linux-kernel - Re: [PATCH 16/16] fix handling of integer constant expressions

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Message-ID: <20070627001803.GJ21478@ftp.linux.org.uk>
Date:	Wed, 27 Jun 2007 01:18:03 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Neil Booth <neil@...kokuya.co.uk>
Cc:	Josh Triplett <josh@...edesktop.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

On Wed, Jun 27, 2007 at 08:32:26AM +0900, Neil Booth wrote:
> Al Viro wrote:-
> 
> > Hopefully correct handling of integer constant expressions.  Please, review.
> 
> Am I invoking sparse wrongly?  ./sparse -W -Wall doesn't diagnose
> the following TU, for example.
> 
> extern int a;
> extern int as1[(a = 2)];

sparse simply doesn't check that.  We don't have anything resembling
support of VLA.  Note that check for integer constant expression
has nothing to do with that;

	int x[(int)(0.6 + 0.6)];

is valid (if stupid).  And yes, footnote in 6.6 contradicts 6.7.5.2(1);
too bad...

We certainly need to do checks on array sizes; however, that part
("if it has static storage duration, it should not be a VLA") is minor.
And then there are gccisms:
size_t foo(int n)
{
	struct {
		int a[n];
		char b;
	} x;
	return offsetof(typeof(x), b);
}

Yes, it's eaten up just fine.  And yes, such structures are silently
accepted even with -pedantic -std=c99, which is a bug.  Sigh...

We'll need to tackle VLAs at some point, but it certainly won't be fun ;-/
-
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