[PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment

[PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment

[Sun Jian]

Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 net/core/dev.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 5933c5dab09e..5c37cf6c4aa1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5517,12 +5517,16 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
 	}
 
 	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
-	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
+	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
+	 * from skb->len before updating data_len, then add the new one back.
 	 */
-	if (xdp_buff_has_frags(xdp))
+	skb->len -= skb->data_len;
+	if (xdp_buff_has_frags(xdp)) {
 		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
-	else
+		skb->len += skb->data_len;
+	} else {
 		skb->data_len = 0;
+	}
 
 	/* check if XDP changed eth hdr such SKB needs update */
 	eth = (struct ethhdr *)xdp->data;
-- 
2.43.0

Re: [PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment

[Mohsin Bashir]

On 7/30/26 8:23 PM, Sun Jian wrote:
> Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
> XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
> xdp_frags_size back to skb->data_len but leaves skb->len containing the
> old fragment contribution.
> 
> After a fragment shrink, this makes skb_headlen() larger than the actual
> linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> copied 1024 bytes past the actual linear tail to userspace, starting at
> struct skb_shared_info. The copied bytes included the affected skb's
> nr_frags, xdp_frags_size and a kernel pointer from
> skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> amount and truncated at the end.
> 
> Subtract the old data_len before replacing it and add the new data_len
> afterwards, keeping skb->len and skb->data_len synchronized.
> 
> A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> corrupted payloads. After the fix, all 10 runs matched the expected
> payload exactly.
> 
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> ---
>   net/core/dev.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 5933c5dab09e..5c37cf6c4aa1 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5517,12 +5517,16 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>   	}
>   
>   	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> +	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> +	 * from skb->len before updating data_len, then add the new one back.
>   	 */
> -	if (xdp_buff_has_frags(xdp))
> +	skb->len -= skb->data_len;
> +	if (xdp_buff_has_frags(xdp)) {
>   		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> -	else
> +		skb->len += skb->data_len;
> +	} else {
>   		skb->data_len = 0;
> +	}
>   
>   	/* check if XDP changed eth hdr such SKB needs update */
>   	eth = (struct ethhdr *)xdp->data;

Reviewed-by: Mohsin Bashir <hmohsin@meta.com>

Re: [PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment

[Lorenzo Bianconi]

[-- Attachment #1: Type: text/plain, Size: 2415 bytes --]

> Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
> XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
> xdp_frags_size back to skb->data_len but leaves skb->len containing the
> old fragment contribution.
> 
> After a fragment shrink, this makes skb_headlen() larger than the actual
> linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> copied 1024 bytes past the actual linear tail to userspace, starting at
> struct skb_shared_info. The copied bytes included the affected skb's
> nr_frags, xdp_frags_size and a kernel pointer from
> skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> amount and truncated at the end.
> 
> Subtract the old data_len before replacing it and add the new data_len
> afterwards, keeping skb->len and skb->data_len synchronized.
> 
> A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> corrupted payloads. After the fix, all 10 runs matched the expected
> payload exactly.
> 
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>

Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>

> ---
>  net/core/dev.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 5933c5dab09e..5c37cf6c4aa1 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5517,12 +5517,16 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>  	}
>  
>  	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> +	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> +	 * from skb->len before updating data_len, then add the new one back.
>  	 */
> -	if (xdp_buff_has_frags(xdp))
> +	skb->len -= skb->data_len;
> +	if (xdp_buff_has_frags(xdp)) {
>  		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> -	else
> +		skb->len += skb->data_len;
> +	} else {
>  		skb->data_len = 0;
> +	}
>  
>  	/* check if XDP changed eth hdr such SKB needs update */
>  	eth = (struct ethhdr *)xdp->data;
> -- 
> 2.43.0
> 

[-- Attachment #2: signature.asc --]
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[PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[Sun Jian]

veth exposes non-linear skb fragments through an xdp_buff. If an XDP
program adjusts the fragment area, veth_xdp_rcv_skb() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

The fragment accounting must run before the linear tail adjustment:
when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
releases all fragments, and __skb_put() requires skb->data_len == 0
by that point.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 drivers/net/veth.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 00e34afd858e..348391e87e14 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 
 	skb_reset_mac_header(skb);
 
