Re: [PATCH v1 net-next] tcp: Initialise ehash secrets during connect() and listen().

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Eric Dumazet <edumazet@google.com>
Date: Tue, 3 Mar 2026 04:42:24 +0100
> On Mon, Mar 2, 2026 at 9:51 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
> >
> > inet_ehashfn() and inet6_ehashfn() initialise random secrets
> > on the first call by net_get_random_once().
> >
> > While the init part is patched out using static keys, with
> > CONFIG_STACKPROTECTOR_STRONG=y, this causes a compiler to
> > generate a stack canary due to an automatic variable,
> > unsigned long ___flags, in the DO_ONCE() macro being passed
> > to __do_once_start().
> >
> > With FDO, this is visible in __inet_lookup_established() and
> > __inet6_lookup_established() too.
> >
> > Let's move net_get_random_once() to the slow paths: inet_hash()
> > for listen(), and inet_hash_connect() and inet6_hash_connect()
> > for connect().
> >
> > Note that IPv6 listener will initialise both IPv4 & IPv6 secrets
> > in inet_hash() for IPv4-mapped IPv6 address.
> >
> > With the patch, the stack size is reduced by 16 bytes (___flags
> >  + a stack canary) and NOPs for the static key go away.
> >
> > Before: __inet6_lookup_established()
> >
> >
> >
> > Suggested-by: Eric Dumazet <edumazet@google.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> > ---
> >  include/net/inet6_hashtables.h |  2 ++
> >  net/ipv4/inet_hashtables.c     | 16 ++++++++++++++--
> >  net/ipv6/inet6_hashtables.c    | 11 ++++++++---
> >  3 files changed, 24 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
> > index 282e29237d93..c17e0d874808 100644
> > --- a/include/net/inet6_hashtables.h
> > +++ b/include/net/inet6_hashtables.h
> > @@ -24,6 +24,8 @@
> >
> >  struct inet_hashinfo;
> >
> > +void inet6_init_ehash_secret(void);
> > +
> >  static inline unsigned int __inet6_ehashfn(const u32 lhash,
> >                                     const u16 lport,
> >                                     const u32 fhash,
> > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > index fca980772c81..2c2cac5b0f1a 100644
> > --- a/net/ipv4/inet_hashtables.c
> > +++ b/net/ipv4/inet_hashtables.c
> > @@ -30,12 +30,15 @@
> >  #include <net/sock_reuseport.h>
> >  #include <net/tcp.h>
> >
> > +static void inet_init_ehash_secret(void)
> > +{
> > +       net_get_random_once(&inet_ehash_secret, sizeof(inet_ehash_secret));
> > +}
> 
> Could this use get_random_sleepable_once()?
> We are running in process context here.
> Same remark for  inet6_init_ehash_secret()

Ah that's better !

I'll squash this to v2, thanks !

---8<---
diff --git a/include/linux/net.h b/include/linux/net.h
index f58b38ab37f8..a8e818de95b3 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -304,6 +304,8 @@ do {									\
 
 #define net_get_random_once(buf, nbytes)			\
 	get_random_once((buf), (nbytes))
+#define net_get_random_sleepable_once(buf, nbytes)		\
+	get_random_sleepable_once((buf), (nbytes))
 
 /*
  * E.g. XFS meta- & log-data is in slab pages, or bcache meta
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 2c2cac5b0f1a..e4c9a1590a74 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -32,7 +32,8 @@
 
 static void inet_init_ehash_secret(void)
 {
-	net_get_random_once(&inet_ehash_secret, sizeof(inet_ehash_secret));
+	net_get_random_sleepable_once(&inet_ehash_secret,
+				      sizeof(inet_ehash_secret));
 }
 
 u32 inet_ehashfn(const struct net *net, const __be32 laddr,
diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
index 054f75d32381..a0e5a8812614 100644
--- a/net/ipv6/inet6_hashtables.c
+++ b/net/ipv6/inet6_hashtables.c
@@ -25,8 +25,10 @@
 
 void inet6_init_ehash_secret(void)
 {
-	net_get_random_once(&inet6_ehash_secret, sizeof(inet6_ehash_secret));
-	net_get_random_once(&tcp_ipv6_hash_secret, sizeof(tcp_ipv6_hash_secret));
+	net_get_random_sleepable_once(&inet6_ehash_secret,
+				      sizeof(inet6_ehash_secret));
+	net_get_random_sleepable_once(&tcp_ipv6_hash_secret,
+				      sizeof(tcp_ipv6_hash_secret));
 }
 
 u32 inet6_ehashfn(const struct net *net,
---8<---