initrd: extend SYSTEMD_IN_INITRD to accept non-ramfs rootfs

Sometimes, non-ramfs initrd root are useful. Eg, for kdump, because
initramfs is memory consuming, so mount a compressed image in earlier
initrd, chroot into it then let systemd do the rest of job is a good
solution.

But systemd doesn't recognize the initrd environment if rootfs is not a
temporary fs. This is a reasonable check, because switch-root in initrd
will wipe the whole rootfs, will be a disaster if there are any
misdetect.

So extend SYSTEMD_IN_INITRD environment variable, now it accepts boolean
value and two extra keyword, "auto" and "lenient". "auto" is same as
before, and it's the default value. "lenient" will let systemd bypass
the rootfs check.

2 files changed, 30 insertions, 12 deletions

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
index 74a71bba93..8eb2b9b9e2 100644
--- a/docs/ENVIRONMENT.md
+++ b/docs/ENVIRONMENT.md
@@ -52,9 +52,13 @@ All tools:
 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
   SystemdOptions EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
 
-* `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
-  This is useful for debugging and testing initrd-only programs in the main
-  system.
+* `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
+  method. Defaults to `auto`. Behavior is defined as follows:
+  `auto`: Checks if `/etc/initrd-release` exists, and a temporary fs is mounted
+          on `/`. If both conditions meet, then it's in initrd.
+  `lenient`: Similiar to `auto`, but the rootfs check is skipped.
+  `0|1`: Simply overrides initrd detection. This is useful for debugging and
+         testing initrd-only programs in the main system.
 
 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
   completion. If no time unit is specified, assumes seconds. The usual other units
diff --git a/src/basic/util.c b/src/basic/util.c
index 907a8d1b2a..7de5341637 100644
--- a/src/basic/util.c
+++ b/src/basic/util.c
@@ -53,11 +53,13 @@ int prot_from_flags(int flags) {
 
 bool in_initrd(void) {
         int r;
+        const char *e;
+        bool lenient = false;
 
         if (saved_in_initrd >= 0)
                 return saved_in_initrd;
 
-        /* We make two checks here:
+        /* We have two checks here:
          *
          * 1. the flag file /etc/initrd-release must exist
          * 2. the root file system must be a memory file system
@@ -65,17 +67,29 @@ bool in_initrd(void) {
          * The second check is extra paranoia, since misdetecting an
          * initrd can have bad consequences due the initrd
          * emptying when transititioning to the main systemd.
+         *
+         * If env var $SYSTEMD_IN_INITRD is not set or set to "auto",
+         * both checks are used. If it's set to "lenient", only check
+         * 1 is used. If set to a booleen value, then the boolean
+         * value is returned.
          */
 
-        r = getenv_bool_secure("SYSTEMD_IN_INITRD");
-        if (r < 0 && r != -ENXIO)
-                log_debug_errno(r, "Failed to parse $SYSTEMD_IN_INITRD, ignoring: %m");
+        e = secure_getenv("SYSTEMD_IN_INITRD");
+        if (e) {
+                if (streq(e, "lenient"))
+                        lenient = true;
+                else if (!streq(e, "auto")) {
+                        r = parse_boolean(e);
+                        if (r >= 0) {
+                                saved_in_initrd = r > 0;
+                                return saved_in_initrd;
+                        }
+                        log_debug_errno(r, "Failed to parse $SYSTEMD_IN_INITRD, ignoring: %m");
+                }
+        }
 
-        if (r >= 0)
-                saved_in_initrd = r > 0;
-        else
-                saved_in_initrd = access("/etc/initrd-release", F_OK) >= 0 &&
-                                  path_is_temporary_fs("/") > 0;
+        saved_in_initrd = (lenient || path_is_temporary_fs("/") > 0) &&
+                          access("/etc/initrd-release", F_OK) >= 0;
 
         return saved_in_initrd;
 }