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Subject[PATCH v4 07/34] lib/printbuf: Heap allocation
Date
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>
---
include/linux/printbuf.h | 120 +++++++++++++++++++++++++++++++++------
lib/Makefile | 2 +-
lib/printbuf.c | 71 +++++++++++++++++++++++
3 files changed, 175 insertions(+), 18 deletions(-)
create mode 100644 lib/printbuf.c

diff --git a/include/linux/printbuf.h b/include/linux/printbuf.h
index 8186c447ca..382863afa7 100644
--- a/include/linux/printbuf.h
+++ b/include/linux/printbuf.h
@@ -4,19 +4,69 @@
#ifndef _LINUX_PRINTBUF_H
#define _LINUX_PRINTBUF_H

-#include <linux/kernel.h>
-#include <linux/string.h>
-
/*
- * Printbufs: String buffer for outputting (printing) to, for vsnprintf
+ * Printbufs: Simple strings for printing to, with optional heap allocation
+ *
+ * This code has provisions for use in userspace, to aid in making other code
+ * portable between kernelspace and userspace.
+ *
+ * Basic example:
+ * struct printbuf buf = PRINTBUF;
+ *
+ * prt_printf(&buf, "foo=");
+ * foo_to_text(&buf, foo);
+ * printk("%s", buf.buf);
+ * printbuf_exit(&buf);
+ *
+ * Or
+ * struct printbuf buf = PRINTBUF_EXTERN(char_buf, char_buf_size)
+ *
+ * We can now write pretty printers instead of writing code that dumps
+ * everything to the kernel log buffer, and then those pretty-printers can be
+ * used by other code that outputs to kernel log, sysfs, debugfs, etc.
+ *
+ * Memory allocation: Outputing to a printbuf may allocate memory. This
+ * allocation is done with GFP_KERNEL, by default: use the newer
+ * memalloc_*_(save|restore) functions as needed.
+ *
+ * Since no equivalent yet exists for GFP_ATOMIC/GFP_NOWAIT, memory allocations
+ * will be done with GFP_NOWAIT if printbuf->atomic is nonzero.
+ *
+ * Memory allocation failures: We don't return errors directly, because on
+ * memory allocation failure we usually don't want to bail out and unwind - we
+ * want to print what we've got, on a best-effort basis. But code that does want
+ * to return -ENOMEM may check printbuf.allocation_failure.
*/

+#include <linux/kernel.h>
+#include <linux/string.h>
+
struct printbuf {
char *buf;
unsigned size;
unsigned pos;
+ /*
+ * If nonzero, allocations will be done with GFP_ATOMIC:
+ */
+ u8 atomic;
+ bool allocation_failure:1;
+ bool heap_allocated:1;
};

+int printbuf_make_room(struct printbuf *, unsigned);
+const char *printbuf_str(const struct printbuf *);
+void printbuf_exit(struct printbuf *);
+
+/* Initializer for a heap allocated printbuf: */
+#define PRINTBUF ((struct printbuf) { .heap_allocated = true })
+
+/* Initializer a printbuf that points to an external buffer: */
+#define PRINTBUF_EXTERN(_buf, _size) \
+((struct printbuf) { \
+ .buf = _buf, \
+ .size = _size, \
+})
+
/*
* Returns size remaining of output buffer:
*/
@@ -49,26 +99,36 @@ static inline bool printbuf_overflowed(struct printbuf *out)

static inline void printbuf_nul_terminate(struct printbuf *out)
{
+ printbuf_make_room(out, 1);
+
if (out->pos < out->size)
out->buf[out->pos] = 0;
else if (out->size)
out->buf[out->size - 1] = 0;
}

-static inline void __prt_char(struct printbuf *out, char c)
+/* Doesn't call printbuf_make_room(), doesn't nul terminate: */
+static inline void __prt_char_reserved(struct printbuf *out, char c)
{
if (printbuf_remaining(out))
out->buf[out->pos] = c;
out->pos++;
}

