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Subject: Re: [for-stable-4.19 PATCH 1/2] vmlinux.lds.h: Create section for
 protection against instrumentation
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nicolas Boichat <drinkcat@chromium.org>
Cc:     stable@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christopher Li <sparse@chrisli.org>,
        Daniel Axtens <dja@axtens.net>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal Marek <michal.lkml@markovi.net>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Paul Mackerras <paulus@samba.org>,
        Sasha Levin <sashal@kernel.org>, linux-arch@vger.kernel.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-sparse@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <20210318235416.794798-1-drinkcat@chromium.org>
 <20210319075410.for-stable-4.19.1.I222f801866f71be9f7d85e5b10665cd4506d78ec@changeid>
 <YFR/fQIePjDQcO5W@kroah.com>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <b5d3d0ed-953e-083d-15f6-4a1e3ed95428@oracle.com>
Date:   Fri, 19 Mar 2021 12:20:22 +0100
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On 3/19/21 11:39 AM, Greg Kroah-Hartman wrote:
> On Fri, Mar 19, 2021 at 07:54:15AM +0800, Nicolas Boichat wrote:
>> From: Thomas Gleixner <tglx@linutronix.de>
>>
>> commit 6553896666433e7efec589838b400a2a652b3ffa upstream.
>>
>> Some code pathes, especially the low level entry code, must be protected
>> against instrumentation for various reasons:
>>
>>   - Low level entry code can be a fragile beast, especially on x86.
>>
>>   - With NO_HZ_FULL RCU state needs to be established before using it.
>>
>> Having a dedicated section for such code allows to validate with tooling
>> that no unsafe functions are invoked.
>>
>> Add the .noinstr.text section and the noinstr attribute to mark
>> functions. noinstr implies notrace. Kprobes will gain a section check
>> later.
>>
>> Provide also a set of markers: instrumentation_begin()/end()
>>
>> These are used to mark code inside a noinstr function which calls
>> into regular instrumentable text section as safe.
>>
>> The instrumentation markers are only active when CONFIG_DEBUG_ENTRY is
>> enabled as the end marker emits a NOP to prevent the compiler from merging
>> the annotation points. This means the objtool verification requires a
>> kernel compiled with this option.
>>
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>> Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
>> Acked-by: Peter Zijlstra <peterz@infradead.org>
>> Link: https://lkml.kernel.org/r/20200505134100.075416272@linutronix.de
>>
>> [Nicolas: context conflicts in:
>> 	arch/powerpc/kernel/vmlinux.lds.S
>> 	include/asm-generic/vmlinux.lds.h
>> 	include/linux/compiler.h
>> 	include/linux/compiler_types.h]
>> Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> 
> Did you build this on x86?
> 
> I get the following build error:
> 
> ld:./arch/x86/kernel/vmlinux.lds:20: syntax error
> 
> And that line looks like:
> 
>   . = ALIGN(8); *(.text.hot .text.hot.*) *(.text .text.fixup) *(.text.unlikely .text.unlikely.*) *(.text.unknown .text.unknown.*) . = ALIGN(8); __noinstr_text_start = .; *(.__attribute__((noinline)) __attribute__((no_instrument_function)) __attribute((__section__(".noinstr.text"))).text) __noinstr_text_end = .; *(.text..refcount) *(.ref.text) *(.meminit.text*) *(.memexit.text*)
> 

In the NOINSTR_TEXT macro, noinstr is expanded with the value of the noinstr
macro from linux/compiler_types.h while it shouldn't.

The problem is possibly that the noinstr macro is defined for assembly. Make
sure that the macro is not defined for assembly e.g.:

#ifndef __ASSEMBLY__

/* Section for code which can't be instrumented at all */
#define noinstr								\
	noinline notrace __attribute((__section__(".noinstr.text")))

#endif

alex.