linux-kernel - Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative

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Date:   Wed, 28 Jun 2017 12:35:10 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     <akpm@...ux-foundation.org>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <minchan@...nel.org>, <mhocko@...e.com>,
        <hpa@...or.com>, <x86@...nel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, zhongjiang <zhongjiang@...wei.com>
Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent
 is negative

Hi,  Ingo

Thank you for the comment.
On 2017/6/22 0:40, Ingo Molnar wrote:
> * zhong jiang <zhongjiang@...wei.com> wrote:
>
>> when shift expoment is negative, left shift alway zero. therefore, we
>> modify the logic to avoid the warining.
>>
>> Signed-off-by: zhong jiang <zhongjiang@...wei.com>
>> ---
>>  arch/x86/include/asm/futex.h | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
>> index b4c1f54..2425fca 100644
>> --- a/arch/x86/include/asm/futex.h
>> +++ b/arch/x86/include/asm/futex.h
>> @@ -49,8 +49,12 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
>>  	int cmparg = (encoded_op << 20) >> 20;
>>  	int oldval = 0, ret, tem;
>>  
>> -	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
>> -		oparg = 1 << oparg;
>> +	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
>> +		if (oparg >= 0)
>> +			oparg = 1 << oparg;
>> +		else
>> +			oparg = 0;
>> +	}
> Could we avoid all these complications by using an unsigned type?
  I think it is not feasible.  a negative shift exponent is likely existence and reasonable.
  as the above case,  oparg is a negative is common. 

 I think it can be avoided by following change. 

  diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
index b4c1f54..3205e86 100644
--- a/arch/x86/include/asm/futex.h
+++ b/arch/x86/include/asm/futex.h
@@ -50,7 +50,7 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
        int oldval = 0, ret, tem;

        if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
-               oparg = 1 << oparg;
+               oparg = safe_shift(1, oparg);

        if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
                return -EFAULT;
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 069fe79..b4edda3 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -190,11 +190,6 @@ char* fb_get_buffer_offset(struct fb_info *info, struct fb_pixmap *buf, u32 size

 #ifdef CONFIG_LOGO

-static inline unsigned safe_shift(unsigned d, int n)
-{
-       return n < 0 ? d >> -n : d << n;
-}
-
 static void fb_set_logocmap(struct fb_info *info,
                                   const struct linux_logo *logo)
 {
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d043ada..f3b8856 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -841,6 +841,10 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
  */
 #define clamp_val(val, lo, hi) clamp_t(typeof(val), val, lo, hi)

+static inline unsigned safe_shift(unsigned d, int n)
+{
+       return n < 0 ? d >> -n : d << n;
+}

Thansk
zhongjiang

> Thanks,
>
> 	Ingo
>
> .
>