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Kernel+runc: Remove the pivot_root functionality in copy_mount syscall

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That functionality seems to be too much complicated.
We shouldn't overengineer how the copy_mount syscall works, so instead
of allowing replacement of the root filesystem, let's make the unshare
file descriptor to be configured via a special ioctl call before we
initialize a new VFSRootContext object.

The special ioctl can either set a new root filesystem for the upcoming
VFSRootContext object, or remove it (by passing fd of -1).
If there's no specified root filesystem, a new RAMFS instance will be
created automatically when invoking the unshare_create syscall.

This also simplifies the code in the boot process, hence making it much
more readable.

It should be noted, that we assumed during pivot_root that the first
mountpoint in a context is the root mountpoint, which is probably a fair
assumption, but we don't assume this anywhere else in the VFSRootContext
code.
If this functionality ever comes back, we should ensure that we make
some effort to not assume this again.
This commit is contained in:
Liav A.
2026-02-14 14:43:26 +02:00
committed by Sönke Holz
parent 2a4a096e0f
commit f6db24dba4
14 changed files with 143 additions and 119 deletions
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17
Kernel/FileSystem/VirtualFileSystem.cpp
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@@ -242,13 +242,6 @@ ErrorOr<void> VirtualFileSystem::copy_mount(Custody& original_custody, VFSRootCo
if (&original_custody.inode() != &original_custody.inode().fs().root_inode())
return EINVAL;
// NOTE: If the user specified the root custody ("/") on the destination context
// then try to `pivot_root` the destination context root mount with the desired
// custody.
auto destination_context_root_custody = destination_context.root_custody().with([](auto& custody) -> NonnullRefPtr<Custody> { return custody; });
if (&new_mount_point == destination_context_root_custody.ptr())
return pivot_root_by_copying_mounted_fs_instance(destination_context, original_custody.inode().fs(), flags);
TRY(destination_context.add_new_mount(VFSRootContext::DoBindMount::No, original_custody.inode(), new_mount_point, flags));
return {};
}
@@ -315,16 +308,6 @@ ErrorOr<void> VirtualFileSystem::remove_mount(Mount& mount, FileBackedFileSystem
return {};
}
ErrorOr<void> VirtualFileSystem::pivot_root_by_copying_mounted_fs_instance(VFSRootContext& context, FileSystem& fs, int root_mount_flags)
{
auto root_mount_point = TRY(Custody::try_create(nullptr, ""sv, fs.root_inode(), root_mount_flags));
auto new_mount = TRY(adopt_nonnull_own_or_enomem(new (nothrow) Mount(fs.root_inode(), root_mount_flags)));
return s_details->file_backed_file_systems_list.with_exclusive([&](auto& file_backed_file_systems_list) -> ErrorOr<void> {
return context.pivot_root(file_backed_file_systems_list, fs, move(new_mount), move(root_mount_point), root_mount_flags);
});
}
ErrorOr<void> VirtualFileSystem::unmount(VFSRootContext& context, Inode& guest_inode, StringView custody_path)
{
return s_details->file_backed_file_systems_list.with_exclusive([&](auto& file_backed_file_systems_list) -> ErrorOr<void> {