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.

PATCH RFC v2:
Note: Skipped need_klp_reloc()/kpatch_create_intermediate_sections()
check for .rela__bug_table section.
Reason: The function symbols that were not referenced by any sections
other than .rela__bug_table were being initialized with include = 0 (via
rela->sym->include = 0). As a result, kpatch_migrate_included_elements()
did not migrate these function symbols into kelf_out. However, later in
kpatch_create_intermediate_sections(), when parsing the .rela__bug_table
relasec and evaluating each symbol in need_klp_reloc(), the
code ended up using the previous rela->sym reference (which had already
been torn down). Since that symbol had its include field set to 0, the
dereference led to a segmentation fault. To prevent this, the
.rela__bug_table section is excluded from consideration in
kpatch_migrate_included_elements(). Additionally, if a function is
modified, the assumption is that, it will be referenced by other
relasec.

[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

[sumanthkorikkar]

Added RFC patch v2