-	/* check if bpf_xdp_adjust_tail was used */
-	off = xdp->data_end - orig_data_end;
-	if (off != 0)
-		__skb_put(skb, off); /* positive on grow, negative on shrink */
-
 	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
-	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
+	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
+	 * from skb->len before updating data_len, then add the new one back.
+	 * This must precede the linear tail adjustment below: a changed
+	 * data_end implies that no fragments remain, and __skb_put() requires
+	 * a linear skb.
 	 */
-	if (xdp_buff_has_frags(xdp))
+	skb->len -= skb->data_len;
+	if (xdp_buff_has_frags(xdp)) {
 		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
-	else
+		skb->len += skb->data_len;
+	} else {
 		skb->data_len = 0;
+	}
+
+	/* check if bpf_xdp_adjust_tail was used */
+	off = xdp->data_end - orig_data_end;
+	if (off != 0)
+		__skb_put(skb, off); /* positive on grow, negative on shrink */
 
 	skb->protocol = eth_type_trans(skb, rq->dev);
 
-- 
2.43.0

Re: [PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[Lorenzo Bianconi]

[-- Attachment #1: Type: text/plain, Size: 3184 bytes --]

> veth exposes non-linear skb fragments through an xdp_buff. If an XDP
> program adjusts the fragment area, veth_xdp_rcv_skb() copies
> xdp_frags_size back to skb->data_len but leaves skb->len containing the
> old fragment contribution.
> 
> After a fragment shrink, this makes skb_headlen() larger than the actual
> linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> copied 1024 bytes past the actual linear tail to userspace, starting at
> struct skb_shared_info. The copied bytes included the affected skb's
> nr_frags, xdp_frags_size and a kernel pointer from
> skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> amount and truncated at the end.
> 
> Subtract the old data_len before replacing it and add the new data_len
> afterwards, keeping skb->len and skb->data_len synchronized.
> 
> The fragment accounting must run before the linear tail adjustment:
> when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
> releases all fragments, and __skb_put() requires skb->data_len == 0
> by that point.
> 
> A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> corrupted payloads. After the fix, all 10 runs matched the expected
> payload exactly.
> 
> Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>

Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>

> ---
>  drivers/net/veth.c | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 00e34afd858e..348391e87e14 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
>  
>  	skb_reset_mac_header(skb);
>  
> -	/* check if bpf_xdp_adjust_tail was used */
> -	off = xdp->data_end - orig_data_end;
> -	if (off != 0)
> -		__skb_put(skb, off); /* positive on grow, negative on shrink */
> -
>  	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> +	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> +	 * from skb->len before updating data_len, then add the new one back.
> +	 * This must precede the linear tail adjustment below: a changed
> +	 * data_end implies that no fragments remain, and __skb_put() requires
> +	 * a linear skb.
>  	 */
> -	if (xdp_buff_has_frags(xdp))
> +	skb->len -= skb->data_len;
> +	if (xdp_buff_has_frags(xdp)) {
>  		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> -	else
> +		skb->len += skb->data_len;
> +	} else {
>  		skb->data_len = 0;
> +	}
> +
> +	/* check if bpf_xdp_adjust_tail was used */
> +	off = xdp->data_end - orig_data_end;
> +	if (off != 0)
> +		__skb_put(skb, off); /* positive on grow, negative on shrink */
>  
>  	skb->protocol = eth_type_trans(skb, rq->dev);
>  
> -- 
> 2.43.0
> 

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[Mohsin Bashir]

