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Date:   Wed, 30 Mar 2022 16:30:29 -0300
From:   Marcelo Schmitt <marcelo.schmitt1@gmail.com>
To:     David Gow <davidgow@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Daniel Latypov <dlatypov@google.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        linux-sparse@vger.kernel.org, cocci@inria.fr,
        smatch@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Dan Carpenter <dan.carpenter@oracle.com>, julia.lawall@inria.fr
Subject: Re: [PATCH v2 2/2] Documentation: dev-tools: Enhance static analysis
 section with discussion
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On 03/30, David Gow wrote:
> On Wed, Mar 30, 2022 at 7:23 AM Marcelo Schmitt
> <marcelo.schmitt1@gmail.com> wrote:
> >
> > Enhance the static analysis tools section with a discussion on when to
> > use each of them.
> >
> > This was mainly taken from Dan Carpenter and Julia Lawall's comments on
> > the previous documentation patch for static analysis tools.
> >
> > Lore: https://lore.kernel.org/linux-doc/20220329090911.GX3293@kadam/T/#mb97770c8e938095aadc3ee08f4ac7fe32ae386e6
> >
> > Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
> > Cc: Dan Carpenter <dan.carpenter@oracle.com>
> > Cc: Julia Lawall <julia.lawall@inria.fr>
> > ---
> 
> Thanks: this sort of "when to use which tool" information is really
> what the testing guide page needs.
> 
> I'm not familiar enough with these tools that I can really review the
> details properly, but nothing stands out as obviously wrong to me.
> I've made a few comments below regardless, but feel free to ignore
> them if they're not quite right.
> 
> Acked-by: David Gow <davidgow@google.com>
> 
> Cheers,
> -- David
> 
> >  Documentation/dev-tools/testing-overview.rst | 33 ++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/Documentation/dev-tools/testing-overview.rst b/Documentation/dev-tools/testing-overview.rst
> > index b5e02dd3fd94..91e479045d3a 100644
> > --- a/Documentation/dev-tools/testing-overview.rst
> > +++ b/Documentation/dev-tools/testing-overview.rst
> > @@ -146,3 +146,36 @@ Documentation/dev-tools/coccinelle.rst documentation page for details.
> >
> >  Beware, though, that static analysis tools suffer from **false positives**.
> >  Errors and warns need to be evaluated carefully before attempting to fix them.
> > +
> > +When to use Sparse and Smatch
> > +-----------------------------
> > +
> > +Sparse is useful for type checking, detecting places that use ``__user``
> > +pointers improperly, or finding endianness bugs. Sparse runs much faster than
> > +Smatch.
> 
> Given that the __user pointer and endianness stuff is found as a
> result of Sparse's type checking support, would rewording this as
> "Sparse does type checking, such as [detecting places...]" or similar
> be more clear?

Myabe. I tried changing it a little while adding a bit of information from
https://lwn.net/Articles/689907/

"Sparse does type checking, such as verifying that annotated variables do not
cause endianness bugs, detecting places that use ``__user`` pointers improperly,
and analyzing the compatibility of symbol initializers."

Does it sound better?

> 
> > +
> > +Smatch does flow analysis and, if allowed to build the function database, it
> > +also does cross function analysis. Smatch tries to answer questions like where
> > +is this buffer allocated? How big is it? Can this index be controlled by the
> > +user? Is this variable larger than that variable?
> > +
> > +It's generally easier to write checks in Smatch than it is to write checks in
> > +Sparse. Nevertheless, there are some overlaps between Sparse and Smatch checks
> > +because there is no reason for re-implementing Sparse's check in Smatch.
> 
> This last sentence isn't totally clear to me. Should this "because" be "so"?

Smatch uses (is shipped with) a modified Sparse implementation which it uses as
a C parser. Apparently, Sparse does some checkings while parsing the code for
Smatch so that's why we have some overlapping between the checks made when we
run Smatch and the ones made when we run Sparse alone.

I didn't dig into the code, but I guess further modifying Sparse to prevent it
from doing some types of cheks wouldn't add much to Smatch. That last saying
should've reflected this fact, but it seems to cause confusion without a proper
context. Reading the sentence back again, I think we could just drop the last
part:

"Nevertheless, there are some overlaps between Sparse and Smatch checks."

> 
> > +
> > +Strong points of Smatch and Coccinelle
> > +--------------------------------------
> > +
> > +Coccinelle is probably the easiest for writing checks. It works before the
> > +pre-compiler so it's easier to check for bugs in macros using Coccinelle.
> > +Coccinelle also writes patches fixes for you which no other tool does.
> > +
> > +With Coccinelle you can do a mass conversion from
> 
> (Maybe start this with "For example," just to make it clear that this
> paragraph is mostly following on from how useful it is that Coccinelle
> produces fixes, not just warnings.)

Will do

> 
> > +``kmalloc(x * size, GFP_KERNEL)`` to ``kmalloc_array(x, size, GFP_KERNEL)``, and
> > +that's really useful. If you just created a Smatch warning and try to push the
> > +work of converting on to the maintainers they would be annoyed. You'd have to
> > +argue about each warning if can really overflow or not.
> > +
> > +Coccinelle does no analysis of variable values, which is the strong point of
> > +Smatch. On the other hand, Coccinelle allows you to do simple things in a simple
> > +way.
> > --
> > 2.35.1
> >