On 6/22/26 1:42 PM, Ido Schimmel wrote:

On Sat, Jun 20, 2026 at 06:18:50PM +0200, Fernando Fernandez Mancera wrote:

quoted

When handling proxy_ndp, if rtnl_net_trylock() fails, the operation is

s/proxy_ndp/force_forwarding/

quoted

retried but the position pointer was already advanced meaning that the
restarted sysctl will read from an incorrect offset.

Fix this by restoring the original position pointer before restarting
the syscall.

In addition, remove the redundant position pointer restoration at the
end of the function.

Fixes: f24987ef6959 ("ipv6: add `force_forwarding` sysctl to enable per-interface forwarding")
Signed-off-by: Fernando Fernandez Mancera <redacted>
---
  net/ipv6/addrconf.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index cbe681de3818..8c0741e9dfcc 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c

@@ -6825,8 +6825,10 @@ static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int wri
  	ret = proc_douintvec_minmax(&tmp_ctl, write, buffer, lenp, ppos);
  
  	if (write && old_val != new_val) {
-		if (!rtnl_net_trylock(net))
+		if (!rtnl_net_trylock(net)) {
+			*ppos = pos;
  			return restart_syscall();
+		}

Are you sure that this is needed?

AFAICT, the position pointer is only advanced if the return value is
positive. From new_sync_write():

kiocb.ki_pos = (ppos ? *ppos : 0);
[...]
ret = filp->f_op->write_iter(&kiocb, &iter);
[...]
if (ret > 0 && ppos)
         *ppos = kiocb.ki_pos;

And restart_syscall() returns '-ERESTARTNOINTR'.

Hm, I think you are right. I was not aware of this check, thanks for 
pointing it out. That means we can get rid of position pointer reset 
from the rest of the code.. the are plenty of sysctl following this 
pattern. I will prepare a batch for net-next.

I am sending a v3 dropping this patch.

Thank you Ido!

quoted

  
  		WRITE_ONCE(*valp, new_val);
  

@@ -6851,8 +6853,6 @@ static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int wri
  		rtnl_net_unlock(net);
  	}
  
-	if (ret)
-		*ppos = pos;
  	return ret;
  }
  

-- 
2.54.0