linux-kernel - Re: [PATCH 1/5] [v2] test_hexdump: avoid string truncation warning

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Message-ID: <fg6iv2dqwladc4d6tqzgg26mcyxz7c5bfn7gscic63c2qmhrz5@zvfm2btcf4fp>
Date: Wed, 10 Apr 2024 21:04:54 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-kbuild@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, 
	Richard Russon <ldm@...tcap.org>, Jens Axboe <axboe@...nel.dk>, 
	Robert Moore <robert.moore@...el.com>, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, 
	Len Brown <lenb@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
	Lin Ming <ming.m.lin@...el.com>, Alexey Starikovskiy <astarikovskiy@...e.de>, 
	linux-ntfs-dev@...ts.sourceforge.net, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-acpi@...r.kernel.org, acpica-devel@...ts.linux.dev, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] [v2] test_hexdump: avoid string truncation warning

On Tue, Apr 09, 2024 at 04:00:54PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> gcc can warn when a string is too long to fit into the strncpy()
> destination buffer, as it is here depending on the function
> arguments:
> 
>     inlined from 'test_hexdump_prepare_test.constprop' at /home/arnd/arm-soc/lib/test_hexdump.c:116:3:
> include/linux/fortify-string.h:108:33: error: '__builtin_strncpy' output truncated copying between 0 and 32 bytes from a string of length 32 [-Werror=stringop-truncation]
>   108 | #define __underlying_strncpy    __builtin_strncpy
>       |                                 ^
> include/linux/fortify-string.h:187:16: note: in expansion of macro '__underlying_strncpy'
>   187 |         return __underlying_strncpy(p, q, size);
>       |                ^~~~~~~~~~~~~~~~~~~~
> 
> The intention here is to copy exactly 'l' bytes without any padding or
> NUL-termination, so the most logical change is to use memcpy(), just as
> a previous change adapted the other output from strncpy() to memcpy().
> 
> Cc: Justin Stitt <justinstitt@...gle.com>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

This looks better than the previous strscpy() usage.

Acked-by: Justin Stitt <justinstitt@...gle.com>

> ---
> ---
>  lib/test_hexdump.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/test_hexdump.c b/lib/test_hexdump.c
> index b916801f23a8..fe2682bb21e6 100644
> --- a/lib/test_hexdump.c
> +++ b/lib/test_hexdump.c
> @@ -113,7 +113,7 @@ static void __init test_hexdump_prepare_test(size_t len, int rowsize,
>  			*p++ = ' ';
>  		} while (p < test + rs * 2 + rs / gs + 1);
>  
> -		strncpy(p, data_a, l);
> +		memcpy(p, data_a, l);
>  		p += l;
>  	}
>  
> -- 
> 2.39.2
>