linux-kernel - Re: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses

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Date:   Tue, 5 Jan 2021 14:29:46 +0530
From:   Dwaipayan Ray <dwaipayanray1@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>
Subject: Re: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses

On Tue, Jan 5, 2021 at 2:14 PM Joe Perches <joe@...ches.com> wrote:
>
> On Tue, 2021-01-05 at 13:53 +0530, Dwaipayan Ray wrote:
> > strcpy() performs no bounds checking on the destination buffer.
> > This could result in linear overflows beyond the end of the buffer.
> >
> > strlcpy() reads the entire source buffer first. This read
> > may exceed the destination size limit. This can be both inefficient
> > and lead to linear read overflows.
> >
> > The safe replacement to both of these is to use strscpy() instead.
> > Add a new checkpatch warning which alerts the user on finding usage of
> > strcpy() or strlcpy().
>
> I do not believe that strscpy is preferred over strcpy.
>
> When the size of the output buffer is known to be larger
> than the input, strcpy is faster.
>
> There are about 2k uses of strcpy.
> Is there a use where strcpy use actually matters?
> I don't know offhand...
>
> But I believe compilers do not optimize away the uses of strscpy
> to a simple memcpy like they do for strcpy with a const from
>
>         strcpy(foo, "bar");
>

Yes the optimization here definitely helps. So in case the programmer
knows that the destination buffer is always larger, then strcpy() should be
preferred? I think the documentation might have been too strict about
strcpy() uses here:

Documentation/process/deprecated.rst:
"strcpy() performs no bounds checking on the destination buffer. This
could result in linear overflows beyond the end of the buffer, leading to
all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various
compiler flags help reduce the risk of using this function, there is
no good reason to add new uses of this function. The safe replacement
is strscpy(),..."


> And lastly there is a existing strlcpy test in checkpatch.
>
> commit 5dbdb2d87c29 ("checkpatch: prefer strscpy to strlcpy")
>
I will drop this patch. Thanks for your view.

Thank you,
Dwaipayan.