On 7/30/26 8:23 PM, Sun Jian wrote:
> veth exposes non-linear skb fragments through an xdp_buff. If an XDP
> program adjusts the fragment area, veth_xdp_rcv_skb() copies
> xdp_frags_size back to skb->data_len but leaves skb->len containing the
> old fragment contribution.
> 
> After a fragment shrink, this makes skb_headlen() larger than the actual
> linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> copied 1024 bytes past the actual linear tail to userspace, starting at
> struct skb_shared_info. The copied bytes included the affected skb's
> nr_frags, xdp_frags_size and a kernel pointer from
> skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> amount and truncated at the end.
> 
> Subtract the old data_len before replacing it and add the new data_len
> afterwards, keeping skb->len and skb->data_len synchronized.
> 
> The fragment accounting must run before the linear tail adjustment:
> when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
> releases all fragments, and __skb_put() requires skb->data_len == 0
> by that point.
> 
> A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> corrupted payloads. After the fix, all 10 runs matched the expected
> payload exactly.
> 
> Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> ---
>   drivers/net/veth.c | 23 +++++++++++++++--------
>   1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 00e34afd858e..348391e87e14 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
>   
>   	skb_reset_mac_header(skb);
>   
> -	/* check if bpf_xdp_adjust_tail was used */
> -	off = xdp->data_end - orig_data_end;
> -	if (off != 0)
> -		__skb_put(skb, off); /* positive on grow, negative on shrink */
> -
>   	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> +	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> +	 * from skb->len before updating data_len, then add the new one back.
> +	 * This must precede the linear tail adjustment below: a changed
> +	 * data_end implies that no fragments remain, and __skb_put() requires
> +	 * a linear skb.
>   	 */
> -	if (xdp_buff_has_frags(xdp))
> +	skb->len -= skb->data_len;
> +	if (xdp_buff_has_frags(xdp)) {
>   		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> -	else
> +		skb->len += skb->data_len;
> +	} else {
>   		skb->data_len = 0;
> +	}
> +
> +	/* check if bpf_xdp_adjust_tail was used */
> +	off = xdp->data_end - orig_data_end;
> +	if (off != 0)
> +		__skb_put(skb, off); /* positive on grow, negative on shrink */
>   
>   	skb->protocol = eth_type_trans(skb, rq->dev);
>   

I am most likely missing something here but what happens if we have 
frags and we attempt to advance data_end while leaving some frags 
present (e.g., bpf_xdp_pull_data())? Looks like, in that case we would 
issue __skb_put(skb, off) with off > 0 and we would hit 
SKB_LINEAR_ASSERT() because skb is still non-linear?

Re: [PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[sun jian]

On Sat, Aug 1, 2026 at 12:14 AM Mohsin Bashir <mohsin.bashr@gmail.com> wrote:
>
>
>
> On 7/30/26 8:23 PM, Sun Jian wrote:
> > veth exposes non-linear skb fragments through an xdp_buff. If an XDP
> > program adjusts the fragment area, veth_xdp_rcv_skb() copies
> > xdp_frags_size back to skb->data_len but leaves skb->len containing the
> > old fragment contribution.
> >
> > After a fragment shrink, this makes skb_headlen() larger than the actual
> > linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> > copied 1024 bytes past the actual linear tail to userspace, starting at
> > struct skb_shared_info. The copied bytes included the affected skb's
> > nr_frags, xdp_frags_size and a kernel pointer from
> > skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> > amount and truncated at the end.
> >
> > Subtract the old data_len before replacing it and add the new data_len
> > afterwards, keeping skb->len and skb->data_len synchronized.
> >
> > The fragment accounting must run before the linear tail adjustment:
> > when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
> > releases all fragments, and __skb_put() requires skb->data_len == 0
> > by that point.
> >
> > A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> > 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> > corrupted payloads. After the fix, all 10 runs matched the expected
> > payload exactly.
> >
> > Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
> > Cc: stable@vger.kernel.org
> > Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> > Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> > ---
> >   drivers/net/veth.c | 23 +++++++++++++++--------
> >   1 file changed, 15 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index 00e34afd858e..348391e87e14 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
> >
> >       skb_reset_mac_header(skb);
> >
> > -     /* check if bpf_xdp_adjust_tail was used */
> > -     off = xdp->data_end - orig_data_end;
> > -     if (off != 0)
> > -             __skb_put(skb, off); /* positive on grow, negative on shrink */
> > -
> >       /* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > -      * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > +      * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> > +      * from skb->len before updating data_len, then add the new one back.
> > +      * This must precede the linear tail adjustment below: a changed
> > +      * data_end implies that no fragments remain, and __skb_put() requires
> > +      * a linear skb.
> >        */
> > -     if (xdp_buff_has_frags(xdp))
> > +     skb->len -= skb->data_len;
> > +     if (xdp_buff_has_frags(xdp)) {
> >               skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > -     else
> > +             skb->len += skb->data_len;
> > +     } else {
> >               skb->data_len = 0;
> > +     }
> > +
> > +     /* check if bpf_xdp_adjust_tail was used */
> > +     off = xdp->data_end - orig_data_end;
> > +     if (off != 0)
> > +             __skb_put(skb, off); /* positive on grow, negative on shrink */
> >
> >       skb->protocol = eth_type_trans(skb, rq->dev);
> >
>
> I am most likely missing something here but what happens if we have
> frags and we attempt to advance data_end while leaving some frags
> present (e.g., bpf_xdp_pull_data())? Looks like, in that case we would
> issue __skb_put(skb, off) with off > 0 and we would hit
> SKB_LINEAR_ASSERT() because skb is still non-linear?

