linux-kernel - Re: [PATCH] lib/vsprintf: add %pT format specifier

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Message-ID: <CAMuHMdVwoQbBVESnJrUviA63Xrp845gg8z0BjYJq9OZzvoodrA@mail.gmail.com>
Date:	Sat, 11 Jan 2014 11:28:15 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Pavel Machek <pavel@....cz>, Joe Perches <joe@...ches.com>,
	Kees Cook <keescook@...omium.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/vsprintf: add %pT format specifier

On Sat, Jan 11, 2014 at 12:59 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
>> +char *comm_name(char *buf, char *end, struct task_struct *tsk,
>> +             struct printf_spec spec, const char *fmt)
>> +{
>> +     char name[TASK_COMM_LEN];
>> +
>> +     /* Caller can pass NULL instead of current. */
>> +     if (!tsk)
>> +             tsk = current;
>> +     /* Not using get_task_comm() in case I'm in IRQ context. */
>> +     memcpy(name, tsk->comm, TASK_COMM_LEN);

So this may copy more bytes than the actual string length of tsk->comm.
As this is a temporary buffer, that just wastes cycles.
And even if it wasn't, data between the string zero terminator and the end of
the buffer wouild be leaked.

>> +     name[sizeof(name) - 1] = '\0';

You can use strlcpy() here instead of memcpy and clear.

> get_task_comm() uses strncpy()?

char *get_task_comm(char *buf, struct task_struct *tsk)
{
        /* buf must be at least sizeof(tsk->comm) in size */
        task_lock(tsk);
        strncpy(buf, tsk->comm, sizeof(tsk->comm));
        task_unlock(tsk);
        return buf;
}

Is get_task_comm() used to prepare data for userspace?
strncpy() fills the remaining of the buffer with zeroes, so it avoids leaking
data.

Note that strncpy() may leave the buffer non-zero-terminated if the source
string is too long, but as set_task_comm() uses strlcpy(), this should never
be the case:

void set_task_comm(struct task_struct *tsk, char *buf)
{
        task_lock(tsk);
        trace_task_rename(tsk, buf);
        strlcpy(tsk->comm, buf, sizeof(tsk->comm));
        task_unlock(tsk);
        perf_event_comm(tsk);
}

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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