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

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Message-ID: <CAOSf1CGJ6ZeowMP8Zjo3TazYyaEGuEab4-QRKRJ2jjixUGGtCA@mail.gmail.com>
Date:   Fri, 5 Feb 2021 09:35:31 +1100
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Mayank Suman <mayanksuman@...e.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 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

> ---
>  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.

> +       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.

>         /*
> @@ -1696,7 +1696,7 @@ static ssize_t eeh_dev_check_write(struct file *filp,
>
>         memset(buf, 0, sizeof(buf));
>         ret = simple_write_to_buffer(buf, sizeof(buf)-1, ppos, user_buf, count);
> -       if (!ret)
> +       if (ret <= 0)
>                 return -EFAULT;
>
>         ret = sscanf(buf, "%x:%x:%x.%x", &domain, &bus, &dev, &fn);
> @@ -1836,7 +1836,7 @@ static ssize_t eeh_dev_break_write(struct file *filp,
>
>         memset(buf, 0, sizeof(buf));
>         ret = simple_write_to_buffer(buf, sizeof(buf)-1, ppos, user_buf, count);
> -       if (!ret)
> +       if (ret <= 0)
>                 return -EFAULT;
>
>         ret = sscanf(buf, "%x:%x:%x.%x", &domain, &bus, &dev, &fn);
> diff --git a/arch/powerpc/platforms/powernv/eeh-powernv.c b/arch/powerpc/platforms/powernv/eeh-powernv.c
> index 89e22c460ebf..36ed2b8f7375 100644
> --- a/arch/powerpc/platforms/powernv/eeh-powernv.c
> +++ b/arch/powerpc/platforms/powernv/eeh-powernv.c
> @@ -76,8 +76,8 @@ static ssize_t pnv_eeh_ei_write(struct file *filp,
>                 return -ENXIO;
>
>         /* Copy over argument buffer */
> -       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);
> +       if (ret <= 0)
>                 return -EFAULT;
>
>         /* Retrieve parameters */
> --
> 2.30.0
>