printk: move dictionary keys to dev_printk_info

Dictionaries are only used for SUBSYSTEM and DEVICE properties. The
current implementation stores the property names each time they are
used. This requires more space than otherwise necessary. Also,
because the dictionary entries are currently considered optional,
it cannot be relied upon that they are always available, even if the
writer wanted to store them. These issues will increase should new
dictionary properties be introduced.

Rather than storing the subsystem and device properties in the
dict ring, introduce a struct dev_printk_info with separate fields
to store only the property values. Embed this struct within the
struct printk_info to provide guaranteed availability.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/87mu1jl6ne.fsf@jogness.linutronix.de

Documentation/admin-guide/kdump/gdbmacros.txt

@@ -172,13 +172,13 @@ end
 
 define dump_record
 	set var $desc = $arg0
-	if ($argc > 1)
-		set var $prev_flags = $arg1
+	set var $info = $arg1
+	if ($argc > 2)
+		set var $prev_flags = $arg2
 	else
 		set var $prev_flags = 0
 	end
 
-	set var $info = &$desc->info
 	set var $prefix = 1
 	set var $newline = 1
 
@@ -237,44 +237,36 @@ define dump_record
 
 	# handle dictionary data
 
-	set var $begin = $desc->dict_blk_lpos.begin % (1U << prb->dict_data_ring.size_bits)
-	set var $next = $desc->dict_blk_lpos.next % (1U << prb->dict_data_ring.size_bits)
-
-	# handle data-less record
-	if ($begin & 1)
-		set var $dict_len = 0
-		set var $dict = ""
-	else
-		# handle wrapping data block
-		if ($begin > $next)
-			set var $begin = 0
-		end
-
-		# skip over descriptor id
-		set var $begin = $begin + sizeof(long)
-
-		# handle truncated message
-		if ($next - $begin < $info->dict_len)
-			set var $dict_len = $next - $begin
-		else
-			set var $dict_len = $info->dict_len
+	set var $dict = &$info->dev_info.subsystem[0]
+	set var $dict_len = sizeof($info->dev_info.subsystem)
+	if ($dict[0] != '\0')
+		printf " SUBSYSTEM="
+		set var $idx = 0
+		while ($idx < $dict_len)
+			set var $c = $dict[$idx]
+			if ($c == '\0')
+				loop_break
+			else
+				if ($c < ' ' || $c >= 127 || $c == '\\')
+					printf "\\x%02x", $c
+				else
+					printf "%c", $c
+				end
+			end
+			set var $idx = $idx + 1
 		end
-
-		set var $dict = &prb->dict_data_ring.data[$begin]
+		printf "\n"
 	end
 
-	if ($dict_len > 0)
+	set var $dict = &$info->dev_info.device[0]
+	set var $dict_len = sizeof($info->dev_info.device)
+	if ($dict[0] != '\0')
+		printf " DEVICE="
 		set var $idx = 0
-		set var $line = 1
 		while ($idx < $dict_len)
-			if ($line)
-				printf " "
-				set var $line = 0
-			end
 			set var $c = $dict[$idx]
 			if ($c == '\0')
-				printf "\n"
-				set var $line = 1
+				loop_break
 			else
 				if ($c < ' ' || $c >= 127 || $c == '\\')
 					printf "\\x%02x", $c
@@ -288,10 +280,10 @@ define dump_record
 	end
 end
 document dump_record
-	Dump a single record. The first parameter is the descriptor
-	sequence number, the second is optional and specifies the
-	previous record's flags, used for properly formatting
-	continued lines.
+	Dump a single record. The first parameter is the descriptor,
+	the second parameter is the info, the third parameter is
+	optional and specifies the previous record's flags, used for
+	properly formatting continued lines.
 end
 
 define dmesg
@@ -311,12 +303,13 @@ define dmesg
 
 	while (1)
 		set var $desc = &prb->desc_ring.descs[$id % $desc_count]
+		set var $info = &prb->desc_ring.infos[$id % $desc_count]
 
