[PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[Gustavo A. R. Silva]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the new TRAILING_OVERLAP() helper to fix the following warning:

drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM)
and a set of members that would otherwise follow it (in this case
`u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
overlays the trailing members (rss_hash_key_data) onto the FAM
(hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after `struct virtnet_info` (no
blank line in between).

Notice that due to tail padding in flexible `struct
virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
(at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
offset 84 in struct virtnet_info) are misaligned by one byte. See
below:

struct virtio_net_rss_config_trailer {
        __le16                     max_tx_vq;            /*     0     2 */
        __u8                       hash_key_length;      /*     2     1 */
        __u8                       hash_key_data[];      /*     3     0 */

        /* size: 4, cachelines: 1, members: 3 */
        /* padding: 1 */
        /* last cacheline: 4 bytes */
};

struct virtnet_info {
...
        struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */

        /* XXX last struct has 1 byte of padding */

        u8                         rss_hash_key_data[40]; /*    84    40 */
...
        /* size: 832, cachelines: 13, members: 48 */
        /* sum members: 801, holes: 8, sum holes: 31 */
        /* paddings: 2, sum paddings: 5 */
};

After changes, those members are correctly aligned at offset 795:

struct virtnet_info {
...
        union {
                struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
                struct {
                        unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
                        u8         rss_hash_key_data[40]; /*   795    40 */
                };                                       /*   792    43 */
        };                                               /*   792    44 */
...
        /* size: 840, cachelines: 14, members: 47 */
        /* sum members: 801, holes: 8, sum holes: 35 */
        /* padding: 4 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 8 bytes */
};

As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
moved to the end, since it seems those three members should stick
around together. :)

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---

This should probably include the following tag:

	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")

but I'd like to hear some feedback, first.

Thanks!

 drivers/net/virtio_net.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 975bdc5dab84..f4964a18a214 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -425,9 +425,6 @@ struct virtnet_info {
 	u16 rss_indir_table_size;
 	u32 rss_hash_types_supported;
 	u32 rss_hash_types_saved;
-	struct virtio_net_rss_config_hdr *rss_hdr;
-	struct virtio_net_rss_config_trailer rss_trailer;
-	u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
 
 	/* Has control virtqueue */
 	bool has_cvq;
@@ -493,7 +490,16 @@ struct virtnet_info {
 	struct failover *failover;
 
 	u64 device_stats_cap;
+
+	struct virtio_net_rss_config_hdr *rss_hdr;
+
+	/* Must be last --ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
+		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
+	);
 };
+static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
+	      offsetof(struct virtnet_info, rss_hash_key_data));
 
 struct padded_vnet_hdr {
 	struct virtio_net_hdr_v1_hash hdr;
-- 
2.43.0

Re: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[Simon Horman]

On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
> 
> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after `struct virtnet_info` (no
> blank line in between).
> 
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
> 
> struct virtio_net_rss_config_trailer {
>         __le16                     max_tx_vq;            /*     0     2 */
>         __u8                       hash_key_length;      /*     2     1 */
>         __u8                       hash_key_data[];      /*     3     0 */
> 
>         /* size: 4, cachelines: 1, members: 3 */
>         /* padding: 1 */
>         /* last cacheline: 4 bytes */
> };
> 
> struct virtnet_info {
> ...
>         struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
> 
>         /* XXX last struct has 1 byte of padding */
> 
>         u8                         rss_hash_key_data[40]; /*    84    40 */
> ...
>         /* size: 832, cachelines: 13, members: 48 */
>         /* sum members: 801, holes: 8, sum holes: 31 */
>         /* paddings: 2, sum paddings: 5 */
> };
> 
> After changes, those members are correctly aligned at offset 795:
> 
> struct virtnet_info {
> ...
>         union {
>                 struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
>                 struct {
>                         unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
>                         u8         rss_hash_key_data[40]; /*   795    40 */
>                 };                                       /*   792    43 */
>         };                                               /*   792    44 */
> ...
>         /* size: 840, cachelines: 14, members: 47 */
>         /* sum members: 801, holes: 8, sum holes: 35 */
>         /* padding: 4 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 8 bytes */
> };
> 
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> 
> This should probably include the following tag:
> 
> 	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> 
> but I'd like to hear some feedback, first.

I tend to agree given that:

On the one hand:

1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
   to write random data to .rss_hash_key_data

2) In virtnet_set_rxfh() key data written to .rss_hash_key_data

While

3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
   including the contents of .hash_key_data based on the length of
   that data provided in .hash_key_length is copied.

It seems to me that step 3 will include 1 byte of uninitialised data
at the start of .hash_key_data. And, correspondingly, truncate
.rss_hash_key_data by one byte.

It's unclear to me what the effect of this - perhaps they key works
regardless. But it doesn't seem intended. And while the result may be
neutral, I do  suspect this reduces the quality of the key. And I more
strongly suspect it doesn't have any positive outcome.

So I would lean towards playing it safe and considering this as a bug.

Of course, other's may have better insight as to the actual effect of this.

