linux-hardening - Re: [PATCH v5] EDAC/mc: Prefer strscpy or scnprintf over strcpy

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Date:   Sun, 29 Aug 2021 09:38:37 -0700
From:   Joe Perches <joe@...ches.com>
To:     Len Baker <len.baker@....com>, Borislav Petkov <bp@...en8.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        James Morse <james.morse@....com>,
        Robert Richter <rric@...nel.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] EDAC/mc: Prefer strscpy or scnprintf over strcpy

On Sun, 2021-08-29 at 18:15 +0200, Len Baker wrote:
> strcpy() performs no bounds checking on the destination buffer. This
> could result in linear overflows beyond the end of the buffer, leading
> to all kinds of misbehaviors. The safe replacement is strscpy() [1].
> 
> However, to simplify and clarify the code, to concatenate labels use
> the scnprintf() function. This way it is not necessary to check the
> return value of strscpy (-E2BIG if the parameter count is 0 or the src
> was truncated) since the scnprintf returns always the number of chars
> written into the buffer. This function returns always a nul-terminated
> string even if it needs to be truncated.
[]
> diff --git a/drivers/edac/edac_mc.c b/drivers/edac/edac_mc.c
[]
> @@ -1113,12 +1116,11 @@ void edac_mc_handle_error(const enum hw_event_mc_err_type type,
>  			p = e->label;
>  			*p = '\0';
>  		} else {
> -			if (p != e->label) {
> -				strcpy(p, OTHER_LABEL);
> -				p += strlen(OTHER_LABEL);
> -			}
> -			strcpy(p, dimm->label);
> -			p += strlen(p);
> +			const char *or = (p != e->label) ? OTHER_LABEL : "";
> +
> +			n = scnprintf(p, len, "%s%s", or, dimm->label);
> +			len -= n;
> +			p += n;

A very common and intelligible mechanism for this is:

	const char *prefix = "";
	int n = 0;
	...
			n += scnprintf(e->label + n, sizeof(e->label) - n,
				       "%s%s", prefix, dimm->label);
			prefix = " or ";