[PATCH v2 1/8] lib/printbuf: New data structure for heap-allocated strings

Kent Overstreet
2022-04-21T23:48:58Z

This adds printbufs: simple heap-allocated strings meant for building up
structured messages, for logging/procfs/sysfs and elsewhere. They've
been heavily used in bcachefs for writing .to_text() functions/methods -
pretty printers, which has in turn greatly improved the overall quality
of error messages.

Basic usage is documented in include/linux/printbuf.h.

The next patches in the series are going to be using printbufs to
implement a .to_text() method for shrinkers, and improving OOM
reporting.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
---
include/linux/printbuf.h | 164 +++++++++++++++++++++++
lib/Makefile | 2 +-
lib/printbuf.c | 274 +++++++++++++++++++++++++++++++++++++++
3 files changed, 439 insertions(+), 1 deletion(-)
create mode 100644 include/linux/printbuf.h
create mode 100644 lib/printbuf.c

diff --git a/include/linux/printbuf.h b/include/linux/printbuf.h
new file mode 100644
index 0000000000..276cdecf08
--- /dev/null
+++ b/include/linux/printbuf.h
@@ -0,0 +1,164 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+/* Copyright (C) 2022 Kent Overstreet */
+
+#ifndef _LINUX_PRINTBUF_H
+#define _LINUX_PRINTBUF_H
+
+/*
+ * Printbufs: Simple heap allocated strings, with some features for structered
+ * formatting.
+ *
+ * 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;
+ *
+ * pr_buf(&buf, "foo=");
+ * foo_to_text(&buf, foo);
+ * printk("%s", buf.buf);
+ * printbuf_exit(&buf);
+ *
+ * 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_ATOMIC 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.
+ *
+ * Indenting, tabstops:
+ *
+ * To aid is writing multi-line pretty printers spread across multiple
+ * functions, printbufs track the current indent level.
+ *
+ * pr_indent_push() and pr_indent_pop() increase and decrease the current indent
+ * level, respectively.
+ *
+ * To use tabstops, set printbuf->tabstops[]; they are in units of spaces, from
+ * start of line. Once set, pr_tab() will output spaces up to the next tabstop.
+ * pr_tab_rjust() will also advance the current line of text up to the next
+ * tabstop, but it does so by shifting text since the previous tabstop up to the
+ * next tabstop - right justifying it.
+ *
+ * Make sure you use pr_newline() instead of \n in the format string for indent
+ * level and tabstops to work corretly.
+ *
+ * Output units: printbuf->units exists to tell pretty-printers how to output
+ * numbers: a raw value (e.g. directly from a superblock field), as bytes, or as
+ * human readable bytes. pr_units() and pr_sectors() obey it.
+ *
+ * Other helpful functions:
+ *
+ * pr_human_readable_u64, pr_human_readable_s64: Print an integer with human
+ * readable units.
+ *
+ * pr_time(): for printing a time_t with strftime in userspace, prints as an
+ * integer number of seconds in the kernel.
+ *
+ * pr_string_option: Given an enumerated value and a string array with names for
+ * each option, prints out the enum names with the selected one indicated with
+ * square brackets.
+ *
+ * pr_bitflags: Given a bitflag and a string array with names for each bit,
+ * prints out the names of the selected bits.
+ */
+
+#include <linux/slab.h>
+#include <linux/string_helpers.h>
+
+enum printbuf_units {
+ PRINTBUF_UNITS_RAW,
+ PRINTBUF_UNITS_BYTES,
+ PRINTBUF_UNITS_HUMAN_READABLE,
+};
+
+struct printbuf {
+ char *buf;
+ unsigned size;
+ unsigned pos;
+ unsigned last_newline;
+ unsigned last_field;
+ unsigned indent;
+ enum printbuf_units units:8;
+ /*
