[RFC PATCH 0/2] elfutils: don't use dlopen() for libebl modules

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

Hi,

I'm developing an application which uses libdwfl. When I tested it in
our production environment, the application hit DWARF parsing errors for
Linux kernel modules. I tracked it down to an issue that ELF relocations
were silently not being applied because the proper libebl_CPU.so could
not be found. That appears to be a bug in relocate_sections() or
check_badreltype() in libdwfl, but that got me looking at getting rid of
the dlopen() entirely. IMO, it simplifies things nicely, and it makes it
possible to use the elfutils libraries in standalone static binaries.

I came across a previous discussion [1] where Mark suggested a hybrid
approach, where one could configure which modules to compile in and
which ones to keep separate. I tried to implement this, but it quickly
turned into automake dependency hell: the backends subdirectory needs to
depend on the libdw subdirectory in order to link libebl_CPU.so against
libdw.so, but the libdw subdirectory needs to depend on the backends
subdirectory in order to include the static backends in libdw.so.

My understanding of the benefit of separate modules is that we don't
need to link all backend modules into every elfutils binary. I did some
measurements to that end:

Dynamic backends (status quo):
44K     ./libasm/libasm.so
380K    ./libdw/libdw.so
120K    ./libelf/libelf.so
72K     ./src/nm
28K     ./src/size
80K     ./src/strip
140K    ./src/elflint
28K     ./src/findtextrel
36K     ./src/addr2line
52K     ./src/elfcmp
56K     ./src/objdump
32K     ./src/strings
48K     ./src/ar
60K     ./src/unstrip
36K     ./src/stack
40K     ./src/elfcompress
32K     ./src/ranlib
272K    ./src/readelf
68K     backends/libebl_i386.so
24K     backends/libebl_sh.so
76K     backends/libebl_x86_64.so
28K     backends/libebl_ia64.so
24K     backends/libebl_alpha.so
36K     backends/libebl_arm.so
44K     backends/libebl_aarch64.so
36K     backends/libebl_sparc.so
36K     backends/libebl_ppc.so
36K     backends/libebl_ppc64.so
32K     backends/libebl_s390.so
24K     backends/libebl_tilegx.so
20K     backends/libebl_m68k.so
24K     backends/libebl_bpf.so
32K     backends/libebl_riscv.so
2.1M    total

Static backends (after this series):
44K     ./libasm/libasm.so
668K    ./libdw/libdw.so
120K    ./libelf/libelf.so
356K    ./src/nm
28K     ./src/size
368K    ./src/strip
428K    ./src/elflint
28K     ./src/findtextrel
36K     ./src/addr2line
340K    ./src/elfcmp
348K    ./src/objdump
32K     ./src/strings
48K     ./src/ar
60K     ./src/unstrip
36K     ./src/stack
40K     ./src/elfcompress
32K     ./src/ranlib
560K    ./src/readelf
3.5M    total

If the size increase is a deal-breaker, one easy solution may be to
export the libebl symbols from libdw so that libebl.a doesn't need to be
linked into the elfutils binaries. Considering that libebl.a is already
shipped by distributions, this doesn't increase the surface area of
exported APIs (and the warning in libebl.h should make it clear that
these APIs are not intended to be used).

Thoughts? Is the size increase palatable? Should I add a third patch to
export the libebl symbols in libdw in order to mitigate the size
increase? Or is this a dead end?

Thanks!

1: https://sourceware.org/ml/elfutils-devel/2018-q2/msg00171.html

Omar Sandoval (2):
  libcpu: merge libcpu_{i386,x86_64,bpf} into one library
  Don't use dlopen() for libebl modules

 ChangeLog                |   6 +
 Makefile.am              |   4 +-
 backends/ChangeLog       |   5 +
 backends/Makefile.am     |  99 ++-------------
 configure.ac             |  12 +-
 libcpu/ChangeLog         |   4 +
 libcpu/Makefile.am       |  12 +-
 libdw/ChangeLog          |   7 +
 libdw/Makefile.am        |  11 +-
 libebl/ChangeLog         |  11 ++
 libebl/Makefile.am       |   7 +-
 libebl/eblclosebackend.c |   4 -
 libebl/eblopenbackend.c  | 267 +++++++++++++++------------------------
 libebl/libeblP.h         |   3 -
 src/ChangeLog            |   7 +
 src/Makefile.am          |  18 +--
 tests/ChangeLog          |   5 +
 tests/Makefile.am        |  34 ++---
 tests/test-subr.sh       |   4 +-
 19 files changed, 208 insertions(+), 312 deletions(-)

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2.22.0