Re: [PATCH v3 0/5] s390/vfio-ap: ap_config sysfs attribute for mdevctl automation

Jason J. Herne
2024-03-21T14:14:47Z

On 3/19/24 7:12 AM, Heiko Carstens wrote:

> With gcc gcc 13.2.0 / binutils 2.40.90.20230730 I get this (defconfig):
>
> CC [M] drivers/s390/crypto/vfio_ap_ops.o
> In file included from ./include/linux/cpumask.h:13,
> from ./include/linux/smp.h:13,
> from ./include/linux/lockdep.h:14,
> from ./include/linux/spinlock.h:63,
> from ./include/linux/mmzone.h:8,
> from ./include/linux/gfp.h:7,
> from ./include/linux/mm.h:7,
> from ./include/linux/scatterlist.h:8,
> from ./include/linux/iommu.h:10,
> from ./include/linux/vfio.h:12,
> from drivers/s390/crypto/vfio_ap_ops.c:12:
> In function ‘bitmap_copy’,
> inlined from ‘ap_matrix_copy’ at drivers/s390/crypto/vfio_ap_ops.c:1672:2,
> inlined from ‘ap_config_store’ at drivers/s390/crypto/vfio_ap_ops.c:1696:2:
> ./include/linux/bitmap.h:253:17: warning: ‘memcpy’ reading 32 bytes from a region of size 0 [-Wstringop-overread]
> 253 | memcpy(dst, src, len);
> | ^~~~~~~~~~~~~~~~~~~~~
> In function ‘ap_config_store’:
> cc1: note: source object is likely at address zero
> In function ‘bitmap_copy’,
> inlined from ‘ap_matrix_copy’ at drivers/s390/crypto/vfio_ap_ops.c:1673:2,
> inlined from ‘ap_config_store’ at drivers/s390/crypto/vfio_ap_ops.c:1696:2:
> ./include/linux/bitmap.h:253:17: warning: ‘memcpy’ reading 32 bytes from a region of size 0 [-Wstringop-overread]
> 253 | memcpy(dst, src, len);
> | ^~~~~~~~~~~~~~~~~~~~~
> In function ‘ap_config_store’:
> cc1: note: source object is likely at address zero
> In function ‘bitmap_copy’,
> inlined from ‘ap_matrix_copy’ at drivers/s390/crypto/vfio_ap_ops.c:1674:2,
> inlined from ‘ap_config_store’ at drivers/s390/crypto/vfio_ap_ops.c:1696:2:
> ./include/linux/bitmap.h:253:17: warning: ‘memcpy’ reading 32 bytes from a region of size 0 [-Wstringop-overread]
> 253 | memcpy(dst, src, len);
> | ^~~~~~~~~~~~~~~~~~~~~
> In function ‘ap_config_store’:
> cc1: note: source object is likely at address zero

I believe that this is a bogus compiler warning. I cannot reproduce it,
fwiw.

gcc: gcc (GCC) 13.2.1 20231205 (Red Hat 13.2.1-6)
binutls binutils-2.40-14.fc39

make W=1 modules

Here is the supposedly offending code.

drivers/s390/crypto/vfio_ap_ops.c:
1670 static void ap_matrix_copy(struct ap_matrix *dst, struct ap_matrix
*src)
1671 {
1672 bitmap_copy(dst->apm, src->apm, AP_DEVICES);
1673 bitmap_copy(dst->aqm, src->aqm, AP_DOMAINS);
1674 bitmap_copy(dst->adm, src->adm, AP_DOMAINS);
1675 }

called from drivers/s390/crypto/vfio_ap_ops.c:
1695 /* Save old state */
1696 ap_matrix_copy(&m_old, &matrix_mdev->matrix);

Definition of struct in drivers/s390/crypto/vfio_ap_private.h:
113 struct ap_matrix_mdev {
114 struct vfio_device vdev;
115 struct list_head node;
116 struct ap_matrix matrix;
117 struct ap_matrix shadow_apcb;
118 struct kvm *kvm;
119 crypto_hook pqap_hook;
120 struct mdev_device *mdev;
121 struct ap_queue_table qtable;
122 struct eventfd_ctx *req_trigger;
123 DECLARE_BITMAP(apm_add, AP_DEVICES);
124 DECLARE_BITMAP(aqm_add, AP_DOMAINS);
125 DECLARE_BITMAP(adm_add, AP_DOMAINS);
126 };

drivers/s390/crypto/vfio_ap_private.h:
76 struct ap_matrix {
77 unsigned long apm_max;
78 DECLARE_BITMAP(apm, AP_DEVICES);
79 unsigned long aqm_max;
80 DECLARE_BITMAP(aqm, AP_DOMAINS);
81 unsigned long adm_max;
82 DECLARE_BITMAP(adm, AP_DOMAINS);
83 };

drivers/s390/crypto/ap_bus.h:
22 #define AP_DEVICES 256 /* Number of AP devices. */
23 #define AP_DOMAINS 256 /* Number of AP domains. */

The source object seems to have a well defined size.

A quick web search seems to indicate gcc throws quite a few
Wstringop-overread warnings for valid code. I suspect this is
another example of that.