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 --- include/linux/seq_buf.h | 60 +++++++++++++++++++++++++++++++++++++++++ lib/seq_buf.c | 35 ++++++++++++++++++++++++ 2 files changed, 95 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..4d56ac71fafe 100644 --- a/lib/seq_buf.c +++ b/lib/seq_buf.c @@ -376,6 +376,41 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt) return cnt - ret; } +/** + * 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 + * + * seq_buf_puts() writes nothing at all if @str doesn't fully fit, + * unlike strlcat()/strscpy(), which copy as much of the source as + * there is room for. That all-or-nothing behavior is usually what's + * wanted for building diagnostic/trace text, but it's the wrong + * choice when converting code that relied on strlcat()'s always-copy- + * what-fits truncation to avoid losing content that was already + * appended. This copies the leading bytes of @str that fit, reserving + * room for the NUL terminator later added by seq_buf_str(). + * + * 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 left = seq_buf_buffer_left(s); + size_t len; + + if (left <= 1) + return 0; + + len = strnlen(str, left - 1); + seq_buf_putmem(s, str, len); + + return len; +} + /** * seq_buf_hex_dump - print formatted hex dump into the sequence buffer * @s: seq_buf descriptor -- 2.55.0.1082.g2b9226bbc0-goog
