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From: David Laight <David.Laight@ACULAB.COM>
To: 'Guenter Roeck' <linux@roeck-us.net>,
	"David S . Miller" <davem@davemloft.net>
Cc: "sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Arnd Bergmann <arnd@kernel.org>,
	Anatoly Pugachev <matorola@gmail.com>,
	Sam Ravnborg <sam@ravnborg.org>
Subject: RE: [PATCH v2] sparc: mdesc: Fix compile error seen with gcc 11.x
Date: Wed, 15 Sep 2021 08:38:43 +0000	[thread overview]
Message-ID: <d50a68b4c2c54b7b9440cbef7fcdc642@AcuMS.aculab.com> (raw)
In-Reply-To: <20210914224709.601156-1-linux@roeck-us.net>

From: Guenter Roeck
> Sent: 14 September 2021 23:47
...
> I am not sure if there was agreement to accept this patch or not, but
> I was asked to resend it with the above change, so here it is. An open
> question was if it is acceptable to have a structure named xxx_hdr
> include an element pointing to the data following that header.

It may be a pragmatic solution to the problem.
But it isn't 'correct'.
OTOH I think gcc is broken.
It ought to at least give a sane method of getting the warning
ignored in specific cases.

> If this patch is not acceptable, the patch in buildbot may be a possible
> alternative to consider.
>     https://git.busybox.net/buildroot/commit/?id=6e1106b4a9aee25d1556310d5cd1cb6dde2e6e3f
> 
>  arch/sparc/kernel/mdesc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/sparc/kernel/mdesc.c b/arch/sparc/kernel/mdesc.c
> index 8e645ddac58e..83e1f699bc32 100644
> --- a/arch/sparc/kernel/mdesc.c
> +++ b/arch/sparc/kernel/mdesc.c
> @@ -39,6 +39,7 @@ struct mdesc_hdr {
>  	u32	node_sz; /* node block size */
>  	u32	name_sz; /* name block size */
>  	u32	data_sz; /* data block size */
> +	char	data[];
>  } __attribute__((aligned(16)));
> 
>  struct mdesc_elem {
> @@ -612,7 +613,7 @@ EXPORT_SYMBOL(mdesc_get_node_info);
> 
>  static struct mdesc_elem *node_block(struct mdesc_hdr *mdesc)
>  {
> -	return (struct mdesc_elem *) (mdesc + 1);
> +	return (struct mdesc_elem *) (mdesc->data);
>  }

In order for gcc to consider (mdesc + 1) to have size 0
I think it must have tracked the pointer from a structure
that has another field (or structure end) following 'mdesc'.
If that is the case then it should also know that the data[]
must also be size 0.
So the warning may reappear with the next gcc version.

The busybox patch has:
+@@ -75,6 +75,7 @@ struct mdesc_handle {
+ 	refcount_t		refcnt;
+ 	unsigned int		handle_size;
+ 	struct mdesc_hdr	mdesc;
++	char			data[];
+ };


Which really ought to be more than enough.
Although the extra space could be considered to even be
outside that structure.
But the gcc folks suggested a completely brain-dead change
that requires taking the offset from the outer structure.
--	return (struct mdesc_elem *) (mdesc + 1);
++	return (struct mdesc_elem *) hp + offsetof(struct mdesc_handle, data);
which is probably missing a (char *) cast.

I wonder if it might be better to 'launder' the pointer
so that gcc can't track its size.
It may be that:
	return (struct mdesc_elem *)(ulong)(mdesc + 1);
is enough.
Otherwise it will need to be passed into an asm block.

But gcc is getting stupid for system programming.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2021-09-15  8:38 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-14 22:47 Guenter Roeck
2021-09-15  8:38 ` David Laight [this message]
2021-09-15 14:04 ` Arnd Bergmann

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