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 47f047458264f..dce4cf55a4b68 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)