Hi Mohsin,

Thanks for catching this. You're right that bpf_xdp_pull_data() can advance
data_end while leaving frags present. I'll investigate this path and follow up.

Thanks,
Sun Jian

Re: [PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[Lorenzo Bianconi]

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> On Sat, Aug 1, 2026 at 12:14 AM Mohsin Bashir <mohsin.bashr@gmail.com> wrote:
> >
> >
> >
> > On 7/30/26 8:23 PM, Sun Jian wrote:
> > > veth exposes non-linear skb fragments through an xdp_buff. If an XDP
> > > program adjusts the fragment area, veth_xdp_rcv_skb() copies
> > > xdp_frags_size back to skb->data_len but leaves skb->len containing the
> > > old fragment contribution.
> > >
> > > After a fragment shrink, this makes skb_headlen() larger than the actual
> > > linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> > > copied 1024 bytes past the actual linear tail to userspace, starting at
> > > struct skb_shared_info. The copied bytes included the affected skb's
> > > nr_frags, xdp_frags_size and a kernel pointer from
> > > skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> > > amount and truncated at the end.
> > >
> > > Subtract the old data_len before replacing it and add the new data_len
> > > afterwards, keeping skb->len and skb->data_len synchronized.
> > >
> > > The fragment accounting must run before the linear tail adjustment:
> > > when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
> > > releases all fragments, and __skb_put() requires skb->data_len == 0
> > > by that point.
> > >
> > > A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> > > 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> > > corrupted payloads. After the fix, all 10 runs matched the expected
> > > payload exactly.
> > >
> > > Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
> > > Cc: stable@vger.kernel.org
> > > Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> > > Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> > > ---
> > >   drivers/net/veth.c | 23 +++++++++++++++--------
> > >   1 file changed, 15 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > > index 00e34afd858e..348391e87e14 100644
> > > --- a/drivers/net/veth.c
> > > +++ b/drivers/net/veth.c
> > > @@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
> > >
> > >       skb_reset_mac_header(skb);
> > >
> > > -     /* check if bpf_xdp_adjust_tail was used */
> > > -     off = xdp->data_end - orig_data_end;
> > > -     if (off != 0)
> > > -             __skb_put(skb, off); /* positive on grow, negative on shrink */
> > > -
> > >       /* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > > -      * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > > +      * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> > > +      * from skb->len before updating data_len, then add the new one back.
> > > +      * This must precede the linear tail adjustment below: a changed
> > > +      * data_end implies that no fragments remain, and __skb_put() requires
> > > +      * a linear skb.
> > >        */
> > > -     if (xdp_buff_has_frags(xdp))
> > > +     skb->len -= skb->data_len;
> > > +     if (xdp_buff_has_frags(xdp)) {
> > >               skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > > -     else
> > > +             skb->len += skb->data_len;
> > > +     } else {
> > >               skb->data_len = 0;
> > > +     }
> > > +
> > > +     /* check if bpf_xdp_adjust_tail was used */
> > > +     off = xdp->data_end - orig_data_end;
> > > +     if (off != 0)
> > > +             __skb_put(skb, off); /* positive on grow, negative on shrink */
> > >
> > >       skb->protocol = eth_type_trans(skb, rq->dev);
> > >
> >
> > I am most likely missing something here but what happens if we have
> > frags and we attempt to advance data_end while leaving some frags
> > present (e.g., bpf_xdp_pull_data())? Looks like, in that case we would
> > issue __skb_put(skb, off) with off > 0 and we would hit
> > SKB_LINEAR_ASSERT() because skb is still non-linear?
> 
> Hi Mohsin,
> 
> Thanks for catching this. You're right that bpf_xdp_pull_data() can advance
> data_end while leaving frags present. I'll investigate this path and follow up.
> 
> Thanks,
> Sun Jian

I guess we should implement the same approach used in
bpf_prog_run_generic_xdp(), something like:

	if (off) {
		skb_set_tail_pointer();
		skb->len += off;
	}

Regards,
Lorenzo

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Re: [PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[sun jian]

