linux-kernel - Re: [PATCH -tip v8 3/6] tracing/probe: Add ustring type for user-space string

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Date:   Tue, 14 May 2019 09:24:26 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>,
        Joel Fernandes <joel@...lfernandes.org>, yhs@...com
Subject: Re: [PATCH -tip v8 3/6] tracing/probe: Add ustring type for
 user-space string

* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> +/* Return the length of string -- including null terminal byte */
> +static nokprobe_inline int
> +fetch_store_strlen_user(unsigned long addr)
> +{
> +	return strnlen_unsafe_user((__force const void __user *)addr,
> +				   MAX_STRING_SIZE);

Pointless line break that doesn't improve readability.

> +/*
> + * Fetch a null-terminated string from user. Caller MUST set *(u32 *)buf
> + * with max length and relative data location.
> + */
> +static nokprobe_inline int
> +fetch_store_string_user(unsigned long addr, void *dest, void *base)
> +{
> +	const void __user *uaddr =  (__force const void __user *)addr;
> +	int maxlen = get_loc_len(*(u32 *)dest);
> +	u8 *dst = get_loc_data(dest, base);
> +	long ret;
> +
> +	if (unlikely(!maxlen))
> +		return -ENOMEM;
> +	ret = strncpy_from_unsafe_user(dst, uaddr, maxlen);
> +
> +	if (ret >= 0)
> +		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);
> +
>  	return ret;

Firstly, why is there a 'dest' and a 'dst' variable name as well - the 
two are very similar and the difference not explained at all.

Secondly, a style nit: if you group statements then please group 
statements based on the usual logic - which is the group them by the flow 
of logic. In the above case you grouped the 'maxlen' check with the 
strncpy_from_unsafe_user() call, while the grouping should be the other 
way around:

	if (unlikely(!maxlen))
		return -ENOMEM;

	ret = strncpy_from_unsafe_user(dst, uaddr, maxlen);
	if (ret >= 0)
		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);

	return ret;

Third, hiding the get_loc_data() call within variable initialization is 
bad style - we usually only put 'trivial' (constant) initializations 
there.

Fourth, 'dst' is independent of 'maxlen', so it should probably 
calculated *after* maxlen.

I.e. the whole sequence should be:

	maxlen = get_loc_len(*(u32 *)dest);
	if (unlikely(!maxlen))
		return -ENOMEM;

	dst = get_loc_data(dest, base);

	ret = strncpy_from_unsafe_user(dst, uaddr, maxlen);
	if (ret >= 0)
		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);

	return ret;

Fifth, we don't actually dereference 'dst', do we? So the whole type 
casting to 'void *' could be avoided by declaring 'dst' (or whatever its 
new, clearer name is) not as u8 *, but as void *.

I.e. these are five problems in a short sequence of code, which it sad to 
see in a v8 submission. :-/

Please review the other patches and the whole code base for similar 
mishaps and small details as well.

Thanks,

	Ingo