linux-kernel - Re: [PATCH AUTOSEL 4.19 5/6] x86/entry: Work around Clang _

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Message-ID: <20221011114215.GA12851@duo.ucw.cz>
Date:   Tue, 11 Oct 2022 13:42:15 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        nathan@...nel.org, ndesaulniers@...gle.com, llvm@...ts.linux.dev
Subject: Re: [PATCH AUTOSEL 4.19 5/6] x86/entry: Work around Clang __bdos()
 bug

Hi!

> From: Kees Cook <keescook@...omium.org>
> 
> [ Upstream commit 3e1730842f142add55dc658929221521a9ea62b6 ]
> 
> Clang produces a false positive when building with CONFIG_FORTIFY_SOURCE=y
> and CONFIG_UBSAN_BOUNDS=y when operating on an array with a dynamic
> offset. Work around this by using a direct assignment of an empty
> instance. Avoids this warning:
> 
> ../include/linux/fortify-string.h:309:4: warning: call to __write_overflow_field declared with 'warn
> ing' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wat
> tribute-warning]
>                         __write_overflow_field(p_size_field, size);
>                         ^
> 
> which was isolated to the memset() call in xen_load_idt().
> 
> Note that this looks very much like another bug that was worked around:
> https://github.com/ClangBuiltLinux/linux/issues/1592

At least in 4.19, there's no UBSAN_BOUNDS. Sounds like we don't need
it in old kernels?

Best regards,
								Pavel
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -752,6 +752,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
>  {
>  	static DEFINE_SPINLOCK(lock);
>  	static struct trap_info traps[257];
> +	static const struct trap_info zero = { };
>  	unsigned out;
>  
>  	trace_xen_cpu_load_idt(desc);
> @@ -761,7 +762,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
>  	memcpy(this_cpu_ptr(&idt_desc), desc, sizeof(idt_desc));
>  
>  	out = xen_convert_trap_info(desc, traps, false);
> -	memset(&traps[out], 0, sizeof(traps[0]));
> +	traps[out] = zero;
>  
>  	xen_mc_flush();
>  	if (HYPERVISOR_set_trap_table(traps))
> -- 
> 2.35.1

-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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