On Mon, Aug 3, 2026 at 4:42 PM Lorenzo Bianconi <lorenzo@kernel.org> wrote:
>
> > On Sat, Aug 1, 2026 at 12:14 AM Mohsin Bashir <mohsin.bashr@gmail.com> wrote:
> > >
> > >
> > >
> > > On 7/30/26 8:23 PM, Sun Jian wrote:
> > > > veth exposes non-linear skb fragments through an xdp_buff. If an XDP
> > > > program adjusts the fragment area, veth_xdp_rcv_skb() copies
> > > > xdp_frags_size back to skb->data_len but leaves skb->len containing the
> > > > old fragment contribution.
> > > >
> > > > After a fragment shrink, this makes skb_headlen() larger than the actual
> > > > linear area. In the reproduced UDP receive path, __skb_datagram_iter()
> > > > copied 1024 bytes past the actual linear tail to userspace, starting at
> > > > struct skb_shared_info. The copied bytes included the affected skb's
> > > > nr_frags, xdp_frags_size and a kernel pointer from
> > > > skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
> > > > amount and truncated at the end.
> > > >
> > > > Subtract the old data_len before replacing it and add the new data_len
> > > > afterwards, keeping skb->len and skb->data_len synchronized.
> > > >
> > > > The fragment accounting must run before the linear tail adjustment:
> > > > when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
> > > > releases all fragments, and __skb_put() requires skb->data_len == 0
> > > > by that point.
> > > >
> > > > A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
> > > > 1024 bytes from its fragment area. Before the fix, all 10 runs produced
> > > > corrupted payloads. After the fix, all 10 runs matched the expected
> > > > payload exactly.
> > > >
> > > > Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
> > > > Cc: stable@vger.kernel.org
> > > > Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
> > > > Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> > > > ---
> > > >   drivers/net/veth.c | 23 +++++++++++++++--------
> > > >   1 file changed, 15 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > > > index 00e34afd858e..348391e87e14 100644
> > > > --- a/drivers/net/veth.c
> > > > +++ b/drivers/net/veth.c
> > > > @@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
> > > >
> > > >       skb_reset_mac_header(skb);
> > > >
> > > > -     /* check if bpf_xdp_adjust_tail was used */
> > > > -     off = xdp->data_end - orig_data_end;
> > > > -     if (off != 0)
> > > > -             __skb_put(skb, off); /* positive on grow, negative on shrink */
> > > > -
> > > >       /* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > > > -      * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > > > +      * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
> > > > +      * from skb->len before updating data_len, then add the new one back.
> > > > +      * This must precede the linear tail adjustment below: a changed
> > > > +      * data_end implies that no fragments remain, and __skb_put() requires
> > > > +      * a linear skb.
> > > >        */
> > > > -     if (xdp_buff_has_frags(xdp))
> > > > +     skb->len -= skb->data_len;
> > > > +     if (xdp_buff_has_frags(xdp)) {
> > > >               skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > > > -     else
> > > > +             skb->len += skb->data_len;
> > > > +     } else {
> > > >               skb->data_len = 0;
> > > > +     }
> > > > +
> > > > +     /* check if bpf_xdp_adjust_tail was used */
> > > > +     off = xdp->data_end - orig_data_end;
> > > > +     if (off != 0)
> > > > +             __skb_put(skb, off); /* positive on grow, negative on shrink */
> > > >
> > > >       skb->protocol = eth_type_trans(skb, rq->dev);
> > > >
> > >
> > > I am most likely missing something here but what happens if we have
> > > frags and we attempt to advance data_end while leaving some frags
> > > present (e.g., bpf_xdp_pull_data())? Looks like, in that case we would
> > > issue __skb_put(skb, off) with off > 0 and we would hit
> > > SKB_LINEAR_ASSERT() because skb is still non-linear?
> >
> > Hi Mohsin,
> >
> > Thanks for catching this. You're right that bpf_xdp_pull_data() can advance
> > data_end while leaving frags present. I'll investigate this path and follow up.
> >
> > Thanks,
> > Sun Jian
>
Hi Lorenzo,

> I guess we should implement the same approach used in
> bpf_prog_run_generic_xdp(), something like:
>
>         if (off) {
>                 skb_set_tail_pointer();
>                 skb->len += off;
>         }
Yes, that makes sense. I confirmed Mohsin's case: bpf_xdp_pull_data()
can advance data_end while keeping frags, so the current v2 can hit
SKB_LINEAR_ASSERT() through __skb_put() on a non-linear skb.

I tested the skb_set_tail_pointer() + skb->len update approach locally
with a forced-tailroom reproducer, and it fixes that path. I will fold
this into v3.

Thanks,
Sun Jian
>
> Regards,
> Lorenzo