linux-kernel - Re: [PATCH] kbuild: Disable KCSAN for autogenerated *.mod.c intermediaries

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Date: Tue, 26 Mar 2024 21:41:50 +0100
From: Marco Elver <elver@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, linux-kbuild@...r.kernel.org, 
	Nikolay Borisov <nik.borisov@...e.com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Paul Menzel <pmenzel@...gen.mpg.de>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	LKML <linux-kernel@...r.kernel.org>, kasan-dev@...glegroups.com, 
	David Kaplan <David.Kaplan@....com>
Subject: Re: [PATCH] kbuild: Disable KCSAN for autogenerated *.mod.c intermediaries

On Tue, 26 Mar 2024 at 21:26, Borislav Petkov <bp@...en8.de> wrote:
>
> 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@...en8.de>
> 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@....com> and Nikolay
> Borisov <nik.borisov@...e.com>.
>
> Reported-by: Paul Menzel <pmenzel@...gen.mpg.de>
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> Link: https://lore.kernel.org/r/0851a207-7143-417e-be31-8bf2b3afb57d@molgen.mpg.de

Reviewed-by: Marco Elver <elver@...gle.com>

Thanks!

> ---
>  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