linux-kernel - Re: [PATCH 3/3] module: Add compile-time check for embedded NUL characters

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Message-ID: <20251210010020.GA2522829@google.com>
Date: Wed, 10 Dec 2025 01:00:20 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: "Luck, Tony" <tony.luck@...el.com>
Cc: Daniel Gomez <da.gomez@...nel.org>, Eric Biggers <ebiggers@...nel.org>,
	Kees Cook <kees@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Petr Pavlu <petr.pavlu@...e.com>,
	"linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
	Malcolm Priestley <tvboxspy@...il.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Hans Verkuil <hverkuil@...nel.org>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH 3/3] module: Add compile-time check for embedded NUL
 characters

On Tue, Dec 09, 2025 at 10:29:06AM -0800, Luck, Tony wrote:
> On Tue, Dec 09, 2025 at 08:45:14AM -0800, Luck, Tony wrote:
> > On Tue, Dec 09, 2025 at 04:20:06PM +0000, Luck, Tony wrote:
> > > >> Likewise, I just got the following kernel test robot report sent to me,
> > > >> where it's warning about MODULE_LICENSE("GPL"):
> > > >> https://lore.kernel.org/all/202512090359.7BkUaiC9-lkp@intel.com/
> > > >
> > > > Can you both confirm which version of sparse are you using?
> > > >
> > > > My understanding was that this patch fixed that problem:
> > > > >https://lore.kernel.org/linux-sparse/CACePvbVG2KrGQq4cNKV=wbO5h=jp3M0RO1SdfX8kV4OukjPG8A@mail.gmail.com/T/#mf838b3e2e3245d88c30a801ea7473d5a5c0eb121
> > > 
> > > > The patch is already merged into the sparse tree, and I was not able to
> > > > reproduce the issue.
> > > 
> > > I pulled the latest sparse source and re-checked before reporting. Top commit I have is the one you mention:
> > > 
> > > fbdde3127b83 ("builtin: implement __builtin_strlen() for constants")
> > > 
> > > I'm building latest Linus tree from the current merge window (well latest as-of yesterday):
> > > 
> > > c2f2b01b74be ("Merge tag 'i3c/for-6.19' of git://git.kernel.org/pub/scm/linux/kernel/git/i3c/linux")
> > 
> > I added a debug trace to the new expand_strlen() function added to
> > sparse. It is being called and doing the right thing. My trace says:
> > 
> > 	len(GPL) = 3
> 
> Simple test case:
> 
> $ cat -n s.c
>      1
>      2  _Static_assert(sizeof("GPL") - 1 == 3, "sizeof");
>      3
>      4  _Static_assert(__builtin_strlen("GPL") == 3, "strlen");
> 
> $ sparse s.c
> s.c:4:40: error: bad integer constant expression
> s.c:4:40: error: static assertion failed: "strlen"
> 
> So the "sizeof" bit is OK. But the __builtin_strlen() isn't.

This looks like a bug in Sparse. The CEF_ICE flag isn't propagated to
the comparison expression, which it presumably should be when both
sides are integer constant expressions.

I'm not really familiar enough with Sparse to know whether this is the
correct place to handle this case, but this quick hack fixes the issue
for me:

diff --git a/expand.c b/expand.c
index f14e7181..71221d35 100644
--- a/expand.c
+++ b/expand.c
@@ -535,6 +535,8 @@ static int expand_compare(struct expression *expr)
 			expr->taint = 0;
 			return 0;
 		}
+		if (left->flags & CEF_ICE && right->flags & CEF_ICE)
+			expr->flags |= CEF_SET_ICE;
 		if (simplify_cmp_binop(expr, left->ctype))
 			return 0;
 		if (simplify_float_cmp(expr, left->ctype))

Sami