linux-kernel - Re: [PATCH 29/32] lib/string_helpers: string_get

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Message-ID: <202307121248.36919B223@keescook>
Date:   Wed, 12 Jul 2023 12:58:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 29/32] lib/string_helpers: string_get_size() now returns
 characters wrote

On Tue, May 09, 2023 at 12:56:54PM -0400, Kent Overstreet wrote:
> From: Kent Overstreet <kent.overstreet@...il.com>
> 
> printbuf now needs to know the number of characters that would have been
> written if the buffer was too small, like snprintf(); this changes
> string_get_size() to return the the return value of snprintf().

Unfortunately, snprintf doesn't return characters written, it return
what it TRIED to write, and can cause a lot of problems[1]. This patch
would be fine with me if the snprintf was also replaced by scnprintf,
which will return the actual string length copied (or 0) *not* including
the trailing %NUL.

> [...]
> @@ -126,8 +126,8 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
>  	else
>  		unit = units_str[units][i];
>  
> -	snprintf(buf, len, "%u%s %s", (u32)size,
> -		 tmp, unit);
> +	return snprintf(buf, len, "%u%s %s", (u32)size,
> +			tmp, unit);

-Kees

[1] https://github.com/KSPP/linux/issues/105

-- 
Kees Cook