Re: [PATCH bpf-next v4 2/6] bpf: Add syscall common attrs support for xdp link_create

On 2026/10/25 09:19, Alexei Starovoitov wrote:

On Sat, Oct 24, 2026 at 08:36 AM Leon Hwang [off-list ref] wrote:

quoted

Instead of passing bpf log or extack to bpf_xdp_link_attach(), pass an
errmsg pointer to receive the kstrdup() error msg, because

[...]

quoted

2) passing extack will access underlying driver string out of dev_put()
   guard, which will crash kernel when the device is unloaded
   concurrently.

Netlink does exactly that.
rtnl_setlink() looks up the dev with __dev_get_by_index(), so no refcnt,
do_setlink() -> dev_change_xdp_fd(dev, extack, ...) calls into the same
drivers that set the same strings, rtnl is unlocked, and only then
netlink_rcv_skb() -> netlink_ack() does strlen(extack->_msg).
All extack users work this way. kstrdup() in one of them fixes nothing.

Oops, I was overthinking about it when self-reviewing the patch. Failed
to verify the describing crash issue just now. I should verify my review
when self-reviewing.

Instead of patches 1 and 2:

Will change like this.

I'm think I gave this suggestion earlier. Why did you go with strdup()?

Sorry for my overthinking. Will verify my review to my patches in the
future.

Thanks,
Leon