Converting some strlcat() call sites seq_buf need behavior seq_buf doesn't currently provide, either directly or without introducing subtle bugs:
- seq_buf_strlen(): seq_buf_used() reports the full buffer size when the buffer is completely filled, even though seq_buf_str() then overwrites the final byte with a NUL terminator, leaving only size - 1 bytes of actual content. Callers that need the true length of the NUL-terminated string have to fall back to strlen(seq_buf_str(s)). seq_buf_strlen() mirrors seq_buf_str()'s NUL-termination logic but returns the resulting string's length directly. - seq_buf_init_append(): seq_buf_init() always clears the buffer it's given via seq_buf_clear(). Code migrating from strlcat(buf, ...), which appends to whatever @buf already contains, can't use seq_buf_init() without silently discarding that existing content. seq_buf_init_append() preserves it and positions the seq_buf to append after it. - seq_buf_puts_trunc(): seq_buf_puts() (like seq_buf_printf() and friends) writes nothing at all if the string doesn't fully fit, whereas strlcat() always copies as much of the source as there is room for. Converting a strlcat() call site that relied on that partial-copy behavior to plain seq_buf_puts() can silently drop content that used to survive truncated. seq_buf_puts_trunc() keeps the leading bytes of the string that fit. Assisted-by: LLM Suggested-by: Kees Cook <[email protected]> Signed-off-by: Bill Wendling <[email protected]> --- v2: Reword the commit message to be clearer and not refer to a series of patches v3: Add EXPORT_SYMBOL_GPL, make sure to overflow when truncating, correct some logic discovered by the KUnit tests, and add KUnit tests. --- include/linux/seq_buf.h | 60 +++++++++++++++++++++++++++++++++++++++ lib/seq_buf.c | 33 +++++++++++++++++++++ lib/tests/seq_buf_kunit.c | 48 +++++++++++++++++++++++++++++++ 3 files changed, 141 insertions(+) diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h index 9f2839e73f8a..a552bcaab07f 100644 --- a/include/linux/seq_buf.h +++ b/include/linux/seq_buf.h @@ -30,6 +30,10 @@ struct seq_buf { .size = SIZE, \ } +/** + * seq_buf_clear - reset the seq_buf to be read / appended from the beginning + * @s: the seq_buf handle + */ static inline void seq_buf_clear(struct seq_buf *s) { s->len = 0; @@ -37,6 +41,14 @@ static inline void seq_buf_clear(struct seq_buf *s) s->buffer[0] = '\0'; } +/** + * seq_buf_init - initialize a seq_buf + * @s: the seq_buf handle + * @buf: pointer to the buffer + * @size: total size of @buf + * + * The contents of the buffer are ignored. + */ static inline void seq_buf_init(struct seq_buf *s, char *buf, unsigned int size) { @@ -45,6 +57,26 @@ 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 clears @buf, 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. @@ -108,6 +140,33 @@ static inline const char *seq_buf_str(struct seq_buf *s) return s->buffer; } +/** + * seq_buf_strlen - get the length of the NUL-terminated string in seq_buf + * @s: the seq_buf handle + * + * Like seq_buf_str(), this makes sure that the buffer in @s is + * NUL-terminated, and returns the length of the resulting string. + * Unlike seq_buf_used(), the returned length is always correct, even + * when the buffer is completely full: in that case seq_buf_used() + * reports @s->size, but the last byte was overwritten with the + * trailing NUL, so only @s->size - 1 bytes of content remain. + * + * Returns: the length of the NUL-terminated string in @s->buffer. + */ +static inline size_t seq_buf_strlen(struct seq_buf *s) +{ + if (WARN_ON(s->size == 0)) + return 0; + + if (seq_buf_buffer_left(s)) { + s->buffer[s->len] = 0; + return s->len; + } + + s->buffer[s->size - 1] = 0; + return s->size - 1; +} + /** * seq_buf_get_buf - get buffer to write arbitrary data to * @s: the seq_buf handle @@ -179,6 +238,7 @@ extern int seq_buf_putmem(struct seq_buf *s, const void *mem, unsigned int len); extern int seq_buf_putmem_hex(struct seq_buf *s, const void *mem, unsigned int len); extern int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc); +extern size_t seq_buf_puts_trunc(struct seq_buf *s, const char *str); extern int seq_buf_hex_dump(struct seq_buf *s, const char *prefix_str, int prefix_type, int rowsize, int groupsize, const void *buf, size_t len, bool ascii); diff --git a/lib/seq_buf.c b/lib/seq_buf.c index a92093f346da..831a1542fb78 100644 --- a/lib/seq_buf.c +++ b/lib/seq_buf.c @@ -199,6 +199,39 @@ int seq_buf_puts(struct seq_buf *s, const char *str) } EXPORT_SYMBOL_GPL(seq_buf_puts); +/** + * seq_buf_puts_trunc - append as much of a string as fits, keeping any of it + * @s: the seq_buf handle + * @str: the string to append + * + * This copies the leading bytes of @str that fit, reserving room for the NUL + * terminator later added by seq_buf_str(). This differs from seq_buf_puts(), + * which writes nothing at all if @str doesn't fully fit. + * + * Unlike seq_buf_puts(), this does NOT NUL-terminate @s->buffer as it + * goes (it copies raw bytes via seq_buf_putmem(), not @str's own + * terminator). Callers MUST call seq_buf_str() or seq_buf_strlen() + * before using @s->buffer as a C string. + * + * Returns: the number of bytes copied from @str. + */ +size_t seq_buf_puts_trunc(struct seq_buf *s, const char *str) +{ + size_t len = strlen(str); + + WARN_ON(s->size == 0); + + if (seq_buf_can_fit(s, len)) + return seq_buf_puts(s, str); + + /* Truncate the string to fit the buffer. */ + len = s->size - s->len; + memcpy(s->buffer + s->len, str, len); + seq_buf_set_overflow(s); + return len; +} +EXPORT_SYMBOL_GPL(seq_buf_puts_trunc); + /** * seq_buf_putc - sequence printing of simple character * @s: seq_buf descriptor diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c index eb466386bbef..a16b1365b503 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -23,6 +23,21 @@ static void seq_buf_init_test(struct kunit *test) KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); } +static void seq_buf_init_append_test(struct kunit *test) +{ + char buf[32] = "hello world"; + struct seq_buf s; + + seq_buf_init_append(&s, buf, sizeof(buf)); + + KUNIT_EXPECT_EQ(test, s.size, 32); + 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), 32 - 11); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 11); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world"); +} + static void seq_buf_declare_test(struct kunit *test) { DECLARE_SEQ_BUF(s, 24); @@ -51,6 +66,36 @@ static void seq_buf_clear_test(struct kunit *test) KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); } +static void seq_buf_strlen_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 16); + + seq_buf_puts(&s, "hello world!"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 12); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world!"); + + seq_buf_puts(&s, " It's a small world!"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), s.size - 1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello world!"); +} + +static void seq_buf_puts_trunc_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 10); + + seq_buf_puts(&s, "hello"); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 5); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello"); + + seq_buf_puts_trunc(&s, " world!"); + KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), s.size - 1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hello wor"); +} + static void seq_buf_puts_test(struct kunit *test) { DECLARE_SEQ_BUF(s, 16); @@ -218,8 +263,11 @@ static void seq_buf_putmem_hex_overflow_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_strlen_test), + KUNIT_CASE(seq_buf_puts_trunc_test), KUNIT_CASE(seq_buf_puts_test), KUNIT_CASE(seq_buf_puts_overflow_test), KUNIT_CASE(seq_buf_putc_test), -- 2.55.0.1082.g2b9226bbc0-goog
