[PATCH] kbuild: Disable KCSAN for autogenerated *.mod.c intermediaries

[Borislav Petkov]

On Tue, Mar 26, 2024 at 08:33:31PM +0100, Marco Elver wrote:
> I think just removing instrumentation from the mod.c files is very reasonable.

Thanks!

@Masahiro: pls send this to Linus now as the commit which adds the
warning is in 6.9 so we should make sure we release it with all issues
fixed.

Thx.

---
From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Date: Tue, 26 Mar 2024 21:11:01 +0100

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 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@xxxxxxx> and Nikolay
Borisov <nik.borisov@xxxxxxxx>.

Reported-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/0851a207-7143-417e-be31-8bf2b3afb57d@xxxxxxxxxxxxx
---
 scripts/Makefile.modfinal | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 8568d256d6fb..79fcf2731686 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



-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette