cherry-pick the fix from release/2.2#183
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In do_pivot_root, the post-pivot call
do_propagation_mount(old_root, MS_REC | MS_PRIVATE)
used old_root.current_path() (a readlink of /proc/self/fd/<old_root>) as
the mount target. old_root is opened on the host root mount *before*
pivot_root(".", "."); after the pivot that readlink resolves to "/"
(i.e. the *new* root X), not to the old root. The recursive MS_PRIVATE
therefore landed on X and its entire subtree, clobbering the propagation
type that /run/media, /sys, etc. had just inherited from their host
sources during do_mounts.
Root cause: pivot_root(".", ".") makes the new rootfs X the namespace
root and parks the old root at the cwd. A subsequent readlink of the
pre-pivot old_root fd returns "/" because the detached old root mount is
reported through the new root, so the MS_REC|MS_PRIVATE recursively
re-privatized X's subtree instead of the old root's.
Fix: drop the fd-based target and use the cwd literal "." —
fchdir(old_root) has just set cwd to the old root, so "." unambiguously
refers to it. The recursive private now only touches the old root's
subtree (host-side mounts), leaving the new root
and its inherited-slave submounts untouched.
Also open new_root with O_PATH instead of O_RDONLY: fchdir accepts
O_PATH descriptors since Linux 3.5 and pivot_root(".", ".") operates on
the cwd, so a read-write descriptor is unnecessary.
Signed-off-by: Yuming He <ComixHe1895@outlook.com>
ComixHe
changed the title
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In do_pivot_root, the post-pivot call
do_propagation_mount(old_root, MS_REC | MS_PRIVATE)
used old_root.current_path() (a readlink of /proc/self/fd/<old_root>) as the mount target. old_root is opened on the host root mount before pivot_root(".", "."); after the pivot that readlink resolves to "/" (i.e. the new root X), not to the old root. The recursive MS_PRIVATE therefore landed on X and its entire subtree, clobbering the propagation type that /run/media, /sys, etc. had just inherited from their host sources during do_mounts.
Root cause: pivot_root(".", ".") makes the new rootfs X the namespace root and parks the old root at the cwd. A subsequent readlink of the pre-pivot old_root fd returns "/" because the detached old root mount is reported through the new root, so the MS_REC|MS_PRIVATE recursively re-privatized X's subtree instead of the old root's.
Fix: drop the fd-based target and use the cwd literal "." — fchdir(old_root) has just set cwd to the old root, so "." unambiguously refers to it. The recursive private now only touches the old root's subtree (host-side mounts), leaving the new root
and its inherited-slave submounts untouched.
Also open new_root with O_PATH instead of O_RDONLY: fchdir accepts O_PATH descriptors since Linux 3.5 and pivot_root(".", ".") operates on the cwd, so a read-write descriptor is unnecessary.