seq_buf_putc() and strings

[Alejandro Colomar <alx+linux-hardening@kernel.org>]

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Hi Steven,


I've been looking at rewriting seq_buf with pointers instead of sizes.
While doing that, I've found one place which writes to seq_buf without
forming a proper string (that is, a null-terminated string).

	$ git show v7.3:lib/seq_buf.c | grepc seq_buf_putc
	(standard input):int seq_buf_putc(struct seq_buf *s, unsigned char c)
	{
		WARN_ON(s->size == 0);

		if (seq_buf_can_fit(s, 1)) {
			s->buffer[s->len++] = c;
			return 0;
		}
		seq_buf_set_overflow(s);
		return -1;
	}

I think a safe API for handling strings would have to make sure that it
*always* holds a string.  I think this function should write a NUL byte
after the byte written, and so, I suggest doing this:

	 		s->buffer[s->len++] = c;
	+		s->buffer[s->len] = '\0';

Are there any reasons to not write that NUL byte?


Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es>

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