scripts/gdb/symbols: make BPF debug info available to GDB

scripts/gdb/symbols: make BPF debug info available to GDB

One can debug BPF programs with QEMU gdbstub by setting a breakpoint on
bpf_prog_kallsyms_add(), waiting for a hit with a matching aux.name, and
then setting a breakpoint on bpf_func.  This is tedious, error-prone, and
also lacks line numbers.

Automate this in a way similar to the existing support for modules in
lx-symbols.

Enumerate and monitor changes to both BPF kallsyms and JITed progs.  For
each ksym, generate and compile a synthetic .s file containing the name,
code, and size.  In addition, if this ksym is also a prog, and not a
trampoline, add line number information.

Ensure that this is a no-op if the kernel is built without BPF support or
if "as" is missing.  In theory the "as" dependency may be dropped by
generating the synthetic .o file manually, but this is too much complexity
for too little benefit.

Now one can debug BPF progs out of the box like this:

    (gdb) lx-symbols -bpf
    (gdb) b bpf_prog_4e612a6a881a086b_arena_list_add
    Breakpoint 2 (bpf_prog_4e612a6a881a086b_arena_list_add) pending.

    # ./test_progs -t arena_list

    Thread 4 hit Breakpoint 2, bpf_prog_4e612a6a881a086b_arena_list_add ()
        at linux/tools/testing/selftests/bpf/progs/arena_list.c:51
    51              list_head = &global_head;
    (gdb) n
    bpf_prog_4e612a6a881a086b_arena_list_add () at linux/tools/testing/selftests/bpf/progs/arena_list.c:53
    53              for (i = zero; i < cnt && can_loop; i++) {

This also works for subprogs.

Link: https://lkml.kernel.org/r/20251106124600.86736-3-iii@linux.ibm.com
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkman <daniel@iogearbox.net>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>