linux-kernel - Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized' issue

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Message-ID: <8361d582-3429-c91d-3f5a-0b988a0e16ed@oracle.com>
Date:   Mon, 2 Sep 2019 17:40:28 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>
Subject: Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized'
 issue


On 2019/9/2 17:35, Borislav Petkov wrote:
> 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@...cle.com>
>> Cc: Kirill A. Shutemov<kirill@...temov.name>
>> Cc: Thomas Gleixner<tglx@...utronix.de>
>> Cc: Ingo Molnar<mingo@...hat.com>
>> Cc: Borislav Petkov<bp@...en8.de>
>> Cc: "H. Peter Anvin"<hpa@...or.com>
>> Cc: x86-ml<x86@...nel.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.

Ok, I see. Thanks for your suggestion.

Zhenzhong