+/* Doesn't nul terminate: */
+static inline void __prt_char(struct printbuf *out, char c)
+{
+ printbuf_make_room(out, 1);
+ __prt_char_reserved(out, c);
+}
+
static inline void prt_char(struct printbuf *out, char c)
{
__prt_char(out, c);
printbuf_nul_terminate(out);
}

-static inline void __prt_chars(struct printbuf *out, char c, unsigned n)
+static inline void __prt_chars_reserved(struct printbuf *out, char c, unsigned n)
{
unsigned i, can_print = min(n, printbuf_remaining(out));

@@ -79,13 +139,18 @@ static inline void __prt_chars(struct printbuf *out, char c, unsigned n)

static inline void prt_chars(struct printbuf *out, char c, unsigned n)
{
- __prt_chars(out, c, n);
+ printbuf_make_room(out, n);
+ __prt_chars_reserved(out, c, n);
printbuf_nul_terminate(out);
}

static inline void prt_bytes(struct printbuf *out, const void *b, unsigned n)
{
- unsigned i, can_print = min(n, printbuf_remaining(out));
+ unsigned i, can_print;
+
+ printbuf_make_room(out, n);
+
+ can_print = min(n, printbuf_remaining(out));

for (i = 0; i < can_print; i++)
out->buf[out->pos++] = ((char *) b)[i];
@@ -101,22 +166,43 @@ static inline void prt_str(struct printbuf *out, const char *str)

static inline void prt_hex_byte(struct printbuf *out, u8 byte)
{
- __prt_char(out, hex_asc_hi(byte));
- __prt_char(out, hex_asc_lo(byte));
+ printbuf_make_room(out, 2);
+ __prt_char_reserved(out, hex_asc_hi(byte));
+ __prt_char_reserved(out, hex_asc_lo(byte));
printbuf_nul_terminate(out);
}

static inline void prt_hex_byte_upper(struct printbuf *out, u8 byte)
{
- __prt_char(out, hex_asc_upper_hi(byte));
- __prt_char(out, hex_asc_upper_lo(byte));
+ printbuf_make_room(out, 2);
+ __prt_char_reserved(out, hex_asc_upper_hi(byte));
+ __prt_char_reserved(out, hex_asc_upper_lo(byte));
printbuf_nul_terminate(out);
}

-#define PRINTBUF_EXTERN(_buf, _size) \
-((struct printbuf) { \
- .buf = _buf, \
- .size = _size, \
-})
+/**
+ * printbuf_reset - re-use a printbuf without freeing and re-initializing it:
+ */
+static inline void printbuf_reset(struct printbuf *buf)
+{
+ buf->pos = 0;
+ buf->allocation_failure = 0;
+}
+
+/**
+ * printbuf_atomic_inc - mark as entering an atomic section
+ */
+static inline void printbuf_atomic_inc(struct printbuf *buf)
+{
+ buf->atomic++;
+}
+
+/**
+ * printbuf_atomic_inc - mark as leaving an atomic section
+ */
+static inline void printbuf_atomic_dec(struct printbuf *buf)
+{
+ buf->atomic--;
+}

#endif /* _LINUX_PRINTBUF_H */
diff --git a/lib/Makefile b/lib/Makefile
index 6b9ffc1bd1..b4609a4258 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -34,7 +34,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
is_single_threaded.o plist.o decompress.o kobject_uevent.o \
earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
nmi_backtrace.o nodemask.o win_minmax.o memcat_p.o \
- buildid.o
+ buildid.o printbuf.o

lib-$(CONFIG_PRINTK) += dump_stack.o
lib-$(CONFIG_SMP) += cpumask.o
diff --git a/lib/printbuf.c b/lib/printbuf.c
new file mode 100644
index 0000000000..8c70128e31
--- /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);
--
2.36.1
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