Re: [PATCH 2/9] btrfs-progs: fix gcc8 default build warning caused by '-Wformat-truncation'

From: David Sterba <dsterba@suse.cz>
To: Qu Wenruo <wqu@suse.com>
Cc: linux-btrfs@vger.kernel.org, Su Yanjun <suyj.fnst@cn.fujitsu.com>
Subject: Re: [PATCH 2/9] btrfs-progs: fix gcc8 default build warning caused by '-Wformat-truncation'
Date: Tue, 4 Dec 2018 12:10:19 +0100	[thread overview]
Message-ID: <20181204111018.GI17773@twin.jikos.cz> (raw)
In-Reply-To: <20181116075426.4142-3-wqu@suse.com>

On Fri, Nov 16, 2018 at 03:54:19PM +0800, Qu Wenruo wrote:
> From: Su Yanjun <suyj.fnst@cn.fujitsu.com>
> 
> When using gcc8 compiles utils.c, it complains as below:
> 
> utils.c:852:45: warning: '%s' directive output may be truncated writing
> up to 4095 bytes into a region of size 4084 [-Wformat-truncation=]
>    snprintf(path, sizeof(path), "/dev/mapper/%s", name);
>                                              ^~   ~~~~
> In file included from /usr/include/stdio.h:873,
>                  from utils.c:20:
> /usr/include/bits/stdio2.h:67:10: note: '__builtin___snprintf_chk'
> output between 13 and 4108 bytes into a destination of size 4096
>    return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
>           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         __bos (__s), __fmt, __va_arg_pack ());
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This isn't a type of warning we care about, particularly when PATH_MAX
> is much less than either.
> 
> Using the GCC option -Wno-format-truncation to disable this for default
> build.
> 
> Signed-off-by: Su Yanjun <suyj.fnst@cn.fujitsu.com>
> [Use cc-disable-warning to fix the not working CFLAGS setting in configure.ac]
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  Makefile           | 1 +
>  Makefile.extrawarn | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index f4ab14ea74c8..252299f8869f 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -49,6 +49,7 @@ CSCOPE_CMD := cscope -u -b -c -q
>  include Makefile.extrawarn
>  
>  EXTRA_CFLAGS :=
> +EXTRA_CFLAGS += $(call cc-disable-warning, format-truncation)

Please don't touch EXTRA_CFLAGS, this is for users who want to override
any defaults that are set by build. This should go to CFLAGS.

>  EXTRA_LDFLAGS :=
>  
>  DEBUG_CFLAGS_DEFAULT = -O0 -U_FORTIFY_SOURCE -ggdb3
> diff --git a/Makefile.extrawarn b/Makefile.extrawarn
> index 5849036fd166..bbb2d5173846 100644
> --- a/Makefile.extrawarn
> +++ b/Makefile.extrawarn
> @@ -53,6 +53,7 @@ warning-1 += -Wold-style-definition
>  warning-1 += $(call cc-option, -Wmissing-include-dirs)
>  warning-1 += $(call cc-option, -Wunused-but-set-variable)
>  warning-1 += $(call cc-disable-warning, missing-field-initializers)
> +warning-1 += $(call cc-disable-warning, format-truncation)

It's ok to disable for W=1 but eg. the W=3 level could take all
previously disabled warnings.

>  
>  warning-2 := -Waggregate-return
>  warning-2 += -Wcast-align
> -- 
> 2.19.1