Re: [PATCH] sparc: mdesc: Fix compile error seen with gcc 11.x

[Guenter Roeck]

On 9/13/21 11:02 AM, Sam Ravnborg wrote:
> Hi Guenter,
>
> On Mon, Sep 13, 2021 at 09:37:12AM -0700, Guenter Roeck wrote:
> > sparc64 images fail to compile with gcc 11.x, reporting the following
> > errors.
> >
> > arch/sparc/kernel/mdesc.c:647:22: error:
> > 	'strcmp' reading 1 or more bytes from a region of size 0
> > arch/sparc/kernel/mdesc.c:692:22: error:
> > 	'strcmp' reading 1 or more bytes from a region of size 0
> > arch/sparc/kernel/mdesc.c:719:21:
> > 	error: 'strcmp' reading 1 or more bytes from a region of size 0
> >
> > The underlying problem is that node_block() returns a pointer beyond
> > the end of struct mdesc_hdr. gcc 11.x detects that and reports the error.
> > Adding an additional zero-length field to struct mdesc_hdr and pointing
> > to that field fixes the problem.
> >
> > Cc: Arnd Bergmann <arnd@xxxxxxxxxx>
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> > My apologies if a similar patch was submitted already; I was unable to find it.
> > I did find the following patch:
> >      https://git.busybox.net/buildroot/commit/?id=6e1106b4a9aee25d1556310d5cd1cb6dde2e6e3f
> > but I failed to find it in patchwork or on lore.kernel.org, and it
> > seems to be more expensive than the solution suggested here.
> >
> >   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..c67bdcc23727 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[0];
> >   } __attribute__((aligned(16)));
>
> I do not think this will works.
> See following comment:
>   * mdesc_hdr and mdesc_elem describe the layout of the data structure
>   * we get from the Hypervisor.
>
> With the above change you increased the size from 16 to 32 bytes,
> and any code using sizeof(struct mdesc_hdr) will now point too far in
> memory for the second and subsequent entries.
>
> I did not take any closer look, but this was from a quick analysis.

Sorry, I didn't realize that a field of size 0 increases the structure size
on sparc. I had checked the size of the old and the new structure with gcc
on x86_64 and didn't see a field size increase.

Guenter

---
Test code I had used:

#include <stddef.h>
#include <stdio.h>

typedef unsigned int u32;

struct mdesc_hdr {
        u32     version; /* Transport version */
        u32     node_sz; /* node block size */
        u32     name_sz; /* name block size */
        u32     data_sz; /* data block size */
} __attribute__((aligned(16)));

struct mdesc_hdr2 {
        u32     version; /* Transport version */
        u32     node_sz; /* node block size */
        u32     name_sz; /* name block size */
        u32     data_sz; /* data block size */
        char    data[0];
} __attribute__((aligned(16)));

int main()
{
	printf("%ld %ld\n", sizeof(struct mdesc_hdr), sizeof(struct mdesc_hdr2));

	return 0;
}