feat(preinit): support alternative roots

In A/B schemas, it is possible to have multiple rootfs.

Thus, it is necessary to let the B kernel preinitialize either the A
rootfs or the B rootfs failing to mount the A rootfs.

Ideally, we should also try to switch to the B rootfs if we cannot start
up the init on the A rootfs, but that's a FUTUREWORK.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

pkgs/preinit/opts.h

@@ -1,5 +1,6 @@
 struct root_opts {
     char *device;
+    char *altdevice; /* For A/B schemas */
     char *fstype;
     char *mount_opts;
 };

pkgs/preinit/parseopts.c

@@ -70,6 +70,7 @@ void parseopts(char * cmdline, struct root_opts *opts) {
 	p = eat_param(p, "root=", &(opts->device));
 	p = eat_param(p, "rootfstype=", &(opts->fstype));
 	p = eat_param(p, "rootflags=", &(opts->mount_opts));
+	p = eat_param(p, "altroot=", &(opts->altdevice));
     };
 }
 
@@ -102,6 +103,14 @@ int main()
     expect_equal(opts.fstype, "ubifs");
     expect_equal(opts.mount_opts, "subvol=1");
 
+    // finds altroot= options
+    buf = strdup("liminix console=ttyS0,115200 panic=10 oops=panic init=/bin/init loglevel=8 root=/dev/ubi0_4 rootfstype=ubifs rootflags=subvol=1 fw_devlink=off mtdparts=phram0:18M(rootfs) phram.phram=phram0,0x40400000,18874368,65536 root=/dev/mtdblock0 altroot=/dev/mtdblock6 foo");
+    memset(&opts, '\0', sizeof opts); parseopts(buf, &opts);
+    expect_equal(opts.device, "/dev/mtdblock0");
+    expect_equal(opts.altdevice, "/dev/mtdblock6");
+    expect_equal(opts.fstype, "ubifs");
+    expect_equal(opts.mount_opts, "subvol=1");
+
     // in case of duplicates, chooses the latter
     // also: works if the option is end of string
     buf = strdup("liminix console=ttyS0,115200 panic=10 oops=panic init=/bin/init loglevel=8 root=/dev/ubi0_4 rootfstype=ubifs fw_devlink=off mtdparts=phram0:18M(rootfs) phram.phram=phram0,0x40400000,18874368,65536 root=/dev/mtdblock0");
@@ -134,13 +143,15 @@ int main()
     if(opts.fstype) die("expected null rootfstype, got \"%s\"", opts.fstype);
     if(opts.device) die("expected null root, got \"%s\"", opts.device);
     if(opts.mount_opts) die("expected null mount_opts, got \"%s\"", opts.mount_opts);
+    if(opts.altdevice) die("expected null altdevice, got \"%s\"", opts.altdevice);
 
     // provides empty strings for empty options
-    buf = strdup("liminix rootfstype= fw_devlink=off root= /dev/hda1");
+    buf = strdup("liminix rootfstype= fw_devlink=off root= altroot= /dev/hda1");
     memset(&opts, '\0', sizeof opts);  parseopts(buf, &opts);
 
     if(strlen(opts.fstype)) die("expected empty rootfstype, got \"%s\"", opts.fstype);
-    if(strlen(opts.device)) die("expected null root, got \"%s\"", opts.device);
+    if(strlen(opts.device)) die("expected empty root, got \"%s\"", opts.device);
+    if(strlen(opts.altdevice)) die("expected empty altroot, got \"%s\"", opts.altdevice);
 
     expect_equal("01", pr_u32(1));
     expect_equal("ab", pr_u32(0xab));

pkgs/preinit/preinit.c

@@ -13,7 +13,7 @@
 #include "opts.h"
 
 #define ERR(x) write(2, x, strlen(x))
-#define AVER(c) do { if(c < 0) { ERR("failed: "  #c ": error=0x" ); pr_u32(errno); ERR("\n"); } } while(0)
+#define AVER(c) do { if(c < 0) { ERR("failed: "  #c ": error=0x" ); pr_u32(errno); ERR ( " - "); ERR(strerror(errno)); ERR("\n"); } } while(0)
 
 char * pr_u32(int32_t input);
 
@@ -88,8 +88,21 @@ int main(int argc, char *argv[], char *envp[])
 	    write(1, ", opts=", 7);
 	    write(1, opts.mount_opts, strlen(opts.mount_opts));
 	}
+	if(opts.altdevice) {
+	    write(1, ", altdevice=", 12);
+	    write(1, opts.altdevice, strlen(opts.altdevice));
+	}
 	write(1, ")\n", 2);
+
+	if(!opts.altdevice) {
 	    AVER(mount(opts.device, "/target/persist", opts.fstype, 0, opts.mount_opts));
+	} else {
+	    if(mount(opts.device, "/target/persist", opts.fstype, 0, opts.mount_opts) < 0) {
+		AVER(mount(opts.altdevice, "/target/persist", opts.fstype, 0, opts.mount_opts));
+	    }
+	}
+
+	// FUTUREWORK: any failure using `opts.device` should force us to consider rerunning this with the alternative rootfs.
 	AVER(mount("/target/persist/nix", "/target/nix",
 		   "bind", MS_BIND, NULL));
 
@@ -102,6 +115,7 @@ int main(int argc, char *argv[], char *envp[])
 
 	argv[0] = "init";
 	argv[1] = NULL;
+
 	AVER(execve("/persist/init", argv, envp));
     }
     die();