Re: [PATCH] ipv4: fib: annotate data-race around nh->nh_saddr

On Mon, Sep 14, 2026 at 12:51 PM Kuniyuki Iwashima [off-list ref] wrote:

On Fri, Sep 11, 2026 at 12:38 AM Linkui Xiao [off-list ref] wrote:

quoted

From: Linkui Xiao <redacted>

fib_select_multipath() compares nexthop_nh->nh_saddr against the flow
source address with no lock held, while fib_info_update_nhc_saddr()
stores a new value from another CPU as soon as the preferred source
address of the egress device changes.

Commit 195374d89368 ("ipv4: fib: annotate races around nh->nh_saddr_genid
and nh->nh_saddr") added WRITE_ONCE() on the store side and READ_ONCE()
in fib_result_prefsrc() after syzbot reported

        BUG: KCSAN: data-race in fib_select_path / fib_select_path

but it only covered that reader. fib_select_multipath(), reached from
fib_select_path(), is a second lockless reader of nh->nh_saddr and was
left bare.

32607a332cfe added the reader after 195374d89368.

Indeed, please put in V2:

Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches
source address")

pw-bot: cr