Several strlcat() call sites being converted to seq_buf need behavior seq_buf doesn't currently provide. The return from seq_buf_used() is not the length of the string in a seq_buf. Once the buffer is full or has overflowed it returns the buffer size, which counts the byte that seq_buf_str() replaces with the NUL, so a caller that needs the string and its length has to call seq_buf_str() and then walk the string with strlen().
Move the termination out of seq_buf_str() into a helper that returns where it put the NUL, and add seq_buf_strlen(), which terminates the buffer in the same way and returns that offset. Add tests comparing seq_buf_strlen() against strlen() of seq_buf_str() for empty, appended, truncated, exactly full, and overflowed buffers, and checking that seq_buf_strlen() alone terminates a full buffer. 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 Reviewed-by: Andy Shevchenko <[email protected]> Signed-off-by: Kees Cook <[email protected]> --- Cc: "Matthew Wilcox (Oracle)" <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Andy Shevchenko <[email protected]> Cc: David Gow <[email protected]> Cc: Petr Mladek <[email protected]> Cc: Shuvam Pandey <[email protected]> Cc: Steven Rostedt <[email protected]> --- include/linux/seq_buf.h | 57 +++++++++++++++++-- lib/tests/seq_buf_kunit.c | 114 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 166 insertions(+), 5 deletions(-) diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h index 0c0a0db04b09..7f025c7a68be 100644 --- a/include/linux/seq_buf.h +++ b/include/linux/seq_buf.h @@ -89,6 +89,27 @@ static inline unsigned int seq_buf_used(struct seq_buf *s) return min(s->len, s->size); } +/* + * NUL-terminate the buffer in @s: directly after the data when there is + * room for it, otherwise in the last byte of the buffer. @s->size must not + * be zero. + * + * Returns: the offset of the NUL. + */ +static inline size_t __seq_buf_terminate(struct seq_buf *s) +{ + size_t end; + + if (seq_buf_buffer_left(s)) + end = s->len; + else + end = s->size - 1; + + s->buffer[end] = 0; + + return end; +} + /** * seq_buf_str - get NUL-terminated C string from seq_buf * @s: the seq_buf handle @@ -98,7 +119,9 @@ static inline unsigned int seq_buf_used(struct seq_buf *s) * * Note, if this is called when the buffer has overflowed, then * the last byte of the buffer is zeroed, and the len will still - * point passed it. + * point passed it. The same happens when the buffer is exactly + * full: the NUL takes the place of the last byte written, which is + * lost, though seq_buf_used() still counts it. * * After this function is called, s->buffer is safe to use * in string operations. @@ -110,14 +133,38 @@ static inline const char *seq_buf_str(struct seq_buf *s) if (WARN_ON(s->size == 0)) return ""; - if (seq_buf_buffer_left(s)) - s->buffer[s->len] = 0; - else - s->buffer[s->size - 1] = 0; + __seq_buf_terminate(s); return s->buffer; } +/** + * seq_buf_strlen - get the length of the NUL-terminated C string in seq_buf + * @s: the seq_buf handle + * + * This makes sure that the buffer in @s is NUL-terminated, exactly as + * seq_buf_str() does, and returns the length of the resulting string + * without walking it. Unlike seq_buf_used(), this does not count the byte + * given up to the NUL when the buffer is full or has overflowed. When the + * buffer is exactly full, that byte is the last one written, and calling + * either function loses it. + * + * After this function is called, s->buffer is safe to use + * in string operations. + * + * Returns: the offset of the NUL that terminates @s->buffer. That is the + * length of the string unless an earlier NUL is in the way, either one the + * data written to @s carried itself, or one seq_buf_set_overflow() left + * behind when it cleared what no writer had claimed. + */ +static inline size_t seq_buf_strlen(struct seq_buf *s) +{ + if (WARN_ON(s->size == 0)) + return 0; + + return __seq_buf_terminate(s); +} + /** * seq_buf_get_buf - get buffer to write arbitrary data to * @s: the seq_buf handle diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c index 852eb645e253..0259c8506b89 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -26,8 +26,10 @@ static void seq_buf_init_test(struct kunit *test) KUNIT_EXPECT_EQ(test, seq_buf_buffer_left(&s), 32); KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 0); KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0); } + static void seq_buf_declare_test(struct kunit *test) { DECLARE_SEQ_BUF(s, 24); @@ -510,6 +512,113 @@ static void seq_buf_path_overflow_test(struct kunit *test) KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), expected); } +static void seq_buf_strlen_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 16); + + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 0); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); + + seq_buf_puts(&s, "hello"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); + + seq_buf_printf(&s, " %s", "world"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 11); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); +} + +static void seq_buf_strlen_printf_overflow_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 16); + DECLARE_SEQ_BUF(t, 8); + + seq_buf_printf(&s, "%s", "1234567890abcdefghij"); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 16); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 15); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "1234567890abcde"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); + + /* Output one byte too long for the NUL. */ + seq_buf_printf(&t, "%s", "12345678"); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&t), 8); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 7); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "1234567"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), strlen(seq_buf_str(&t))); +} + +static void seq_buf_strlen_full_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 4); + DECLARE_SEQ_BUF(t, 8); + char *buf; + size_t len; + + /* Filled exactly, with no room left for a NUL, but not overflowed. */ + seq_buf_putc(&s, 'a'); + seq_buf_putc(&s, 'b'); + seq_buf_putc(&s, 'c'); + seq_buf_putc(&s, 'd'); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 3); + /* seq_buf_strlen() terminates the buffer by itself. */ + KUNIT_EXPECT_EQ(test, s.buffer[3], '\0'); + KUNIT_EXPECT_EQ(test, strnlen(s.buffer, s.size), 3); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); + + /* A printf into a full buffer writes nothing. */ + KUNIT_EXPECT_EQ(test, seq_buf_printf(&s, "%s", "x"), -1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 3); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); + + len = seq_buf_get_buf(&t, &buf); + KUNIT_ASSERT_EQ(test, len, 8); + memset(buf, 'z', len); + seq_buf_commit(&t, len); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&t)); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 7); + KUNIT_EXPECT_EQ(test, t.buffer[7], '\0'); + KUNIT_EXPECT_EQ(test, strnlen(t.buffer, t.size), 7); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "zzzzzzz"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), strlen(seq_buf_str(&t))); +} + +static void seq_buf_strlen_puts_overflow_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 16); + + /* A puts that does not fit copies as much as fits. */ + seq_buf_puts(&s, "hello"); + KUNIT_EXPECT_EQ(test, seq_buf_puts(&s, " this does not fit"), -1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 15); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello this does"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), strlen(seq_buf_str(&s))); +} + + +static void seq_buf_strlen_embedded_nul_test(struct kunit *test) +{ + static const char data[] = "ab\0cd"; + DECLARE_SEQ_BUF(s, 16); + + /* + * seq_buf_strlen() reports where it put the terminator, not where + * the first NUL is, so data carrying a NUL of its own makes the two + * disagree. That is expected, and is what the documented caveat is + * about. + */ + seq_buf_putmem(&s, data, sizeof(data) - 1); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 5); + KUNIT_EXPECT_EQ(test, strlen(seq_buf_str(&s)), 2); +} + static struct kunit_case seq_buf_test_cases[] = { KUNIT_CASE(seq_buf_init_test), KUNIT_CASE(seq_buf_declare_test), @@ -527,6 +636,11 @@ static struct kunit_case seq_buf_test_cases[] = { KUNIT_CASE(seq_buf_putmem_partial_overflow_test), KUNIT_CASE(seq_buf_putmem_hex_partial_overflow_test), KUNIT_CASE(seq_buf_path_overflow_test), + KUNIT_CASE(seq_buf_strlen_test), + KUNIT_CASE(seq_buf_strlen_printf_overflow_test), + KUNIT_CASE(seq_buf_strlen_full_test), + KUNIT_CASE(seq_buf_strlen_puts_overflow_test), + KUNIT_CASE(seq_buf_strlen_embedded_nul_test), KUNIT_CASE(seq_buf_do_printk_test), {} }; -- 2.34.1