+ * If nonzero, allocations will be done with GFP_ATOMIC:
+ */
+ u8 atomic;
+ bool allocation_failure:1;
+ /* SI units (10^3), or 2^10: */
+ enum string_size_units human_readable_units:1;
+ u8 tabstop;
+ u8 tabstops[4];
+};
+
+#define PRINTBUF ((struct printbuf) { .human_readable_units = STRING_UNITS_2 })
+
+/**
+ * printbuf_exit - exit a printbuf, freeing memory it owns and poisoning it
+ * against accidental use.
+ */
+static inline void printbuf_exit(struct printbuf *buf)
+{
+ kfree(buf->buf);
+ buf->buf = ERR_PTR(-EINTR); /* poison value */
+}
+
+/**
+ * printbuf_reset - re-use a printbuf without freeing and re-initializing it:
+ */
+static inline void printbuf_reset(struct printbuf *buf)
+{
+ buf->pos = 0;
+ buf->last_newline = 0;
+ buf->last_field = 0;
+ buf->indent = 0;
+ buf->tabstop = 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--;
+}
+
+void pr_buf(struct printbuf *out, const char *fmt, ...)
+ __attribute__ ((format (printf, 2, 3)));
+
+void pr_char(struct printbuf *buf, char c);
+void pr_newline(struct printbuf *);
+void pr_indent_push(struct printbuf *, unsigned);
+void pr_indent_pop(struct printbuf *, unsigned);
+void pr_tab(struct printbuf *);
+void pr_tab_rjust(struct printbuf *);
+void pr_human_readable_u64(struct printbuf *, u64);
+void pr_human_readable_s64(struct printbuf *, s64);
+void pr_units(struct printbuf *, s64, s64);
+void pr_sectors(struct printbuf *, u64);
+void pr_time(struct printbuf *, u64);
+void pr_uuid(struct printbuf *, u8 *);
+void pr_string_option(struct printbuf *, const char * const list[], size_t);
+void pr_bitflags(struct printbuf *, const char * const list[], u64);
+const char *printbuf_str(const struct printbuf *);
+
+#endif /* _LINUX_PRINTBUF_H */
diff --git a/lib/Makefile b/lib/Makefile
index c588a126a3..31a3904eda 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..e0dfa82cda
--- /dev/null
+++ b/lib/printbuf.c
@@ -0,0 +1,274 @@
+// 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/log2.h>
+#include <linux/printbuf.h>
+#include <linux/string_helpers.h>
+
+static inline size_t printbuf_remaining(struct printbuf *buf)
+{
+ return buf->size - buf->pos;
+}
+
+static inline size_t printbuf_linelen(struct printbuf *buf)
+{
+ return buf->pos - buf->last_newline;
+}
+
+static int printbuf_realloc(struct printbuf *out, unsigned extra)
+{
+ unsigned new_size;
+ char *buf;
+
+ if (out->pos + extra + 1 < out->size)
+ return 0;
+
+ new_size = roundup_pow_of_two(out->size + extra);
+ buf = krealloc(out->buf, new_size, !out->atomic ? GFP_KERNEL : GFP_ATOMIC);
+
+ if (!buf) {
+ out->allocation_failure = true;
+ return -ENOMEM;
+ }
+
+ out->buf = buf;
+ out->size = new_size;
+ return 0;
+}
+
+void pr_buf(struct printbuf *out, const char *fmt, ...)
+{
+ va_list args;
+ int len;
+
+ do {
+ va_start(args, fmt);
+ len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args);
+ va_end(args);
+ } while (len + 1 >= printbuf_remaining(out) &&
+ !printbuf_realloc(out, len + 1));
+
+ len = min_t(size_t, len,
+ printbuf_remaining(out) ? printbuf_remaining(out) - 1 : 0);
+ out->pos += len;
+}
+EXPORT_SYMBOL(pr_buf);
+
+void pr_char(struct printbuf *buf, char c)
+{
+ if (!printbuf_realloc(buf, 1)) {
+ buf->buf[buf->pos++] = c;
+ buf->buf[buf->pos] = 0;
+ }
+}
+EXPORT_SYMBOL(pr_char);
+
+void pr_newline(struct printbuf *buf)
+{
+ unsigned i;
+
+ pr_char(buf, '\n');
+
+ buf->last_newline = buf->pos;
+
+ for (i = 0; i < buf->indent; i++)
+ pr_char(buf, ' ');
+
+ buf->last_field = buf->pos;
+ buf->tabstop = 0;
+}
+EXPORT_SYMBOL(pr_newline);
+
+void pr_indent_push(struct printbuf *buf, unsigned spaces)
+{
