linux-kernel - Re: [PATCH] arch:powerpc simple_write_to

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Date:   Fri, 5 Feb 2021 11:43:12 +0530
From:   Mayank Suman <mayanksuman@...e.com>
To:     Oliver O'Halloran <oohall@...il.com>
Cc:     Russell Currey <ruscur@...sell.cc>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch:powerpc simple_write_to_buffer return check

On 05/02/21 4:05 am, Oliver O'Halloran wrote:
> On Fri, Feb 5, 2021 at 5:17 AM Mayank Suman <mayanksuman@...e.com> wrote:
>>
>> Signed-off-by: Mayank Suman <mayanksuman@...e.com>
> 
> commit messages aren't optional

Sorry. I will include the commit message in PATCH v2.

> 
>> ---
>>  arch/powerpc/kernel/eeh.c                    | 8 ++++----
>>  arch/powerpc/platforms/powernv/eeh-powernv.c | 4 ++--
>>  2 files changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/eeh.c b/arch/powerpc/kernel/eeh.c
>> index 813713c9120c..2dbe1558a71f 100644
>> --- a/arch/powerpc/kernel/eeh.c
>> +++ b/arch/powerpc/kernel/eeh.c
>> @@ -1628,8 +1628,8 @@ static ssize_t eeh_force_recover_write(struct file *filp,
>>         char buf[20];
>>         int ret;
>>
>> -       ret = simple_write_to_buffer(buf, sizeof(buf), ppos, user_buf, count);
>> -       if (!ret)
>> +       ret = simple_write_to_buffer(buf, sizeof(buf)-1, ppos, user_buf, count);
> 
> We should probably be zeroing the buffer. Reading to sizeof(buf) - 1
> is done in a few places to guarantee that the string is nul
> terminated, but without the preceeding memset() that isn't actually
> guaranteed.

Yes, the buffer should be zeroed out first. I have included memset() in Patch v2.

> 
>> +       if (ret <= 0)
>>                 return -EFAULT;
> 
> EFAULT is supposed to be returned when the user supplies a buffer to
> write(2) which is outside their address space. I figured letting the
> sscanf() in the next step fail if the user passes writes a zero-length
> buffer and returning EINVAL made more sense. That said, the exact
> semantics around zero length writes are pretty handwavy so I guess
> this isn't wrong, but I don't think it's better either.
> 

simple_write_to_buffer may return negative value on fail.
So, -EFAULT should be return in case of negative return value. 
The conditional (!ret) was not sufficient to catch negative return value.