From 2aacc6912b78e7d3158f072cd9c1b3c5d0bddc22 Mon Sep 17 00:00:00 2001
From: William Manley <will@williammanley.net>
Date: Wed, 31 Aug 2016 17:15:48 +0100
Subject: [PATCH] ostree-prepare-root: Fix running with musl

musl libc's implementation of `realpath` works by opening the path and then
doing a lookup in `/proc/self/fd` to find the canonical path.  This fails
if `/proc` is not mounted.  This causes problems for us if
`ostree-prepare-root` is `init` as `/proc` won't be mounted.

We have to mount `/proc` anyway for `/proc/cmdline` so this fix just
expands the scope over which `/proc` is mounted to include both our
`realpath` calls.

See also:

* http://www.openwall.com/lists/musl/2016/06/08/2 and
* http://git.musl-libc.org/cgit/musl/tree/src/misc/realpath.c?id=e738b8cbe64b6dd3ed9f47b6d4cd7eb2c422b38d

Closes: #485
Approved by: cgwalters
---
 src/switchroot/ostree-prepare-root.c | 37 +++++++++++++++-------------
 tests/test-switchroot.sh             |  9 ++++++-
 2 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/src/switchroot/ostree-prepare-root.c b/src/switchroot/ostree-prepare-root.c
index ea7333b5..ce48a91e 100644
--- a/src/switchroot/ostree-prepare-root.c
+++ b/src/switchroot/ostree-prepare-root.c
@@ -80,26 +80,10 @@ parse_ostree_cmdline (void)
   char *cmdline = NULL;
   const char *iter;
   char *ret = NULL;
-  int tmp_errno;
 
   cmdline = read_proc_cmdline ();
   if (!cmdline)
-    {
-      // Mount proc
-      if (mount ("proc", "/proc", "proc", 0, NULL) < 0)
-        err (EXIT_FAILURE, "failed to mount proc on /proc");
-
-      cmdline = read_proc_cmdline ();
-      tmp_errno = errno;
-
-      /* Leave the filesystem in the state that we found it: */
-      if (umount ("/proc"))
-        err (EXIT_FAILURE, "failed to umount proc from /proc");
-
-      errno = tmp_errno;
-      if (!cmdline)
-        err (EXIT_FAILURE, "failed to read /proc/cmdline");
-    }
+    err (EXIT_FAILURE, "failed to read /proc/cmdline");
 
   iter = cmdline;
   while (iter != NULL)
@@ -178,17 +162,36 @@ main(int argc, char *argv[])
   char *deploy_path = NULL;
   char srcpath[PATH_MAX];
   struct stat stbuf;
+  int we_mounted_proc = 0;
 
   if (argc < 2)
     root_arg = "/";
   else
     root_arg = argv[1];
 
+  if (stat ("/proc/cmdline", &stbuf) < 0)
+    {
+      if (errno != ENOENT)
+        err (EXIT_FAILURE, "stat(\"/proc/cmdline\") failed");
+      /* We need /proc mounted for /proc/cmdline and realpath (on musl) to
+       * work: */
+      if (mount ("proc", "/proc", "proc", 0, NULL) < 0)
+        err (EXIT_FAILURE, "failed to mount proc on /proc");
+      we_mounted_proc = 1;
+    }
+
   root_mountpoint = realpath (root_arg, NULL);
   if (root_mountpoint == NULL)
     err (EXIT_FAILURE, "realpath(\"%s\")", root_arg);
   deploy_path = resolve_deploy_path (root_mountpoint);
 
+  if (we_mounted_proc)
+    {
+      /* Leave the filesystem in the state that we found it: */
+      if (umount ("/proc"))
+        err (EXIT_FAILURE, "failed to umount proc from /proc");
+    }
+
   /* Work-around for a kernel bug: for some reason the kernel
    * refuses switching root if any file systems are mounted
    * MS_SHARED. Hence remount them MS_PRIVATE here as a
diff --git a/tests/test-switchroot.sh b/tests/test-switchroot.sh
index 96c849e5..fd5a30d0 100755
--- a/tests/test-switchroot.sh
+++ b/tests/test-switchroot.sh
@@ -4,7 +4,14 @@ this_script="${BASH_SOURCE:-$(readlink -f "$0")}"
 
 setup_bootfs() {
 	mkdir -p "$1/proc" "$1/bin"
-	echo "quiet ostree=/ostree/boot.0 ro" >"$1/proc/cmdline"
+
+	# We need the real /proc mounted here so musl's realpath will work, but we
+	# want to be able to override /proc/cmdline, so bind mount.
+	mount -t proc proc "$1/proc"
+
+	echo "quiet ostree=/ostree/boot.0 ro" >"$1/override_cmdline"
+	mount --bind "$1/override_cmdline" "$1/proc/cmdline"
+
 	touch "$1/this_is_bootfs"
 	cp "$(dirname "$this_script")/../ostree-prepare-root" "$1/bin"
 }