Re: [PATCH net] tcp: avoid bogus gcc-7 array-bounds warning

[David Miller]

From: Arnd Bergmann 
Date: Fri, 28 Jul 2017 16:41:37 +0200

> When using CONFIG_UBSAN_SANITIZE_ALL, the TCP code produces a
> false-positive warning:
> 
> net/ipv4/tcp_output.c: In function 'tcp_connect':
> net/ipv4/tcp_output.c:2207:40: error: array subscript is below array bounds 
> [-Werror=array-bounds]
>tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
> ^~
> net/ipv4/tcp_output.c:2207:40: error: array subscript is below array bounds 
> [-Werror=array-bounds]
>tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
>~^
> 
> I have opened a gcc bug for this, but distros have already shipped
> compilers with this problem, and it's not clear yet whether there is
> a way for gcc to avoid the warning. As the problem is related to the
> bitfield access, this introduces a temporary variable to store the old
> enum value.
> 
> I did not notice this warning earlier, since UBSAN is disabled when
> building with COMPILE_TEST, and that was always turned on in both
> allmodconfig and randconfig tests.
> 
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81601
> Signed-off-by: Arnd Bergmann 

Applied, thanks Arnd.

RE: [PATCH net] tcp: avoid bogus gcc-7 array-bounds warning

[David Laight]

From: Arnd Bergmann
> Sent: 28 July 2017 15:42
...
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2202,9 +2202,10 @@ static bool tcp_small_queue_check(struct sock *sk, 
> const struct sk_buff *skb,
>  static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
>  {
>   const u32 now = tcp_jiffies32;
> + enum tcp_chrono old = tp->chrono_type;
> 
> - if (tp->chrono_type > TCP_CHRONO_UNSPEC)
> - tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
> + if (old > TCP_CHRONO_UNSPEC)
> + tp->chrono_stat[old - 1] += now - tp->chrono_start;
>   tp->chrono_start = now;
>   tp->chrono_type = new;

What a horrid combination of enum and integers.
Also have u32 chrono_stat[3]; - should probably be [__TCP_CHRONO_MAX - 1]
(or - CHRONO_FIRST which is defined to be 1).

Checking if (old != 0) would make the code more readable.

David