+ buf->indent += spaces;
+ while (spaces--)
+ pr_char(buf, ' ');
+}
+EXPORT_SYMBOL(pr_indent_push);
+
+void pr_indent_pop(struct printbuf *buf, unsigned spaces)
+{
+ if (buf->last_newline + buf->indent == buf->pos) {
+ buf->pos -= spaces;
+ buf->buf[buf->pos] = 0;
+ }
+ buf->indent -= spaces;
+}
+EXPORT_SYMBOL(pr_indent_pop);
+
+void pr_tab(struct printbuf *buf)
+{
+ BUG_ON(buf->tabstop > ARRAY_SIZE(buf->tabstops));
+
+ while (printbuf_remaining(buf) > 1 &&
+ printbuf_linelen(buf) < buf->tabstops[buf->tabstop])
+ pr_char(buf, ' ');
+
+ buf->last_field = buf->pos;
+ buf->tabstop++;
+}
+EXPORT_SYMBOL(pr_tab);
+
+void pr_tab_rjust(struct printbuf *buf)
+{
+ BUG_ON(buf->tabstop > ARRAY_SIZE(buf->tabstops));
+
+ if (printbuf_linelen(buf) < buf->tabstops[buf->tabstop]) {
+ unsigned move = buf->pos - buf->last_field;
+ unsigned shift = buf->tabstops[buf->tabstop] -
+ printbuf_linelen(buf);
+
+ printbuf_realloc(buf, shift);
+
+ if (buf->last_field + shift + 1 < buf->size) {
+ move = min(move, buf->size - 1 - buf->last_field - shift);
+
+ memmove(buf->buf + buf->last_field + shift,
+ buf->buf + buf->last_field,
+ move);
+ memset(buf->buf + buf->last_field, ' ', shift);
+ buf->pos += shift;
+ buf->buf[buf->pos] = 0;
+ }
+ }
+
+ buf->last_field = buf->pos;
+ buf->tabstop++;
+}
+EXPORT_SYMBOL(pr_tab_rjust);
+
+void pr_human_readable_u64(struct printbuf *buf, u64 v)
+{
+ printbuf_realloc(buf, 10);
+ string_get_size(v, 1, buf->human_readable_units, buf->buf + buf->pos,
+ printbuf_remaining(buf));
+ buf->pos += strlen(buf->buf + buf->pos);
+}
+EXPORT_SYMBOL(pr_human_readable_u64);
+
+void pr_human_readable_s64(struct printbuf *buf, s64 v)
+{
+ if (v < 0)
+ pr_char(buf, '-');
+ pr_human_readable_u64(buf, abs(v));
+}
+EXPORT_SYMBOL(pr_human_readable_s64);
+
+void pr_units(struct printbuf *out, s64 raw, s64 bytes)
+{
+ switch (out->units) {
+ case PRINTBUF_UNITS_RAW:
+ pr_buf(out, "%llu", raw);
+ break;
+ case PRINTBUF_UNITS_BYTES:
+ pr_buf(out, "%llu", bytes);
+ break;
+ case PRINTBUF_UNITS_HUMAN_READABLE:
+ pr_human_readable_s64(out, bytes);
+ break;
+ }
+}
+EXPORT_SYMBOL(pr_units);
+
+void pr_sectors(struct printbuf *out, u64 v)
+{
+ pr_units(out, v, v << 9);
+}
+EXPORT_SYMBOL(pr_sectors);
+
+#ifdef __KERNEL__
+
+void pr_time(struct printbuf *out, u64 time)
+{
+ pr_buf(out, "%llu", time);
+}
+EXPORT_SYMBOL(pr_time);
+
+void pr_uuid(struct printbuf *out, u8 *uuid)
+{
+ pr_buf(out, "%pUb", uuid);
+}
+EXPORT_SYMBOL(pr_uuid);
+
+#else
+
+#include <time.h>
+#include <uuid.h>
+
+void pr_time(struct printbuf *out, u64 _time)
+{
+ char time_str[64];
+ time_t time = _time;
+ struct tm *tm = localtime(&time);
+ size_t err = strftime(time_str, sizeof(time_str), "%c", tm);
+
+ if (!err)
+ pr_buf(out, "(formatting error)");
+ else
+ pr_buf(out, "%s", time_str);
+}
+
+void pr_uuid(struct printbuf *out, u8 *uuid)
+{
+ char uuid_str[40];
+
+ uuid_unparse_lower(uuid, uuid_str);
+ pr_buf(out, uuid_str);
+}
+
+#endif
+
+void pr_string_option(struct printbuf *out,
+ const char * const list[],
+ size_t selected)
+{
+ size_t i;
+
+ for (i = 0; list[i]; i++)
+ pr_buf(out, i == selected ? "[%s] " : "%s ", list[i]);
+}
+EXPORT_SYMBOL(pr_string_option);
+
+void pr_bitflags(struct printbuf *out,
+ const char * const list[], u64 flags)
+{
+ unsigned bit, nr = 0;
+ bool first = true;
+
+ while (list[nr])
+ nr++;
+
+ while (flags && (bit = __ffs(flags)) < nr) {
+ if (!first)
+ pr_buf(out, ",");
+ first = false;
+ pr_buf(out, "%s", list[bit]);
+ flags ^= 1 << bit;
+ }
+}
+EXPORT_SYMBOL(pr_bitflags);
+
+/**
+ * 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);
--
2.35.2