ubase

commit 4f5837147a14e0fb8ec1a2a46a9c4045bdb75696
parent 11d2d1faaf9cd0e2b6796c17390137dedcdc90bb
Author: Ioan-Adrian Ratiu <adi@adirat.com>
Date:   Sat,  7 Apr 2018 05:33:30 +0300

mount: don't call realpath on root target

Musl libc realpath implementation calls readlink on a procfs
path it computes via __procfdname (code at [1] & [2]).

This is problematic if ubase mount is used in a PID 1 because procfs
is not mounted and the kernel passes the rootfs mounted read-only, so
the first step is to read-write remount the rootfs, which can't be
done because procfs is not mounted. Thus we are in a dependency cycle:
procfs can't be mounted because the root is read-only and so on.

To break this cycle, don't call readlink on "/" (it doesn't really make
sense anyway) so the rootfs can be remounted read-write, after which
proc itself can be mounted and the rest of mount calls will succeed
so systems running ubase + musl can succesfully boot into userspace.

[1] https://git.musl-libc.org/cgit/musl/tree/src/misc/realpath.c?h=v1.1.19
[2] https://git.musl-libc.org/cgit/musl/tree/src/internal/procfdname.c?h=v1.1.19

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>

Diffstat:
M
mount.c
 | 
8
+++++---

1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/mount.c b/mount.c
@@ -242,9 +242,11 @@ main(int argc, char *argv[])
 	if (!target) {
 		target = argv[0];
 		source = NULL;
-		if (!(resolvpath = realpath(target, NULL)))
-			eprintf("realpath %s:", target);
-		target = resolvpath;
+		if (strcmp(target, "/") != 0) {
+			if (!(resolvpath = realpath(target, NULL)))
+				eprintf("realpath %s:", target);
+			target = resolvpath;
+		}
 	}
 
 	for (i = 0; files[i]; i++) {