Re: [PATCH v1 bpf-next] selftest: bpf: Correct mssind comparison in test_tcp_custom_syncookie.c.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 8/19/24 12:42 PM, Kuniyuki Iwashima wrote:
> Smatch reported a possible off-by-one in tcp_validate_cookie().
> 
> However, it's false positive because the possible range of mssind is
> limited from 0 to 3 by the preceding calculation.
> 
>    mssind = (cookie & (3 << 6)) >> 6;
> 
> There's no real issue, but let's make Smatch happy to suppress the same
> reports.
> 
> Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
> Closes: https://lore.kernel.org/bpf/6ae12487-d3f1-488b-9514-af0dac96608f@stanley.mountain/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> ---
>   tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> index 44ee0d037f95..36b842133033 100644
> --- a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> +++ b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> @@ -487,12 +487,12 @@ static int tcp_validate_cookie(struct tcp_syncookie *ctx)
>   
>   	mssind = (cookie & (3 << 6)) >> 6;

This should have bound the mssind.

>   	if (ctx->ipv4) {
> -		if (mssind > ARRAY_SIZE(msstab4))
> +		if (mssind >= ARRAY_SIZE(msstab4))

Does the verifier complain without this if check?

>   			goto err;
>   
>   		ctx->attrs.mss = msstab4[mssind];
>   	} else {
> -		if (mssind > ARRAY_SIZE(msstab6))
> +		if (mssind >= ARRAY_SIZE(msstab6))
>   			goto err;
>   
>   		ctx->attrs.mss = msstab6[mssind];