 		# skip non-committed record
 		set var $state = 3 & ($desc->state_var.counter >> $desc_flags_shift)
 		if ($state == $desc_committed || $state == $desc_finalized)
-			dump_record $desc $prev_flags
-			set var $prev_flags = $desc->info.flags
+			dump_record $desc $info $prev_flags
+			set var $prev_flags = $info->flags
 		end
 
 		set var $id = ($id + 1) & $id_mask

drivers/base/core.c

@@ -3815,22 +3815,21 @@ void device_shutdown(void)
  */
 
 #ifdef CONFIG_PRINTK
-static int
-create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
+static void
+set_dev_info(const struct device *dev, struct dev_printk_info *dev_info)
 {
 	const char *subsys;
-	size_t pos = 0;
+
+	memset(dev_info, 0, sizeof(*dev_info));
 
 	if (dev->class)
 		subsys = dev->class->name;
 	else if (dev->bus)
 		subsys = dev->bus->name;
 	else
-		return 0;
+		return;
 
-	pos += snprintf(hdr + pos, hdrlen - pos, "SUBSYSTEM=%s", subsys);
-	if (pos >= hdrlen)
-		goto overflow;
+	strscpy(dev_info->subsystem, subsys, sizeof(dev_info->subsystem));
 
 	/*
 	 * Add device identifier DEVICE=:
@@ -3846,41 +3845,28 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
 			c = 'b';
 		else
 			c = 'c';
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=%c%u:%u",
-				c, MAJOR(dev->devt), MINOR(dev->devt));
+
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "%c%u:%u", c, MAJOR(dev->devt), MINOR(dev->devt));
 	} else if (strcmp(subsys, "net") == 0) {
 		struct net_device *net = to_net_dev(dev);
 
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=n%u", net->ifindex);
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "n%u", net->ifindex);
 	} else {
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=+%s:%s", subsys, dev_name(dev));
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "+%s:%s", subsys, dev_name(dev));
 	}
-
-	if (pos >= hdrlen)
-		goto overflow;
-
-	return pos;
-
-overflow:
-	dev_WARN(dev, "device/subsystem name too long");
-	return 0;
 }
 
 int dev_vprintk_emit(int level, const struct device *dev,
 		     const char *fmt, va_list args)
 {
-	char hdr[128];
-	size_t hdrlen;
+	struct dev_printk_info dev_info;
 
-	hdrlen = create_syslog_header(dev, hdr, sizeof(hdr));
+	set_dev_info(dev, &dev_info);
 
-	return vprintk_emit(0, level, hdrlen ? hdr : NULL, hdrlen, fmt, args);
+	return vprintk_emit(0, level, &dev_info, fmt, args);
 }
 EXPORT_SYMBOL(dev_vprintk_emit);
 

include/linux/dev_printk.h

@@ -21,6 +21,14 @@
 
 struct device;
 
+#define PRINTK_INFO_SUBSYSTEM_LEN	16
+#define PRINTK_INFO_DEVICE_LEN		48
+
+struct dev_printk_info {
+	char subsystem[PRINTK_INFO_SUBSYSTEM_LEN];
+	char device[PRINTK_INFO_DEVICE_LEN];
+};
+
 #ifdef CONFIG_PRINTK
 
 __printf(3, 0) __cold

include/linux/printk.h

@@ -158,10 +158,12 @@ static inline void printk_nmi_direct_enter(void) { }
 static inline void printk_nmi_direct_exit(void) { }
 #endif /* PRINTK_NMI */
 
+struct dev_printk_info;
+
 #ifdef CONFIG_PRINTK
-asmlinkage __printf(5, 0)
+asmlinkage __printf(4, 0)
 int vprintk_emit(int facility, int level,
-		 const char *dict, size_t dictlen,
+		 const struct dev_printk_info *dev_info,
 		 const char *fmt, va_list args);
 
 asmlinkage __printf(1, 0)

kernel/printk/internal.h

@@ -14,9 +14,9 @@
 
 extern raw_spinlock_t logbuf_lock;
 
