linux-kernel - Re: [PATCH] locking/lockdep: fix format-truncation compiler-warning

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Date:   Thu, 12 Oct 2023 00:11:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Lucy Mielke <lucymielke@...oud.com>
Cc:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/lockdep: fix format-truncation compiler-warning


* Lucy Mielke <lucymielke@...oud.com> wrote:

> Compiler: gcc x86_64 v13.2.1
> Config: allyesconfig, "treat warnings as errors" unset
> 
> This fixes a warning emitted by gcc, stating the output may be
> truncated. The fix included increasing the buffer size to the one
> denoted by gcc.

Mind including the output by GCC in the changelog?

>  static void seq_time(struct seq_file *m, s64 time)
>  {
> -	char num[15];
> +	char num[22];
>  
>  	snprint_time(num, sizeof(num), time);
>  	seq_printf(m, " %14s", num);

Fun.

So this:

static void snprint_time(char *buf, size_t bufsiz, s64 nr)
{
        s64 div;
        s32 rem;

        nr += 5; /* for display rounding */
        div = div_s64_rem(nr, 1000, &rem);
        snprintf(buf, bufsiz, "%lld.%02d", (long long)div, (int)rem/10);
}

... actually needs 21+1 bytes?

Which I suppose is true - longest s64 is "-9223372036854775808"-ish, which 
converted to the fixed-point float format above is "-9223372036854775.80": 
21 bytes, plus termination is another byte: 22.

Maybe put this into the changelog too, instead of relying on magic GCC 
calculations only. :-)

Thanks,

	Ingo