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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 13 Oct 2022 08:37:08 -0700
Message-ID: <CANn89iKJQCmEPxf=296_dGt9Wb3MD=0mp0vP6X4exWUPML=UUw@mail.gmail.com>
Subject: Re: [PATCH] net/atm: fix proc_mpc_write 1 byte less calculated
To:     Xiaobo Liu <cppcoffee@gmail.com>
Cc:     davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Oct 13, 2022 at 8:19 AM Xiaobo Liu <cppcoffee@gmail.com> wrote:
>
> Then the input contains '\0' or '\n', proc_mpc_write has read them,
> so the return value needs +1.
>
> Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
> ---
>  net/atm/mpoa_proc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/atm/mpoa_proc.c b/net/atm/mpoa_proc.c
> index 829db9eba..444ceda60 100755
> --- a/net/atm/mpoa_proc.c
> +++ b/net/atm/mpoa_proc.c
> @@ -224,8 +224,11 @@ static ssize_t proc_mpc_write(struct file *file, const char __user *buff,
>                         free_page((unsigned long)page);
>                         return -EFAULT;
>                 }
> -               if (*p == '\0' || *p == '\n')
> +
> +               if (*p == '\0' || *p == '\n') {
> +                       len += 1
>                         break;
> +               }
>         }
>
>         *p = '\0';
> --
> 2.21.0 (Apple Git-122.2)
>

Hi Xiaobo

Can you submit a v2, with this added tag ?

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

Also, I would switch to something cleaner like

diff --git a/net/atm/mpoa_proc.c b/net/atm/mpoa_proc.c
index 829db9eba0cb95ac9cfe775e8eaad712943a8dbe..df530e9725fa63820a7adcd44e750db0733f9d94
100644
--- a/net/atm/mpoa_proc.c
+++ b/net/atm/mpoa_proc.c
@@ -219,11 +219,12 @@ static ssize_t proc_mpc_write(struct file *file,
const char __user *buff,
        if (!page)
                return -ENOMEM;

-       for (p = page, len = 0; len < nbytes; p++, len++) {
+       for (p = page, len = 0; len < nbytes; p++) {
                if (get_user(*p, buff++)) {
                        free_page((unsigned long)page);
                        return -EFAULT;
                }
+               len++;
                if (*p == '\0' || *p == '\n')
                        break;
        }