[Devel] [PATCH v2 VZ10 1/1] fs: namespace: transform mount flags to comma separated values
Vladimir Riabchun
vladimir.riabchun at virtuozzo.com
Tue Jun 2 19:44:18 MSK 2026
On 6/2/26 16:44, 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 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 | 114 ++++++++++++++++++++++++++++++++++++++++++++++++
> fs/internal.h | 8 ++++
> fs/namespace.c | 32 +++++++++++---
> 3 files changed, 147 insertions(+), 7 deletions(-)
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 76f34f3d468e..ae6ea684d596 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -81,6 +81,120 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
> return -ENOPARAM;
> }
>
> +#ifdef CONFIG_VE
> +/*
> + * Append s to the buffer at dst, preceded by a comma if len > 0.
> + * cap is the total buffer capacity (excluding the null terminator slot).
> + * Updates *len and returns the new write position, or ERR_PTR(-ENOSPC).
> + */
> +static char *append_entry(char *dst, size_t *len, size_t cap, const char *s)
> +{
> + size_t s_len = strlen(s);
> + size_t write_chars = (*len > 0) + s_len;
> +
> + if (*len + write_chars > cap)
> + return ERR_PTR(-ENOSPC);
> + if (*len)
> + *dst++ = ',';
> + memcpy(dst, s, s_len);
> + *len += write_chars;
> + return dst + s_len;
> +}
> +
> +/*
> + * Append fc->sb_flags as comma-separated option names into [*dstp, cap).
> + * Updates *dstp and *lenp on success.
> + * Matches vfs_parse_sb_flag() / common_{set,clear}_sb_flag tables.
> + */
> +static int vfs_format_sb_flags(struct fs_context *fc,
> + char **dstp, size_t *lenp, size_t cap)
> +{
> + const struct constant_table *p;
> + unsigned int flags = fc->sb_flags;
> + unsigned int mask = fc->sb_flags_mask;
> + char *dst = *dstp;
> +
> + for (p = common_set_sb_flag; p->name; p++) {
> + if (!(flags & p->value))
> + continue;
> + dst = append_entry(dst, lenp, cap, p->name);
> + if (IS_ERR(dst))
> + return PTR_ERR(dst);
> + }
> +
> + for (p = common_clear_sb_flag; p->name; p++) {
> + if (!(mask & p->value) || (flags & p->value))
> + continue;
> + dst = append_entry(dst, lenp, cap, p->name);
> + if (IS_ERR(dst))
> + return PTR_ERR(dst);
> + }
> +
> + *dstp = dst;
> + return 0;
> +}
> +
> +/*
> + * 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();
> + char *page, *dst;
> + size_t len = 0;
> + int err;
> +
My idea here was to allocate page and copy everything from data
here. Something like:
page = (char *)__get_free_page(GFP_KERNEL);
dst = page;
if (!page)
return ERR_PTR(-ENOMEM);
if (data)
dst = append_entry(dst, &len, PAGE_SIZE - 1, (char *)data);
and then always return new page.
I think it won't be a big overhead - only one temporal page for a big event - mount.
> + 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);
> +
> + dst = page;
> +
> + if (data && *(char *)data) {
> + dst = append_entry(dst, &len, PAGE_SIZE - 1, (char *)data);
> + if (IS_ERR(dst)) {
> + err = PTR_ERR(dst);
> + goto err_free;
> + }
> + }
> +
> + err = vfs_format_sb_flags(fc, &dst, &len, PAGE_SIZE - 1);
> + if (err)
> + goto err_free;
> +
> + if (!len) {
We can just return page, it's fine that it is empty, we already did
the hardest part - allocation.
> + free_page((unsigned long)page);
> + return data;
> + }
> +
> + *dst = '\0';
> +
> + return page;
> +
> +err_free:
> + free_page((unsigned long)page);
> + return ERR_PTR(err);
> +}
> +#endif /* CONFIG_VE */
> +
> /**
> * 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..120c239df394 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -46,6 +46,14 @@ 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 *);
> +#ifdef CONFIG_VE
> +void *legacy_merge_mount_data(struct fs_context *fc, void *data);
> +#else
> +static inline void *legacy_merge_mount_data(struct fs_context *fc, void *data)
> +{
> + return data;
> +}
> +#endif
> 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..b2defe2006ca 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 we always return new page, we can just do free_page without any checks.
> + if (mnt_data && 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)
--
Best regards, Riabchun Vladimir
Linux Kernel Developer, Virtuozzo
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