linux-kernel - Re: Using memcpy instead of copy_to

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Message-ID: <CAM=k+5p4hF1_d+XUR8xUm18Y=BJQPYG2==Tq=7HOY8Z2rwRUxg@mail.gmail.com>
Date:   Tue, 25 Jan 2022 17:56:57 -0600
From:   Aleksey Senin <aleksey-linux-kernel@...in.name>
To:     David Laight <david.laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Using memcpy instead of copy_to_user. xprtrdma code.

Hi,
It seems clue is in new proc_sys_call_handler code that allocates the
kernel buffer first, calls to particular function and than copying
obtained buffer to user space using copy_to_user. Right?

On Tue, Jan 25, 2022 at 4:21 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Aleksey Senin <aleksey-linux-kernel@...in.name>
> > Sent: 25 January 2022 18:04
> >
> > This specific patch -  - implements using memcpy instead of
> > copy_to_user. Why is it considered to be safe in this specific case?
> > All readings about how to copy data are mentioning to use
> > copy_to_user/copy_from_user. Why use direct copy here? What prevents
> > the kernel from failure if the page is not present or doesn't have
> > required access rights?
> >
> > @@ -103,8 +102,8 @@ static int read_reset_stat(struct ctl_table
> > *table, int write,
> >                 len -= *ppos;
> >                 if (len > *lenp)
> >                         len = *lenp;
> > -               if (len && copy_to_user(buffer, str_buf, len))
> > -                       return -EFAULT;
> > +               if (len)
> > +                       memcpy(buffer, str_buf, len);
> >
> > Reference to the specific patch in the services of commits:
> > https://lkml.org/lkml/2020/4/17/60
>
> Read the commit message.
>
> >
> > Commit itself:
> > 32927393dc1ccd60fb2bdc05b9e8e88753761469
>
> -
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