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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Andrei Vagin <avagin@gmail.com>
Cc:     linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org
References: <20210615162346.16032-1-avagin@gmail.com>
Date:   Tue, 15 Jun 2021 14:33:20 -0500
In-Reply-To: <20210615162346.16032-1-avagin@gmail.com> (Andrei Vagin's message
        of "Tue, 15 Jun 2021 09:23:46 -0700")
Message-ID: <877diuq5xb.fsf@disp2133>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH] exec/binfmt_script: trip zero bytes from the buffer
Precedence: bulk

Andrei Vagin <avagin@gmail.com> writes:

> Without this fix, if we try to run a script that contains only the
> interpreter line, the interpreter is executed with one extra empty
> argument.
>
> The code is written so that i_end has to be set to the end of valuable
> data in the buffer.

Out of curiosity how did you spot this change in behavior?

> Fixes: ccbb18b67323 ("exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC")
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Signed-off-by: Andrei Vagin <avagin@gmail.com>
> ---
>  fs/binfmt_script.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> index 1b6625e95958..e242680f96e1 100644
> --- a/fs/binfmt_script.c
> +++ b/fs/binfmt_script.c
> @@ -68,6 +68,9 @@ static int load_script(struct linux_binprm *bprm)
>  		if (!next_terminator(i_end, buf_end))
>  			return -ENOEXEC;
>  		i_end = buf_end;
> +		/* Trim zero bytes from i_end */
> +		while (i_end[-1] == 0)
> +			i_end--;
>  	}
>  	/* Trim any trailing spaces/tabs from i_end */
>  	while (spacetab(i_end[-1]))