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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>
This commit is contained in:
Ioan-Adrian Ratiu 2018-04-07 05:33:30 +03:00 committed by sin
parent 11d2d1faaf
commit 4f5837147a
1 changed files with 5 additions and 3 deletions
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8
mount.c
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@ -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++) {