Files
linux/include/net
Kuniyuki Iwashima d4d8c6e6fd tcp: Initialise ehash secrets during connect() and listen().
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 initialise the secrets by get_random_sleepable_once()
in 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()

       ...
       push   %rbx
       sub    $0x38,%rsp                # stack is 56 bytes
       mov    %edx,%ebx                 # sport
       mov    %gs:0x299419f(%rip),%rax  # load stack canary
       mov    %rax,0x30(%rsp)              and store it onto stack
       mov    0x440(%rdi),%r15          # net->ipv4.tcp_death_row.hashinfo
       nop
 32:   mov    %r8d,%ebp                 # hnum
       shl    $0x10,%ebp                # hnum << 16
       nop
 3d:   mov    0x70(%rsp),%r14d          # sdif
       or     %ebx,%ebp                 # INET_COMBINED_PORTS(sport, hnum)
       mov    0x11a8382(%rip),%eax      # inet6_ehashfn() ...

After: __inet6_lookup_established()

       ...
       push   %rbx
       sub    $0x28,%rsp                # stack is 40 bytes
       mov    0x60(%rsp),%ebp           # sdif
       mov    %r8d,%r14d                # hnum
       shl    $0x10,%r14d               # hnum << 16
       or     %edx,%r14d                # INET_COMBINED_PORTS(sport, hnum)
       mov    0x440(%rdi),%rax          # net->ipv4.tcp_death_row.hashinfo
       mov    0x1194f09(%rip),%r10d     # inet6_ehashfn() ...

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260303235424.3877267-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2026-03-05 18:50:05 -08:00
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