...

Re: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[Gustavo A. R. Silva]

On 9/4/25 11:13, Simon Horman wrote:
> On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>> getting ready to enable it, globally.
>>
>> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>>
>> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>>
>> This helper creates a union between a flexible-array member (FAM)
>> and a set of members that would otherwise follow it (in this case
>> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
>> overlays the trailing members (rss_hash_key_data) onto the FAM
>> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
>> The static_assert() ensures this alignment remains, and it's
>> intentionally placed inmediately after `struct virtnet_info` (no
>> blank line in between).
>>
>> Notice that due to tail padding in flexible `struct
>> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
>> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
>> offset 84 in struct virtnet_info) are misaligned by one byte. See
>> below:
>>
>> struct virtio_net_rss_config_trailer {
>>          __le16                     max_tx_vq;            /*     0     2 */
>>          __u8                       hash_key_length;      /*     2     1 */
>>          __u8                       hash_key_data[];      /*     3     0 */
>>
>>          /* size: 4, cachelines: 1, members: 3 */
>>          /* padding: 1 */
>>          /* last cacheline: 4 bytes */
>> };
>>
>> struct virtnet_info {
>> ...
>>          struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
>>
>>          /* XXX last struct has 1 byte of padding */
>>
>>          u8                         rss_hash_key_data[40]; /*    84    40 */
>> ...
>>          /* size: 832, cachelines: 13, members: 48 */
>>          /* sum members: 801, holes: 8, sum holes: 31 */
>>          /* paddings: 2, sum paddings: 5 */
>> };
>>
>> After changes, those members are correctly aligned at offset 795:
>>
>> struct virtnet_info {
>> ...
>>          union {
>>                  struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
>>                  struct {
>>                          unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
>>                          u8         rss_hash_key_data[40]; /*   795    40 */
>>                  };                                       /*   792    43 */
>>          };                                               /*   792    44 */
>> ...
>>          /* size: 840, cachelines: 14, members: 47 */
>>          /* sum members: 801, holes: 8, sum holes: 35 */
>>          /* padding: 4 */
>>          /* paddings: 1, sum paddings: 4 */
>>          /* last cacheline: 8 bytes */
>> };
>>
>> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
>> moved to the end, since it seems those three members should stick
>> around together. :)
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>
>> This should probably include the following tag:
>>
>> 	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
>>
>> but I'd like to hear some feedback, first.
> 
> I tend to agree given that:
> 
> On the one hand:
> 
> 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
>     to write random data to .rss_hash_key_data
> 
> 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
> 
> While
> 
> 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
>     including the contents of .hash_key_data based on the length of
>     that data provided in .hash_key_length is copied.
> 
> It seems to me that step 3 will include 1 byte of uninitialised data
> at the start of .hash_key_data. And, correspondingly, truncate
> .rss_hash_key_data by one byte.
> 
> It's unclear to me what the effect of this - perhaps they key works
> regardless. But it doesn't seem intended. And while the result may be
> neutral, I do  suspect this reduces the quality of the key. And I more
> strongly suspect it doesn't have any positive outcome.
> 
> So I would lean towards playing it safe and considering this as a bug.
> 
> Of course, other's may have better insight as to the actual effect of this.

Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
tags.

Thanks for the feedback!
-Gustavo

Re: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[Michael S. Tsirkin]

