linux-kernel - Re: [PATCH v2 1/4] fs/nls: Fix utf16 to utf8 conversion

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Message-ID: <aScaVgVAk_tH_v-w@black.igk.intel.com>
Date: Wed, 26 Nov 2025 16:18:46 +0100
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Armin Wolf <W_Armin@....de>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, hansg@...nel.org,
	ilpo.jarvinen@...ux.intel.com, superm1@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v2 1/4] fs/nls: Fix utf16 to utf8 conversion

On Tue, Nov 11, 2025 at 02:11:22PM +0100, Armin Wolf wrote:
> Currently the function responsible for converting between utf16 and
> utf8 strings will ignore any characters that cannot be converted. This
> however also includes multi-byte characters that do not fit into the
> provided string buffer.
> 
> This can cause problems if such a multi-byte character is followed by
> a single-byte character. In such a case the multi-byte character might
> be ignored when the provided string buffer is too small, but the
> single-byte character might fit and is thus still copied into the
> resulting string.
> 
> Fix this by stop filling the provided string buffer once a character
> does not fit. In order to be able to do this extend utf32_to_utf8()
> to return useful errno codes instead of -1.

Can you also update utf8_to_utf32() to return meaningful error codes?
Without that done we have inconsistent APIs.

-- 
With Best Regards,
Andy Shevchenko