[Devel] [PATCH v4 VZ10 1/1] fs: namespace: transform mount flags to comma separated values
Konstantin Khorenko
khorenko at virtuozzo.com
Fri Jun 19 22:20:23 MSK 2026
Main concern: remount injects negative flags and likely breaks ordinary remounts
do_remount() builds the context as:
fc = fs_context_for_reconfigure(path->dentry, sb_flags, MS_RMT_MASK);
so fc->sb_flags_mask = MS_RMT_MASK (the fixed mask RDONLY|SYNCHRONOUS|MANDLOCK|I_VERSION|LAZYTIME). And
vfs_format_sb_flags() emits the negative table based on fc->sb_flags_mask & ~fc->sb_flags. So on any remount where the
user did not set ro/sync/mand/lazytime, the data gets rw,async,nomand,nolazytime appended (all the clear-names for the
MS_RMT_MASK bits that are off).
Then ve_devmnt_check() requires every token to be present in allowed_options. allowed_options is supplied by the host via
VE_CONFIGURE_MOUNT_OPTIONS, and until this commit flags were never checked against it, so a typical list almost certainly
does not contain rw/async/nomand/nolazytime. Result:
▎ a container mount -o remount (e.g. systemd/runc remounting a disk) on a device whose allowed_options does not contain
▎ rw/async/nomand/nolazytime will now return -EPERM, where it previously succeeded.
This is also inconsistent with the new-mount path: there fs_context_for_mount() sets sb_flags_mask = 0, so the negative
branch yields 0 & ~flags = 0 - no negative flags are emitted at all. So the very same device mounts fine but fails to
remount. It also contradicts the claimed symmetry with fsconfig (there only explicitly-set options end up in the data, no
auto-negatives).
For the security goal the negative names aren't even needed: the bypass is about enabling restricted options (the
positives: ro/sync/mand/dirsync/lazytime), whereas rw/async/... are the permissive defaults - no reason to whitelist them.
So my suggestion: don't emit the negative flags at all (keep only common_set_sb_flag). That (1) closes the same bypass,
(2) removes the remount regression, and (3) makes remount consistent with new-mount and fsconfig.
If the negative emission is intentional, then it requires a coordinated userspace change (libvzctl must add
rw,async,nomand,nolazytime to allowed_options), and that dependency should be stated explicitly in the commit/ticket.
Could you confirm that an ordinary container remount was tested against a device with a narrow allowed_options?
Minor
The comment on __vfs_format_flags misstates the return code:
* Returns 0 on success or -ENOSPC if the buffer is too small.
The function returns -E2BIG, not -ENOSPC. Fix the comment (or the code, but -E2BIG is the better fit here and matches
strscpy).
--
Best regards,
Konstantin Khorenko,
Virtuozzo Linux Kernel Team
On 6/3/26 17:03, Vasileios Almpanis wrote:
> In legacy mount callpaths, userspace might pass mount options as
> flags. These flags escape our checks in ve_devmnt_process allowing
> devices to be mounted inside containers with options not specified in
> the allowed field. Introduce helpers that take these flags and
> already existing tables of flag -> string representation to construct
> a comma separated value string from them, and append them to userspace
> provided data. Then pass this string to parse_monolithic_mount_data
> enforcing the same checks symmetrically in both mount and fsconfig
> syscalls.
>
> In the remount path, run legacy_merge_mount_data() before
> ve_devmnt_process() so container device mount policy sees MS_* flags
> from the legacy mount(2) API, not only the user-supplied option string.
> Keep ve_prepare_mount_options() for legacy parsers that do not use
> generic_parse_monolithic().
>
> https://virtuozzo.atlassian.net/browse/VSTOR-132330
> Signed-off-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
>
> Feature: ve: ve generic structures
> ---
> Changes since v3:
> - Drop excess length check in legacy_merge_mount_data
>
> Changes since v2:
> - Remove legacy_merge_mount_data guard in fs/internal.h. All helpers
> don't use anything that would break build and just unchanged pointer
> will be returned.
> - Add __vfs_format_flags helper and use it in vfs_format_sb_flags
> - Fix inconsistent error code: replace -ENOSPC with -E2BIG for the
> buffer-too-small case
>
> Changes since v1:
> - Replace open-coded flag loops with append_entry() helper (pointer-
> advancing style) that unifies the comma-insert-copy pattern across
> all three call sites
> - Rework legacy_merge_mount_data() to allocate the page upfront, write
> user data first then append sb flags via vfs_format_sb_flags(); this
> eliminates the intermediate flags_buf[128], the total size calculation,
> and the +1/+2 arithmetic
> - Fix inconsistent error code: replace -EINVAL with -ENOSPC for the
> buffer-too-small case
> - Add comment on FS_BINARY_MOUNTDATA explaining why those filesystems
> are skipped
> - Add blank line before return in legacy_merge_mount_data()
> - Remove excess blank line after parse_monolithic_mount_data() in
> do_remount()
>
> fs/fs_context.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++++
> fs/internal.h | 1 +
> fs/namespace.c | 32 +++++++++++----
> 3 files changed, 132 insertions(+), 7 deletions(-)
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 76f34f3d468e..d5129e4a919a 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -81,6 +81,112 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
> return -ENOPARAM;
> }
>
> +/*
> + * Emit, into @buff at *@off, the comma-separated names of every entry in @p
> + * whose bit is set in @flags. Advances *@off past the written text.
> + * Returns 0 on success or -ENOSPC if the buffer is too small.
