smaller journal fixes (#3124)

* sd-journal: detect earlier if we try to read an object from an invalid offset

Specifically, detect early if we try to read from offset 0, i.e. are using
uninitialized offset data.

* journal: when dumping journal contents, react nicer to lines we can't read

If journal files are not cleanly closed it might happen that intermediaery
journal entries cannot be read. Handle this nicely, skip over the unreadable
entries, and log a debug message about it; after all we generally follow the
logic that we try to make the best of corrupted files.

* journal-file: always generate the same error when encountering corrupted files

Let's make sure EBADMSG is the one error we throw when we encounter corrupted
data, so that we can neatly test for it.

* journal-file: when iterating through a partly corruped journal file, treat error like EOF

When we linearly iterate through a corrupted journal file, and we encounter a
read error, don't consider this fatal, but merely as EOF condition (and log
about it).

* journal-file: make seeking in corrupted files work

Previously, when we used a bisection table for seeking through a corrupted
file, and the end of the bisection table was corrupted we'd most likely fail
the entire seek operation. Improve the situation: if we encounter invalid
entries in a bisection table, linearly go backwards until we find a working
entry again.

* man: elaborate on the automatic systemd-journald.socket service dependencies

Fixes: #1603

3 files changed, 40 insertions, 14 deletions

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index 4ed62dbada..fea42ebd31 100644
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -94,11 +94,10 @@
     required to access <filename>/tmp</filename> and
     <filename>/var/tmp</filename>.</para>
 
-    <para>Units whose output standard output or error output is
-    connected to any other sink but <option>null</option>,
-    <option>tty</option> and <option>socket</option> automatically
-    acquire dependencies of type <varname>After=</varname> on
-    <filename>journald.socket</filename>.</para>
+    <para>Units whose output standard output or error output is connected to <option>journal</option>,
+    <option>syslog</option> or <option>kmsg</option> (or their combinations with console output, see below)
+    automatically acquire dependencies of type <varname>After=</varname> on
+    <filename>systemd-journald.socket</filename>.</para>
   </refsect1>
 
   <refsect1>
@@ -470,6 +469,10 @@
         similar to the same option of
         <varname>StandardInput=</varname>.</para>
 
+        <para>If the standard output (or error output, see below) of a unit is connected with the journal, syslog or
+        the kernel log buffer the unit will implicitly gain a dependency of type <varname>After=</varname> on
+        <filename>systemd-journald.socket</filename> (also see the automatic dependencies section above).</para>
+
         <para>This setting defaults to the value set with
         <option>DefaultStandardOutput=</option> in
         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
diff --git a/src/journal/journal-file.c b/src/journal/journal-file.c
index ac6c30f9f2..ff01e5aa94 100644
--- a/src/journal/journal-file.c
+++ b/src/journal/journal-file.c
@@ -707,7 +707,11 @@ int journal_file_move_to_object(JournalFile *f, ObjectType type, uint64_t offset
 
         /* Objects may only be located at multiple of 64 bit */
         if (!VALID64(offset))
-                return -EFAULT;
+                return -EBADMSG;
+
+        /* Object may not be located in the file header */
+        if (offset < le64toh(f->header->header_size))
+                return -EBADMSG;
 
         r = journal_file_move_to(f, type, false, offset, sizeof(ObjectHeader), &t);
         if (r < 0)
@@ -1980,9 +1984,14 @@ static int generic_array_bisect(
                 i = right - 1;
                 lp = p = le64toh(array->entry_array.items[i]);
                 if (p <= 0)
-                        return -EBADMSG;
-
-                r = test_object(f, p, needle);
+                        r = -EBADMSG;
+                else
+                        r = test_object(f, p, needle);
+                if (r == -EBADMSG) {
+                        log_debug_errno(r, "Encountered invalid entry while bisecting, cutting algorithm short. (1)");
+                        n = i;
+                        continue;
+                }
                 if (r < 0)
                         return r;
 
@@ -2058,9 +2067,14 @@ static int generic_array_bisect(
 
                                 p = le64toh(array->entry_array.items[i]);
                                 if (p <= 0)
-                                        return -EBADMSG;
-
-                                r = test_object(f, p, needle);
+                                        r = -EBADMSG;
+                                else
+                                        r = test_object(f, p, needle);
+                                if (r == -EBADMSG) {
+                                        log_debug_errno(r, "Encountered invalid entry while bisecting, cutting algorithm short. (2)");
+                                        right = n = i;
+                                        continue;
+                                }
                                 if (r < 0)
                                         return r;
 
@@ -2465,12 +2479,18 @@ int journal_file_next_entry(
                               le64toh(f->header->entry_array_offset),
                               i,
                               ret, &ofs);
+        if (r == -EBADMSG && direction == DIRECTION_DOWN) {
+                /* Special case: when we iterate throught the journal file linearly, and hit an entry we can't read,
+                 * consider this the end of the journal file. */
+                log_debug_errno(r, "Encountered entry we can't read while iterating through journal file. Considering this the end of the file.");
+                return 0;
+        }
         if (r <= 0)
                 return r;
 
         if (p > 0 &&
             (direction == DIRECTION_DOWN ? ofs <= p : ofs >= p)) {
-                log_debug("%s: entry array corrupted at entry %"PRIu64, f->path, i);
+                log_debug("%s: entry array corrupted at entry %" PRIu64, f->path, i);
                 return -EBADMSG;
         }
 
diff --git a/src/shared/logs-show.c b/src/shared/logs-show.c
index cd3d53669c..9351b85eed 100644
--- a/src/shared/logs-show.c
+++ b/src/shared/logs-show.c
@@ -287,7 +287,10 @@ static int output_short(
                 if (r < 0)
                         return r;
         }
-
+        if (r == -EBADMSG) {
+                log_debug_errno(r, "Skipping message we can't read: %m");
+                return 0;
+        }
         if (r < 0)
                 return log_error_errno(r, "Failed to get journal fields: %m");