On Thu, Sep 04, 2025 at 08:53:31PM +0200, Gustavo A. R. Silva wrote:
> 
> 
> On 9/4/25 11:13, Simon Horman wrote:
> > On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> > > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > > getting ready to enable it, globally.
> > > 
> > > Use the new TRAILING_OVERLAP() helper to fix the following warning:
> > > 
> > > drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > 
> > > This helper creates a union between a flexible-array member (FAM)
> > > and a set of members that would otherwise follow it (in this case
> > > `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> > > overlays the trailing members (rss_hash_key_data) onto the FAM
> > > (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> > > The static_assert() ensures this alignment remains, and it's
> > > intentionally placed inmediately after `struct virtnet_info` (no
> > > blank line in between).
> > > 
> > > Notice that due to tail padding in flexible `struct
> > > virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> > > (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> > > offset 84 in struct virtnet_info) are misaligned by one byte. See
> > > below:
> > > 
> > > struct virtio_net_rss_config_trailer {
> > >          __le16                     max_tx_vq;            /*     0     2 */
> > >          __u8                       hash_key_length;      /*     2     1 */
> > >          __u8                       hash_key_data[];      /*     3     0 */
> > > 
> > >          /* size: 4, cachelines: 1, members: 3 */
> > >          /* padding: 1 */
> > >          /* last cacheline: 4 bytes */
> > > };
> > > 
> > > struct virtnet_info {
> > > ...
> > >          struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
> > > 
> > >          /* XXX last struct has 1 byte of padding */
> > > 
> > >          u8                         rss_hash_key_data[40]; /*    84    40 */
> > > ...
> > >          /* size: 832, cachelines: 13, members: 48 */
> > >          /* sum members: 801, holes: 8, sum holes: 31 */
> > >          /* paddings: 2, sum paddings: 5 */
> > > };
> > > 
> > > After changes, those members are correctly aligned at offset 795:
> > > 
> > > struct virtnet_info {
> > > ...
> > >          union {
> > >                  struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
> > >                  struct {
> > >                          unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
> > >                          u8         rss_hash_key_data[40]; /*   795    40 */
> > >                  };                                       /*   792    43 */
> > >          };                                               /*   792    44 */
> > > ...
> > >          /* size: 840, cachelines: 14, members: 47 */
> > >          /* sum members: 801, holes: 8, sum holes: 35 */
> > >          /* padding: 4 */
> > >          /* paddings: 1, sum paddings: 4 */
> > >          /* last cacheline: 8 bytes */
> > > };
> > > 
> > > As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> > > moved to the end, since it seems those three members should stick
> > > around together. :)
> > > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > > ---
> > > 
> > > This should probably include the following tag:
> > > 
> > > 	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> > > 
> > > but I'd like to hear some feedback, first.
> > 
> > I tend to agree given that:
> > 
> > On the one hand:
> > 
> > 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
> >     to write random data to .rss_hash_key_data
> > 
> > 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
> > 
> > While
> > 
> > 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
> >     including the contents of .hash_key_data based on the length of
> >     that data provided in .hash_key_length is copied.
> > 
> > It seems to me that step 3 will include 1 byte of uninitialised data
> > at the start of .hash_key_data. And, correspondingly, truncate
> > .rss_hash_key_data by one byte.
> > 
> > It's unclear to me what the effect of this - perhaps they key works
> > regardless. But it doesn't seem intended. And while the result may be
> > neutral, I do  suspect this reduces the quality of the key. And I more
> > strongly suspect it doesn't have any positive outcome.
> > 
> > So I would lean towards playing it safe and considering this as a bug.
> > 
> > Of course, other's may have better insight as to the actual effect of this.
> 
> Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
> tags.
> 
> Thanks for the feedback!
> -Gustavo
> 
> 


I agree. It looks like that commit completely broke RSS
configuration. Akihiko do you mind sharing how that was
tested? Maybe help testing the fix? Thanks!

Re: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning

[Michael S. Tsirkin]

On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
> 
> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after `struct virtnet_info` (no
> blank line in between).
> 
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
> 
> struct virtio_net_rss_config_trailer {
>         __le16                     max_tx_vq;            /*     0     2 */
>         __u8                       hash_key_length;      /*     2     1 */
>         __u8                       hash_key_data[];      /*     3     0 */
> 
>         /* size: 4, cachelines: 1, members: 3 */
>         /* padding: 1 */
>         /* last cacheline: 4 bytes */
> };
> 
> struct virtnet_info {
> ...
>         struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
> 
>         /* XXX last struct has 1 byte of padding */
> 
>         u8                         rss_hash_key_data[40]; /*    84    40 */
> ...
>         /* size: 832, cachelines: 13, members: 48 */
>         /* sum members: 801, holes: 8, sum holes: 31 */
>         /* paddings: 2, sum paddings: 5 */
> };
> 
> After changes, those members are correctly aligned at offset 795:
> 
> struct virtnet_info {
> ...
>         union {
>                 struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
>                 struct {
>                         unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
>                         u8         rss_hash_key_data[40]; /*   795    40 */
>                 };                                       /*   792    43 */
>         };                                               /*   792    44 */
> ...
>         /* size: 840, cachelines: 14, members: 47 */
>         /* sum members: 801, holes: 8, sum holes: 35 */
>         /* padding: 4 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 8 bytes */
> };
> 
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> 
> This should probably include the following tag:
> 
> 	Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> 
> but I'd like to hear some feedback, first.
> 
> Thanks!
> 


I would add:

as a result, the RSS key passed to the device is shifted by 1
byte: the last byte is cut off, and instead a (possibly uninitialized) byte
is added at the beginning.

>  drivers/net/virtio_net.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 975bdc5dab84..f4964a18a214 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -425,9 +425,6 @@ struct virtnet_info {
>  	u16 rss_indir_table_size;
>  	u32 rss_hash_types_supported;
>  	u32 rss_hash_types_saved;
> -	struct virtio_net_rss_config_hdr *rss_hdr;
> -	struct virtio_net_rss_config_trailer rss_trailer;
> -	u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
>  
>  	/* Has control virtqueue */
>  	bool has_cvq;
> @@ -493,7 +490,16 @@ struct virtnet_info {
>  	struct failover *failover;
>  
>  	u64 device_stats_cap;
> +
> +	struct virtio_net_rss_config_hdr *rss_hdr;
> +
> +	/* Must be last --ends in a flexible-array member. */
> +	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
> +		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> +	);
>  };
> +static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
> +	      offsetof(struct virtnet_info, rss_hash_key_data));
>  
>  struct padded_vnet_hdr {
>  	struct virtio_net_hdr_v1_hash hdr;
> -- 
> 2.43.0