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

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Message-Id: <20251104204540.13931-2-W_Armin@gmx.de>
Date: Tue,  4 Nov 2025 21:45:37 +0100
From: Armin Wolf <W_Armin@....de>
To: viro@...iv.linux.org.uk,
	brauner@...nel.org,
	hansg@...nel.org,
	ilpo.jarvinen@...ux.intel.com
Cc: jack@...e.cz,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	platform-driver-x86@...r.kernel.org
Subject: [PATCH 1/4] fs/nls: Fix utf16 to utf8 conversion

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.

Fixes: 74675a58507e ("NLS: update handling of Unicode")
Signed-off-by: Armin Wolf <W_Armin@....de>
---
 fs/nls/nls_base.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/nls/nls_base.c b/fs/nls/nls_base.c
index 18d597e49a19..d434c4463a8f 100644
--- a/fs/nls/nls_base.c
+++ b/fs/nls/nls_base.c
@@ -94,7 +94,7 @@ int utf32_to_utf8(unicode_t u, u8 *s, int maxout)
 
 	l = u;
 	if (l > UNICODE_MAX || (l & SURROGATE_MASK) == SURROGATE_PAIR)
-		return -1;
+		return -EILSEQ;
 
 	nc = 0;
 	for (t = utf8_table; t->cmask && maxout; t++, maxout--) {
@@ -110,7 +110,7 @@ int utf32_to_utf8(unicode_t u, u8 *s, int maxout)
 			return nc;
 		}
 	}
-	return -1;
+	return -EOVERFLOW;
 }
 EXPORT_SYMBOL(utf32_to_utf8);
 
@@ -217,8 +217,16 @@ int utf16s_to_utf8s(const wchar_t *pwcs, int inlen, enum utf16_endian endian,
 				inlen--;
 			}
 			size = utf32_to_utf8(u, op, maxout);
-			if (size == -1) {
-				/* Ignore character and move on */
+			if (size < 0) {
+				if (size == -EILSEQ) {
+					/* Ignore character and move on */
+					continue;
+				}
+				/*
+				 * Stop filling the buffer with data once a character
+				 * does not fit anymore.
+				 */
+				break;
 			} else {
 				op += size;
 				maxout -= size;
-- 
2.39.5