[PATCH 6.8 119/336] kbuild: Disable KCSAN for autogenerated *.mod.c intermediaries

Greg Kroah-Hartman
2024-05-14T10:33:40Z

From: Borislav Petkov (AMD) <bp@alien8.de>

[ Upstream commit 54babdc0343fff2f32dfaafaaa9e42c4db278204 ]

When KCSAN and CONSTRUCTORS are enabled, one can trigger the

"Unpatched return thunk in use. This should not happen!"

catch-all warning.

Usually, when objtool runs on the .o objects, it does generate a section
.return_sites which contains all offsets in the objects to the return
thunks of the functions present there. Those return thunks then get
patched at runtime by the alternatives.

KCSAN and CONSTRUCTORS add this to the object file's .text.startup
section:

-------------------
Disassembly of section .text.startup:

...

0000000000000010 <_sub_I_00099_0>:
10: f3 0f 1e fa endbr64
14: e8 00 00 00 00 call 19 <_sub_I_00099_0+0x9>
15: R_X86_64_PLT32 __tsan_init-0x4
19: e9 00 00 00 00 jmp 1e <__UNIQUE_ID___addressable_cryptd_alloc_aead349+0x6>
1a: R_X86_64_PLT32 __x86_return_thunk-0x4
-------------------

which, if it is built as a module goes through the intermediary stage of
creating a <module>.mod.c file which, when translated, receives a second
constructor:

-------------------
Disassembly of section .text.startup:

0000000000000010 <_sub_I_00099_0>:
10: f3 0f 1e fa endbr64
14: e8 00 00 00 00 call 19 <_sub_I_00099_0+0x9>
15: R_X86_64_PLT32 __tsan_init-0x4
19: e9 00 00 00 00 jmp 1e <_sub_I_00099_0+0xe>
1a: R_X86_64_PLT32 __x86_return_thunk-0x4

...

0000000000000030 <_sub_I_00099_0>:
30: f3 0f 1e fa endbr64
34: e8 00 00 00 00 call 39 <_sub_I_00099_0+0x9>
35: R_X86_64_PLT32 __tsan_init-0x4
39: e9 00 00 00 00 jmp 3e <__ksymtab_cryptd_alloc_ahash+0x2>
3a: R_X86_64_PLT32 __x86_return_thunk-0x4
-------------------

in the .ko file.

Objtool has run already so that second constructor's return thunk cannot
be added to the .return_sites section and thus the return thunk remains
unpatched and the warning rightfully fires.

Drop KCSAN flags from the mod.c generation stage as those constructors
do not contain data races one would be interested about.

Debugged together with David Kaplan <David.Kaplan@amd.com> and Nikolay
Borisov <nik.borisov@suse.com>.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Closes: https://lore.kernel.org/r/0851a207-7143-417e-be31-8bf2b3afb57d@molgen.mpg.de
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de> # Dell XPS 13
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
scripts/Makefile.modfinal | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 8568d256d6fbf..79fcf27316864 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -23,7 +23,7 @@ modname = $(notdir $(@:.mod.o=))
part-of-module = y

quiet_cmd_cc_o_c = CC [M] $@
- cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI) $(CFLAGS_GCOV), $(c_flags)) -c -o $@ $<
+ cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI) $(CFLAGS_GCOV) $(CFLAGS_KCSAN), $(c_flags)) -c -o $@ $<

%.mod.o: %.mod.c FORCE
$(call if_changed_dep,cc_o_c)
--
2.43.0