-__printf(5, 0)
+__printf(4, 0)
 int vprintk_store(int facility, int level,
-		  const char *dict, size_t dictlen,
+		  const struct dev_printk_info *dev_info,
 		  const char *fmt, va_list args);
 
 __printf(1, 0) int vprintk_default(const char *fmt, va_list args);

kernel/printk/printk_ringbuffer.h

@@ -4,6 +4,7 @@
 #define _KERNEL_PRINTK_RINGBUFFER_H
 
 #include <linux/atomic.h>
+#include <linux/dev_printk.h>
 
 /*
  * Meta information about each stored message.
@@ -21,6 +22,8 @@ struct printk_info {
 	u8	flags:5;	/* internal record flags */
 	u8	level:3;	/* syslog level */
 	u32	caller_id;	/* thread id or processor id */
+
+	struct dev_printk_info	dev_info;
 };
 
 /*

kernel/printk/printk_safe.c

@@ -375,7 +375,7 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	    raw_spin_trylock(&logbuf_lock)) {
 		int len;
 
-		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
+		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
 		raw_spin_unlock(&logbuf_lock);
 		defer_console_output();
 		return len;

scripts/gdb/linux/dmesg.py

@@ -52,6 +52,12 @@ class LxDmesg(gdb.Command):
         addr = utils.read_ulong(desc_ring, off)
         descs = utils.read_memoryview(inf, addr, desc_sz * desc_ring_count).tobytes()
 
+        # read in info array
+        info_sz = printk_info_type.get_type().sizeof
+        off = prb_desc_ring_type.get_type()['infos'].bitpos // 8
+        addr = utils.read_ulong(desc_ring, off)
+        infos = utils.read_memoryview(inf, addr, info_sz * desc_ring_count).tobytes()
+
         # read in text data ring structure
         off = printk_ringbuffer_type.get_type()['text_data_ring'].bitpos // 8
         addr = prb_addr + off
@@ -73,9 +79,8 @@ class LxDmesg(gdb.Command):
         begin_off = off + (prb_data_blk_lpos_type.get_type()['begin'].bitpos // 8)
         next_off = off + (prb_data_blk_lpos_type.get_type()['next'].bitpos // 8)
 
-        off = prb_desc_type.get_type()['info'].bitpos // 8
-        ts_off = off + printk_info_type.get_type()['ts_nsec'].bitpos // 8
-        len_off = off + printk_info_type.get_type()['text_len'].bitpos // 8
+        ts_off = printk_info_type.get_type()['ts_nsec'].bitpos // 8
+        len_off = printk_info_type.get_type()['text_len'].bitpos // 8
 
         # definitions from kernel/printk/printk_ringbuffer.h
         desc_committed = 1
@@ -95,6 +100,7 @@ class LxDmesg(gdb.Command):
         while True:
             ind = did % desc_ring_count
             desc_off = desc_sz * ind
+            info_off = info_sz * ind
 
             # skip non-committed record
             state = 3 & (utils.read_u64(descs, desc_off + sv_off +
@@ -119,7 +125,7 @@ class LxDmesg(gdb.Command):
                 # skip over descriptor id
                 text_start = begin + utils.get_long_type().sizeof
 
-                text_len = utils.read_u16(descs, desc_off + len_off)
+                text_len = utils.read_u16(infos, info_off + len_off)
 
                 # handle truncated message
                 if end - text_start < text_len:
@@ -128,7 +134,7 @@ class LxDmesg(gdb.Command):
                 text = text_data[text_start:text_start + text_len].decode(
                     encoding='utf8', errors='replace')
 
-            time_stamp = utils.read_u64(descs, desc_off + ts_off)
+            time_stamp = utils.read_u64(infos, info_off + ts_off)
 
             for line in text.splitlines():
                 msg = u"[{time:12.6f}] {line}\n".format(