linux-kernel - Re: Arrays of variable length

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Message-ID: <20170305100116.GH29622@ZenIV.linux.org.uk>
Date:   Sun, 5 Mar 2017 10:01:35 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Tomas Winkler <tomasw@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        sparse@...r.kernel.org
Subject: Re: Arrays of variable length

On Sun, Mar 05, 2017 at 11:44:33AM +0200, Tomas Winkler wrote:
> Sparse complains for arrays declared with variable length
> 
> 'warning: Variable length array is used'
> 
> Prior to c99 this was not allowed but lgcc (c99) doesn't have problem
> with that  https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html.
> And also Linux kernel compilation with W=1 doesn't complain.
> 
> Since sparse is used extensively would like to ask what is the correct
> usage of arrays of variable length
> within Linux Kernel.

That depends.  For structure members the answer is simply "don't, it's
not a valid C to start with".  Note that this is about actual VLA, not
struct foo {
	int bar;
	struct baz[];
}
- that is valid C99 and sparse is just fine with it.  For local variables...
keep in mind that kernel stack is _small_, so any VLA there needs to be
done very carefully.  For heap it's more or less usable, but keep in mind
that gcc support of VLA (and variably-modified types in general) has
seriously unpleasant corner cases, especially when combined with the ({...}) 
thing.  IOW, "doesn't have problem" is overoptimistic; use with care.