From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B624D4CE68A; Mon, 5 Oct 2026 15:57:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791215843; cv=none; b=MqIY7vYEmPX0yipZVcMR8vMhW0oE8i1ilEegbr/ZmO2hMeGVYGymCUYs36R1HGbyqIoDkNh7JZAQyBpaVfOEzUQkLJBpUiUw3SDC4pNakWnj26e2F2j7bgIlAnfPJBvSAx+pyF3iOYFz6E4hNVQmeBoSHOUBVkWN1GtWudCiFPo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791215843; c=relaxed/simple; bh=37rf/xNNVRb/1KiSTnxygbv5+NQ+4rfo7S3qzkrgWtI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jwLnfTZAIgh92UhY+NkDzLt4RdPJ64DOdIvWLrBoteuDGMBdrUhC/PzONwfZq3d3cua5wkNbJ8ILp8qN18KrQ6MbwzlWiiKWZo27JcCH+0pPMTkz5KtGF4M/daKzhKg2MeTkh8sEuT76R88mSGo17TT12OLADgckWujtkjccu8M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IdMOiM5n; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IdMOiM5n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E1FB21F0089F; Mon, 5 Oct 2026 15:57:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791215838; bh=7SuJY3k2kmNsMJSwSZXA04J0ONqXZrkivO9Klq0IODE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=IdMOiM5ngwnkH/2W6k6/OgamD2PV4FyT1eokpq5++vW3aVWfCsAVwxh/CdIp2513f LUIXb9pVrTKhz4HQSm7688LjfT/F1KDp/PWDoUfVksvsSylFa9cCphodMt/jd6jsOx 2WD/5IJpJoSoMsYxDEDuv0BjVi6jhOkBJyX3uJ+thaSB13vIFF22yxv8LjjTOnvhgw rNx8G0I49zxvdXH+mZUXwLCh59cqnpmPGnr1V9JdTKot+zU+VAb3tt1+eQhjv0m+Zt mzgfb9TDGsdruNVfogahxuiNsktQE2PrPSk9mDrHEV2WMCGHPgUjNyTThKclyYpxGZ 3Iyqi9GTQbbOg== From: Kees Cook To: Bill Wendling Cc: Kees Cook , "Matthew Wilcox (Oracle)" , Andrew Morton , Andy Shevchenko , David Gow , Petr Mladek , Shuvam Pandey , Steven Rostedt , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH v5 09/12] seq_buf: Add seq_buf_init_append() Date: Mon, 5 Oct 2026 08:56:59 -0700 Message-ID: <20261005155708.1471260-9-kees@kernel.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20261005155653.late.426-kees@kernel.org> References: <20261005155653.late.426-kees@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5786; i=kees@kernel.org; h=from:subject; bh=MsK9B4qxyOrArSkifRFg9tmcYn/2ynAQ2jzEol3ugWk=; b=owGbwMvMwCVmps19z/KJym7G02pJDFmHT5x98Nzm3ieBlheZvP5Hd+gu/1S31O0Ie21+UCfT0 fl3fYT/d5SyMIhxMciKKbIE2bnHuXi8bQ93n6sIM4eVCWQIAxenAEwk2pWR4f//SwenJrfd5OxL u3l1bsokmzuSAWuD9S2kDSfJHHzEtobhf8WpnyZWFe1uH5MWP3v536daeekdduU7AQIL1y7nuf5 jJRMA X-Developer-Key: i=kees@kernel.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit From: Bill Wendling Several strlcat() call sites being converted to seq_buf need behavior seq_buf doesn't currently provide. The normal seq_buf_init() always empties the buffer (sets its "len" to 0) via seq_buf_clear(). Code migrating from strlcat(buf, ...), which appends to whatever buf already contains, can't use seq_buf_init() without discarding that existing content. Add seq_buf_init_append(), which preserves the existing contents and positions the seq_buf to append after it. A buffer with no NUL within its size starts out overflowed, as strlcat() treats it as already truncated. Add KUnit tests for behavior coverage. Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y, and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0. Assisted-by: LLM Signed-off-by: Bill Wendling Reviewed-by: Andy Shevchenko Co-developed-by: Kees Cook Signed-off-by: Kees Cook --- include/linux/seq_buf.h | 25 ++++++++++++++ lib/tests/seq_buf_kunit.c | 69 +++++++++++++++++++++++++++++++++++++++ 2 files changed, 94 insertions(+) diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h index 9830f905f0ee..be979089228f 100644 --- a/include/linux/seq_buf.h +++ b/include/linux/seq_buf.h @@ -71,6 +71,31 @@ seq_buf_set_overflow(struct seq_buf *s) s->len = s->size + 1; } +/** + * seq_buf_init_append - initialize a seq_buf over a buffer that may + * already hold NUL-terminated content + * @s: the seq_buf handle + * @buf: pointer to the (possibly non-empty) buffer + * @size: total size of @buf + * + * Unlike seq_buf_init(), which always starts @buf at len=0, this + * preserves whatever NUL-terminated content @buf already holds and + * positions @s to append after it. Useful for converting code that used + * to append to an existing buffer with strlcat()/scnprintf() and friends. + * + * If @buf holds no NUL within @size, @s starts out overflowed, as + * strlcat() treats such a buffer as already truncated. + */ +static inline void +seq_buf_init_append(struct seq_buf *s, char *buf, unsigned int size) +{ + s->buffer = buf; + s->size = size; + s->len = strnlen(buf, size); + if (s->len == size) + seq_buf_set_overflow(s); +} + /* * How much buffer is left on the seq_buf? */ diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c index 33d351d0be64..fc53e9ef844c 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -28,6 +28,74 @@ static void seq_buf_init_test(struct kunit *test) KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0); } +static void seq_buf_init_append_test(struct kunit *test) +{ + char buf[32] = "hello"; + struct seq_buf s; + + /* Initial string contents match. */ + seq_buf_init_append(&s, buf, sizeof(buf)); + KUNIT_EXPECT_EQ(test, s.size, 32); + KUNIT_EXPECT_EQ(test, s.len, 5); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 32 - 5); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 5); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5); + + /* Appending with space works. */ + seq_buf_puts(&s, " world"); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11); + + /* No truncation when space for NUL is present. */ + seq_buf_init_append(&s, buf, 12); + KUNIT_EXPECT_EQ(test, s.size, 12); + KUNIT_EXPECT_EQ(test, s.len, 11); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11); + + /* + * No NUL within the size: the content is already truncated, as + * strlcat() would see it, so @s starts out overflowed. The content + * stays, and appending adds nothing. + */ + seq_buf_init_append(&s, buf, 11); + KUNIT_EXPECT_EQ(test, s.size, 11); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 0); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11); + KUNIT_EXPECT_MEMEQ(test, buf, "hello world", 11); + seq_buf_puts(&s, "!"); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_MEMEQ(test, buf, "hello world", 11); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello worl"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 10); + + /* + * The size bounds the scan: the string runs past it, so there is + * no NUL within the size either. + */ + seq_buf_init_append(&s, buf, 5); + KUNIT_EXPECT_EQ(test, s.size, 5); + /* Overflowed at the size; an unbounded scan would find more. */ + KUNIT_EXPECT_EQ(test, s.len, 5 + 1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 0); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell"); + + /* With no room even for a NUL, @s is overflowed and @buf untouched. */ + seq_buf_init_append(&s, buf, 0); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 0); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); + KUNIT_EXPECT_EQ(test, buf[0], 'h'); +} + static void seq_buf_declare_test(struct kunit *test) { DECLARE_SEQ_BUF(s, 24); @@ -669,6 +737,7 @@ static void seq_buf_terminate_test(struct kunit *test) static struct kunit_case seq_buf_test_cases[] = { KUNIT_CASE(seq_buf_init_test), + KUNIT_CASE(seq_buf_init_append_test), KUNIT_CASE(seq_buf_declare_test), KUNIT_CASE(seq_buf_clear_test), KUNIT_CASE(seq_buf_puts_test), -- 2.55.0