
	(find_aux_sym): Likewise.
	(find_dw): Likewise.
	* libdwfl/dwfl_segment_report_module.c
	(dwfl_segment_report_module): Likewise.
	* libdwfl/dwfl_report_elf.c (__libdwfl_report_elf): Likewise.

https://sourceware.org/bugzilla/show_bug.cgi?id=33958

Signed-off-by: Mark Wielaard <mark@klomp.org>