seq_buf: Do not pop from an overflowed seq_buf

When a seq_buf has overflowed, its len is size + 1, so seq_buf_pop()
decrements len to size and reads buffer[size], one byte past the end of
the buffer. It also leaves len equal to size, which no longer counts as
overflowed, so a truncated seq_buf then looks like a complete, full one.

An overflowed seq_buf logically has no last character to pop: the
length of what was written has been lost, and the last byte of the
buffer may be the NUL written by vsnprintf() or bytes that were never
committed. Return -1 for an overflowed seq_buf, as for an empty one,
and leave it overflowed, as the rest of the seq_buf API does until
seq_buf_clear() or seq_buf_init().

The current callers do not reach this, e.g. trace_syscalls only calls
trace_seq_pop() when the trace_seq it pops from has not overflowed, and
kernel/bpf/diagnostics.c sets the length from strnlen() before popping.

Add tests for the pop corner cases.

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.

Fixes: 32e0f607ac ("tracing: Add trace_seq_pop() and seq_buf_pop()")
Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20261005155708.1471260-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

include/linux/seq_buf.h

@@ -155,11 +155,11 @@ static inline void seq_buf_commit(struct seq_buf *s, int num)
  *
  * Removes the last written character to the seq_buf @s.
  *
- * Returns the last character or -1 if it is empty.
+ * Returns the last character, or -1 if @s is empty or has overflowed.
  */
 static inline int seq_buf_pop(struct seq_buf *s)
 {
-	if (!s->len)
+	if (!s->len || seq_buf_has_overflowed(s))
 		return -1;
 
 	s->len--;

include/linux/trace_seq.h

@@ -86,7 +86,10 @@ static inline bool trace_seq_has_overflowed(struct trace_seq *s)
  *
  * Removes the last written character to the trace_seq @s.
  *
- * Returns the last character or -1 if it is empty.
+ * Returns the last character, or -1 if the underlying seq_buf is empty or
+ * has overflowed. Note that only that buffer is consulted: a @s marked
+ * full by a write that did not fit, which trace_seq_has_overflowed()
+ * reports as overflowed, still pops the last character written.
  */
 static inline int trace_seq_pop(struct trace_seq *s)
 {

lib/tests/seq_buf_kunit.c

@@ -115,6 +115,47 @@ static void seq_buf_putc_test(struct kunit *test)
 	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "");
 }
 
+static void seq_buf_pop_test(struct kunit *test)
+{
+	DECLARE_SEQ_BUF(s, 8);
+	struct seq_buf t;
+	char *buf;
+
+	/* Nothing to pop. */
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), -1);
+	KUNIT_EXPECT_EQ(test, s.len, 0);
+
+	seq_buf_puts(&s, "hello");
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'o');
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell");
+
+	/* A 0xff byte must not be mistaken for an empty buffer. */
+	seq_buf_putc(&s, 0xff);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 0xff);
+
+	/* A full buffer pops its last byte. */
+	seq_buf_puts(&s, "abc");
+	seq_buf_putc(&s, 'd');
+	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 8);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'd');
+	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 7);
+	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hellabc");
+
+	/*
+	 * An overflowed buffer has nothing to pop, and stays overflowed. Use
+	 * a buffer allocated at its exact size, so that KASAN reports any
+	 * read past its end.
+	 */
+	buf = kunit_kmalloc(test, 16, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, buf);
+	seq_buf_init(&t, buf, 16);
+	KUNIT_EXPECT_EQ(test, seq_buf_printf(&t, "%s", "longer than sixteen"), -1);
+	KUNIT_EXPECT_EQ(test, seq_buf_pop(&t), -1);
+	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
+}
+
 static void seq_buf_printf_test(struct kunit *test)
 {
 	DECLARE_SEQ_BUF(s, 32);
@@ -354,6 +395,7 @@ static struct kunit_case seq_buf_test_cases[] = {
 	KUNIT_CASE(seq_buf_puts_test),
 	KUNIT_CASE(seq_buf_puts_overflow_test),
 	KUNIT_CASE(seq_buf_putc_test),
+	KUNIT_CASE(seq_buf_pop_test),
 	KUNIT_CASE(seq_buf_printf_test),
 	KUNIT_CASE(seq_buf_printf_overflow_test),
 	KUNIT_CASE(seq_buf_get_buf_commit_test),