lib/printbuf: Heap allocation

This makes printbufs optionally heap allocated: a printbuf initialized
with the PRINTBUF initializer will automatically heap allocate and
resize as needed.

Allocations are done with GFP_KERNEL: code should use e.g.
memalloc_nofs_save()/restore() as needed. Since we do not currently have
memalloc_nowait_save()/restore(), in contexts where it is not safe to
block we provide the helpers

  printbuf_atomic_inc()
  printbuf_atomic_dec()

When the atomic count is nonzero, memory allocations will be done with
GFP_NOWAIT.

On memory allocation failure, output will be truncated. Code that wishes
to check for memory allocation failure (in contexts where we should
return -ENOMEM) should check if printbuf->allocation_failure is set.
Since printbufs are expected to be typically used for log messages and
on a best effort basis, we don't return errors directly.

Other helpers provided by this patch:

 - printbuf_make_room(buf, extra)
   Reallocates if necessary to make room for @extra bytes (not including
   terminating null).

 - printbuf_str(buf)
   Returns a null terminated string equivalent to the contents of @buf.
   If @buf was never allocated (or allocation failed), returns a
   constant empty string.

 - printbuf_exit(buf)
   Releases memory allocated by a printbuf.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

1 files changed, 71 insertions, 0 deletions

diff --git a/lib/printbuf.c b/lib/printbuf.c
new file mode 100644
index 000000000000..8c70128e319c
--- /dev/null
+++ b/lib/printbuf.c
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: LGPL-2.1+
+/* Copyright (C) 2022 Kent Overstreet */
+
+#ifdef __KERNEL__
+#include <linux/export.h>
+#include <linux/kernel.h>
+#else
+#define EXPORT_SYMBOL(x)
+#endif
+
+#include <linux/err.h>
+#include <linux/slab.h>
+#include <linux/printbuf.h>
+
+int printbuf_make_room(struct printbuf *out, unsigned extra)
+{
+	unsigned new_size;
+	char *buf;
+
+	if (!out->heap_allocated)
+		return 0;
+
+	/* Reserved space for terminating nul: */
+	extra += 1;
+
+	if (out->pos + extra < out->size)
+		return 0;
+
+	new_size = roundup_pow_of_two(out->size + extra);
+	buf = krealloc(out->buf, new_size, !out->atomic ? GFP_KERNEL : GFP_NOWAIT);
+
+	if (!buf) {
+		out->allocation_failure = true;
+		return -ENOMEM;
+	}
+
+	out->buf	= buf;
+	out->size	= new_size;
+	return 0;
+}
+EXPORT_SYMBOL(printbuf_make_room);
+
+/**
+ * printbuf_str - returns printbuf's buf as a C string, guaranteed to be null
+ * terminated
+ */
+const char *printbuf_str(const struct printbuf *buf)
+{
+	/*
+	 * If we've written to a printbuf then it's guaranteed to be a null
+	 * terminated string - but if we haven't, then we might not have
+	 * allocated a buffer at all:
+	 */
+	return buf->pos
+		? buf->buf
+		: "";
+}
+EXPORT_SYMBOL(printbuf_str);
+
+/**
+ * printbuf_exit - exit a printbuf, freeing memory it owns and poisoning it
+ * against accidental use.
+ */
+void printbuf_exit(struct printbuf *buf)
+{
+	if (buf->heap_allocated) {
+		kfree(buf->buf);
+		buf->buf = ERR_PTR(-EINTR); /* poison value */
+	}
+}
+EXPORT_SYMBOL(printbuf_exit);