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 4A2004C33D3; Wed, 30 Sep 2026 23:52:53 +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=1790812375; cv=none; b=bDbJi8z8f+da709+9YDZphSsnJTpW/cv5l6T8lTZmS7hwORqS7Y1s7eUZa60/yYnuQv4VPKvlI6Wkx5KVQ3miqpkC+GhHFTJsdQEUU2CebEijCJwKp03uXMIgoDHfehzFzoOKTz24ljYI7lIlCpHLGPZ3sml5FPWyxun0eNdsPY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790812375; c=relaxed/simple; bh=2pWbibimYJjEXAHdaatZyQiBGge4J8y7TGlnBYdtUpk=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=AQs24sLcp6etvdsr8uHwjc2iZt3zXt7Vwz31vJdyDRQv/PWseyc/a7UWASNWe9xaNiB2OX4qSk8/poL7UPq7hUKH92muiVprremwqz+nyl1ogNQBzsDVVXkHNk7W9ZMSx2AcN0rS8QU2mmB6u0gGU8cCAS+uBZzBHv1Tc6aB9vw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fTqH4x8u; 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="fTqH4x8u" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 15CD71F0089D; Wed, 30 Sep 2026 23:52:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790812371; bh=0rGd0l6phK5RjyIfaqlGeNty+f5gO7jzOqYdPkB7HTY=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=fTqH4x8uFZnteW656odC+z6VqmAhSEJnkaKwZ7XfXo/zIuNLr/pYWyNOz2UbATeK6 e2HUvDwyphEucY0fsAGcTzqmZ3prg3i8/r92B0TpZK8O8gj79LBiDrO3iLW+NablZN I3gU7hIBSqilk5HJyWd548AK5e6BUyM5fqEX/up4TC3Z51HZU+fF0je5V9YydCVpLF Ci9HLLDFfZSPORayHWSajwFmv35DdTyqP0oHp5voPCz9Gx5Clqvqt5EHuH3Un/vZzd uQPQQyPpi8XsrhHuGYTB2C3ToFN7K/ag0g3dS+6h2w6bICSv6QuMLoGEQh0w8aXC7J W+910TYT3RyZw== From: Kees Cook To: Bill Wendling Cc: Kees Cook , "Matthew Wilcox (Oracle)" , Andrew Morton , Andy Shevchenko , David Gow , Petr Mladek , Shuvam Pandey , Steven Rostedt , nikitash.mariiaw@gmail.com, Greg KH , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH v3 08/11] seq_buf: Add seq_buf_init_append() Date: Wed, 30 Sep 2026 16:52:36 -0700 Message-Id: <20260930235247.157921-8-kees@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260930235231.out.387-kees@kernel.org> References: <20260930235231.out.387-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=4828; i=kees@kernel.org; h=from:subject; bh=BwcCXVyHW3lUCTkHo6Pgo9IWy8KBkwjAuRDkuIELs1I=; b=owGbwMvMwCVmps19z/KJym7G02pJDFl7FxxbJ9bQb2f/75B2rFc2x9Y1jn9UFt2/1RD8OHraT I7NtW31HaUsDGJcDLJiiixBdu5xLh5v28Pd5yrCzGFlAhnCwMUpABOZL8PI8ExX7vMMsxdx659O 7mqre+lldEh02y/7OYfTF/M6VBwUO8bwz672/u2NSW35VjOd/vpt+lXE3LJdfXGY6ZYFSV4r26r ecQIA 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 sets the new buffer size 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. 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 16.1.0. Assisted-by: LLM Signed-off-by: Bill Wendling Co-developed-by: Kees Cook Signed-off-by: Kees Cook --- include/linux/seq_buf.h | 20 +++++++++++++++ lib/tests/seq_buf_kunit.c | 53 +++++++++++++++++++++++++++++++++++++++ 2 files changed, 73 insertions(+) diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h index 195e612a212a..4a2415fcb185 100644 --- a/include/linux/seq_buf.h +++ b/include/linux/seq_buf.h @@ -46,6 +46,26 @@ seq_buf_init(struct seq_buf *s, char *buf, unsigned int size) seq_buf_clear(s); } +/** + * 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. + */ +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); +} + /* * seq_buf have a buffer that might overflow. When this happens * len is set to be greater than size. diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c index 8e879a4082ad..84caa552870b 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -29,6 +29,58 @@ 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); + + /* Check for required truncation when full. */ + seq_buf_init_append(&s, buf, 11); + KUNIT_EXPECT_EQ(test, s.size, 11); + KUNIT_EXPECT_EQ(test, s.len, 11); + KUNIT_EXPECT_FALSE(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_STREQ(test, seq_buf_str(&s), "hello worl"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 10); + + /* + * The size bounds the scan: the string reaches past it, so an + * unbounded strlen() would report 10 here and leave @s overflowed. + */ + seq_buf_init_append(&s, buf, 5); + KUNIT_EXPECT_EQ(test, s.size, 5); + 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), 0); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell"); +} static void seq_buf_declare_test(struct kunit *test) { @@ -664,6 +716,7 @@ 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), -- 2.34.1