create-diff-object: Remove undefined function symbols

[sumanthkorikkar]

When building shadow-pid.patch on a debug kernel, it generates __bug_table, which contains an array of struct bug_entries.

.rela__bug_table contains references to bug address, line number and column.

create-diff-object identifies that .text.kernel_clone has changed and it includes .rela.text.kernel_clone rela section. Then later, it includes all symbols (in kpatch_include_symbols()) associated with it, which ends up including __bug_table and its rela section .rela__bug_table. Then, all the function symbols associated with .rela__bug_table is included irrespective of whether it's section is included or not.

This leads to the following modpost errors:
kernel/fork.o: changed function: kernel_clone
kernel/exit.o: changed function: do_exit
fs/proc/array.o: changed function: proc_pid_status make -C /root/linux M=/root/.kpatch/tmp/patch CFLAGS_MODULE='' make[1]: Entering directory '/root/linux'
make[2]: Entering directory '/root/.kpatch/tmp/patch'
LDS kpatch.lds
CC [M] patch-hook.o
LD [M] test-shadow-newpid.o
MODPOST Module.symvers
WARNING: modpost: missing MODULE_DESCRIPTION() in test-shadow-newpid.o
ERROR: modpost: "replace_mm_exe_file" [test-shadow-newpid.ko] undefined!
ERROR: modpost: "put_task_stack" [test-shadow-newpid.ko] undefined!
ERROR: modpost: "release_task" [test-shadow-newpid.ko] undefined!
ERROR: modpost: "set_mm_exe_file" [test-shadow-newpid.ko] undefined!

Examining the /root/.kpatch/patch/ directory reveals, these symbols are never referenced in any relas.

readelf -Ws output.o |
grep -E 'put_task_stack|replace_mm_exe_file|release_task|set_mm_exe_file'
27: 0000000000000000 0 SECTION LOCAL DEFAULT 36 .rodata.release_task.str1.2
45: 0000000000000000 0 SECTION LOCAL DEFAULT 55 .rodata.set_mm_exe_file.str1.2
47: 0000000000000000 0 SECTION LOCAL DEFAULT 57 .rodata.replace_mm_exe_file.str1.2
234: 0000000000000000 0 FUNC GLOBAL DEFAULT UND replace_mm_exe_file
254: 0000000000000000 0 FUNC GLOBAL DEFAULT UND put_task_stack
263: 0000000000000000 0 FUNC GLOBAL DEFAULT UND release_task
269: 0000000000000000 0 FUNC GLOBAL DEFAULT UND set_mm_exe_file

readelf -Wr output.o |
grep -E 'put_task_stack|replace_mm_exe_file|release_task|set_mm_exe_file'

Hence, exclude these unreferenced symbols to avoid modpost errors.

Fix:

• Identify all function symbols present in __bug_table and track their symbol indices.
• Exclude .rela__bug_table and .rela__mcount_loc, and for all other relocation sections, check whether any of these symbol indices are actually referenced.
• If a symbol index is never referenced in any relevant relocation section and the symbol’s section is not included in the patch, exclude the symbol from being added.

[sumanthkorikkar]

The dynamic-debug-jump-label-issue-1253 test is failing with this PR. I will investigate the failing test. Let me know, if there are better approaches or suggestions to solve this problem. Thank you