DLPX-87560 sdb: dmesg broken for 5.15 kernels (#340)

This patch replaces our hand-written implementation of dmesg
which doesn't work for most vanilla kernel versions after 5.10
with the drgn helper which stays up to date more often.

In order for this to work I had to change the type of our
dmesg command from a Locator+PrettyPrinter to a standard
command. This is not a big deal as the only reason we would
ever walk and filter dmesg would be to filter the kernel logs
by level. I've added this as an argument to the command.

= Github Issue Tracker Automation

Closes #336

sdb/commands/linux/dmesg.py

@@ -16,43 +16,64 @@
 
 # pylint: disable=missing-docstring
 
+import argparse
 from typing import Iterable
 
 import drgn
+from drgn.helpers.linux.printk import get_printk_records
 import sdb
 
+# pylint: disable=line-too-long
 
-class DMesg(sdb.Locator, sdb.PrettyPrinter):
 
-    names = ["dmesg"]
-    load_on = [sdb.Kernel()]
-
-    input_type = "struct printk_log *"
-    output_type = "struct printk_log *"
+class DMesg(sdb.Command):
+    """
+    DESCRIPTION
 
-    def no_input(self) -> Iterable[drgn.Object]:
-        log_idx = sdb.get_object("log_first_idx")
-        log_seq = sdb.get_object("clear_seq")
-        log_end = sdb.get_object("log_next_seq")
-        log_buf = sdb.get_object("log_buf")
+        Get contents from kernel log buffer formatted like dmesg(1).
 
-        while log_seq < log_end:
-            entry = drgn.cast('struct printk_log *', log_buf + log_idx)
+    EXAMPLES
 
-            yield entry
+        sdb> dmesg ! tail
+        [   30.544756] AVX2 version of gcm_enc/dec engaged.
+        [   30.545019] AES CTR mode by8 optimization enabled
+        [   38.855043] Rounding down aligned max_sectors from 4294967295 to 4294967288
+        [   38.863427] db_root: cannot open: /etc/target
+        [   39.822443] aufs 5.15.5-20211129
+        [   40.344495] NFSD: Using UMH upcall client tracking operations.
+        [   40.344501] NFSD: starting 20-second grace period (net f0000000)
+        [   40.978893] EXT4-fs (zd0): mounted filesystem with ordered data mode. Opts: (null). Quota mode: none.
+        [  176.825888] bpfilter: Loaded bpfilter_umh pid 4662
+        [  176.826272] Started bpfilter
 
-            if entry.len == 0:
-                log_idx = 0
-            else:
-                log_idx += entry.len
-            log_seq += 1
+        sdb> dmesg -l 3
+        [   38.863427] db_root: cannot open: /etc/target
+    """
 
-    def pretty_print(self, objs: Iterable[drgn.Object]) -> None:
-        for obj in objs:
-            secs = int(obj.ts_nsec.value_() / 1000000000)
-            usecs = int((obj.ts_nsec.value_() % 1000000000) / 1000)
+    names = ["dmesg"]
+    # input_type = None
+    load_on = [sdb.Kernel()]
 
-            message = drgn.cast("char *", obj) + obj.type_.type.size
-            text = message.string_().decode('utf-8', 'ignore')
+    @classmethod
+    def _init_parser(cls, name: str) -> argparse.ArgumentParser:
+        parser = super()._init_parser(name)
+        #
+        # #define KERN_EMERG	KERN_SOH "0"	/* system is unusable */
+        # #define KERN_ALERT	KERN_SOH "1"	/* action must be taken immediately */
+        # #define KERN_CRIT	KERN_SOH "2"	/* critical conditions */
+        # #define KERN_ERR	KERN_SOH "3"	/* error conditions */
+        # #define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
+        # #define KERN_NOTICE	KERN_SOH "5"	/* normal but significant condition */
+        # #define KERN_INFO	KERN_SOH "6"	/* informational */
+        # #define KERN_DEBUG	KERN_SOH "7"	/* debug-level messages */
+        #
+        parser.add_argument('--level', '-l', nargs="?", type=int, default=7)
+        return parser
 
-            print(f"[{secs:5d}.{usecs:06d}]: {text}")
+    def _call(self, objs: Iterable[drgn.Object]) -> None:
+        for record in get_printk_records(sdb.get_prog()):
+            if self.args.level >= record.level:
+                secs = record.timestamp // 1000000000
+                sub_secs = record.timestamp % 1000000000 // 1000
+                msg = record.text.decode('utf-8')
+                print(f"[{secs: 5d}.{sub_secs:06d}] {msg}")

tests/integration/data/regression_output/dump.201912060006/linux/dmesg -l 3

@@ -0,0 +1,9 @@
+[    3.685278] piix4_smbus 0000:00:07.3: SMBus Host Controller not enabled!
+[    4.346112] sd 2:0:0:0: [sda] Assuming drive cache: write through
+[    4.356113] sd 2:0:1:0: [sdb] Assuming drive cache: write through
+[    4.363435] sd 2:0:2:0: [sdc] Assuming drive cache: write through
+[    4.366085] sd 2:0:3:0: [sdd] Assuming drive cache: write through
+[   48.368009] db_root: cannot open: /etc/target
+[  305.340853] Kernel panic - not syncing: sysrq triggered crash
+@#$ EXIT CODE $#@
+0