> + */
> +static int __vfs_format_flags(const struct constant_table *p, unsigned int flags,
> + char *buff, size_t size, size_t *off)
> +{
> + for (; p->name; p++) {
> + ssize_t ret;
> +
> + if (!(flags & p->value))
> + continue;
> +
> + if (*off) {
> + if (*off + 1 >= size)
> + return -E2BIG;
> + buff[(*off)++] = ',';
> + }
> +
> + ret = strscpy(buff + *off, p->name, size - *off);
> + if (ret < 0)
> + return -E2BIG;
> + *off += ret;
> + }
> + return 0;
> +}
> +
> +/*
> + * Append fc's sb flags as comma-separated option names into @buff, advancing
> + * *@off. Matches vfs_parse_sb_flag() / common_{set,clear}_sb_flag tables.
> + */
> +static int vfs_format_sb_flags(struct fs_context *fc, char *buff, size_t size,
> + size_t *off)
> +{
> + int ret;
> +
> + /* Positive names (ro, sync, ...) for bits that are on. */
> + ret = __vfs_format_flags(common_set_sb_flag, fc->sb_flags,
> + buff, size, off);
> + if (ret)
> + return ret;
> +
> + /*
> + * Negative names (rw, async, ...) only for bits being turned off: in
> + * the mask but off in the value. __vfs_format_flags() emits when
> + * (arg & bit), so the emit set is "mask & ~flags".
> + */
> + return __vfs_format_flags(common_clear_sb_flag,
> + fc->sb_flags_mask & ~fc->sb_flags,
> + buff, size, off);
> +}
> +
> +/*
> + * For legacy mount(2), MS_* mount flags are folded into fc->sb_flags and are
> + * not present in the monolithic data string. Build a page with user data
> + * followed by those flags for ve_devmnt checks in vfs_parse_monolithic_sep.
> + *
> + * Returns @data when nothing needs to be added, a new page otherwise, or
> + * ERR_PTR() on failure. The caller must free_page() when the result != @data.
> + */
> +void *legacy_merge_mount_data(struct fs_context *fc, void *data)
> +{
> + struct ve_struct *ve = get_exec_env();
> + size_t off = 0;
> + char *page;
> + int err;
> +
> + if (ve_is_super(ve))
> + return data;
> +
> + if (!fc->fs_type || !(fc->fs_type->fs_flags & FS_REQUIRES_DEV))
> + return data;
> +
> + /*
> + * Filesystems with binary mount data (e.g. btrfs) bypass option
> + * string parsing entirely, so our checks cannot apply here.
> + */
> + if (fc->fs_type->fs_flags & FS_BINARY_MOUNTDATA)
> + return data;
> +
> + page = (char *)__get_free_page(GFP_KERNEL);
> + if (!page)
> + return ERR_PTR(-ENOMEM);
> +
> + if (data && *(char *)data) {
> + ssize_t ret = strscpy(page, data, PAGE_SIZE);
> +
> + if (ret < 0) {
> + err = -E2BIG;
> + goto err_free;
> + }
> + off = ret;
> + }
> +
> + err = vfs_format_sb_flags(fc, page, PAGE_SIZE, &off);
> + if (err)
> + goto err_free;
> +
> + return page;
> +
> +err_free:
> + free_page((unsigned long)page);
> + return ERR_PTR(err);
> +}
> +
> /**
> * vfs_parse_fs_param_source - Handle setting "source" via parameter
> * @fc: The filesystem context to modify
> diff --git a/fs/internal.h b/fs/internal.h
> index 0064345cb9d0..147130cab046 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -46,6 +46,7 @@ extern void __init chrdev_init(void);
> */
> extern const struct fs_context_operations legacy_fs_context_ops;
> extern int parse_monolithic_mount_data(struct fs_context *, void *);
> +extern void *legacy_merge_mount_data(struct fs_context *fc, void *data);
> extern void vfs_clean_context(struct fs_context *fc);
> extern int finish_clean_context(struct fs_context *fc);
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index acd4507e1247..d4a419a0bca9 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3295,6 +3295,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
> struct super_block *sb = path->mnt->mnt_sb;
> struct mount *mnt = real_mount(path->mnt);
> struct fs_context *fc;
> + void *mnt_data = NULL;
>
> if (!check_mnt(mnt))
> return -EINVAL;
> @@ -3315,13 +3316,17 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
> */
> fc->oldapi = true;
>
> - err = ve_prepare_mount_options(fc, data);
> - if (err) {
> - put_fs_context(fc);
> - return err;
> + mnt_data = legacy_merge_mount_data(fc, data);
> + if (IS_ERR(mnt_data)) {
> + err = PTR_ERR(mnt_data);
> + goto err;
> }
>
> - err = parse_monolithic_mount_data(fc, data);
> + err = ve_prepare_mount_options(fc, mnt_data);
> + if (err)
> + goto free_mnt_data;
> +
> + err = parse_monolithic_mount_data(fc, mnt_data);
> if (!err) {
> down_write(&sb->s_umount);
> err = -EPERM;
> @@ -3338,6 +3343,10 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
>
> mnt_warn_timestamp_expiry(path, &mnt->mnt);
>
> +free_mnt_data:
> + if (mnt_data != data)
> + free_page((unsigned long)mnt_data);
> +err:
> put_fs_context(fc);
> return err;
> }
> @@ -3774,8 +3783,17 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
> subtype, strlen(subtype));
> if (!err && name)
> err = vfs_parse_fs_string(fc, "source", name, strlen(name));
> - if (!err)
> - err = parse_monolithic_mount_data(fc, data);
> + if (!err) {
> + void *mnt_data = legacy_merge_mount_data(fc, data);
> +
> + if (IS_ERR(mnt_data)) {
> + err = PTR_ERR(mnt_data);
> + } else {
> + err = parse_monolithic_mount_data(fc, mnt_data);
> + if (mnt_data != data)
> + free_page((unsigned long)mnt_data);
> + }
> + }
> if (!err && !mount_capable(fc))
> err = -EPERM;
> if (!err)
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