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Date:   Mon, 2 Sep 2019 11:35:34 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc:     linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86-ml <x86@kernel.org>
Subject: Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized'
 issue
Message-ID: <20190902093534.GB9605@zn.tnic>
References: <1567416624-26727-1-git-send-email-zhenzhong.duan@oracle.com>
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On Mon, Sep 02, 2019 at 05:30:24PM +0800, Zhenzhong Duan wrote:
> Builds kernel with "make bzImage EXTRA_CFLAGS=-Wall", gcc complains:
> 
> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
> arch/x86/boot/compressed/pgtable_64.c:92:7: warning: 'new' may be used uninitialized in this function [-Wmaybe-uninitialized]
>    new = round_down(new, PAGE_SIZE);
> 
> In theory a random value of variable new may pass all the check and be
> assigned to bios_start, fixing it by assigning it an initial value.
> 
> Fixes: 0a46fff2f910 ("x86/boot/compressed/64: Fix boot on machines with broken E820 table")
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: x86-ml <x86@kernel.org>
> ---
>  arch/x86/boot/compressed/pgtable_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index 2faddeb..c886269 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -72,7 +72,7 @@ static unsigned long find_trampoline_placement(void)
>  
>  	/* Find the first usable memory region under bios_start. */
>  	for (i = boot_params->e820_entries - 1; i >= 0; i--) {
> -		unsigned long new;
> +		unsigned long new = bios_start;
>  
>  		entry = &boot_params->e820_table[i];
>  
> -- 

https://git.kernel.org/tip/c96e8483cb2da6695c8b8d0896fe7ae272a07b54

In the future, use tip/master when preparing fixes because it has the
latest state of the x86 tree and you won't have to duplicate effort,
like in this case.

Thx.

-- 
Regards/Gruss,
    Boris.

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