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    Subject[PATCH 04/17] fsinfo: Allow retrieval of superblock devname, options and stats [ver #20]
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    Provide fsinfo() attributes to retrieve superblock device name, options,
    and statistics in string form. The following attributes are defined:

    FSINFO_ATTR_SOURCE - Mount-specific device name
    FSINFO_ATTR_CONFIGURATION - Mount options
    FSINFO_ATTR_FS_STATISTICS - Filesystem statistics

    FSINFO_ATTR_SOURCE could be made indexable by params->Nth to handle the
    case where there is more than one source (e.g. the bcachefs filesystem).

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    fs/fsinfo.c | 39 +++++++++++++++++++++++++++++++++++++++
    fs/internal.h | 2 ++
    fs/namespace.c | 41 +++++++++++++++++++++++++++++++++++++++++
    include/uapi/linux/fsinfo.h | 3 +++
    samples/vfs/test-fsinfo.c | 4 ++++
    5 files changed, 89 insertions(+)

    diff --git a/fs/fsinfo.c b/fs/fsinfo.c
    index 79c222d465d8..aef7a736e8fc 100644
    --- a/fs/fsinfo.c
    +++ b/fs/fsinfo.c
    @@ -231,6 +231,42 @@ static int fsinfo_generic_volume_id(struct path *path, struct fsinfo_context *ct
    return fsinfo_string(path->dentry->d_sb->s_id, ctx);
    }

    +/*
    + * Retrieve the superblock configuration (mount options) as a comma-separated
    + * string. The initial comma is stripped off and NUL termination is added.
    + */
    +static int fsinfo_generic_seq_read(struct path *path, struct fsinfo_context *ctx)
    +{
    + struct super_block *sb = path->dentry->d_sb;
    + struct seq_file m = {
    + .buf = ctx->buffer,
    + .size = ctx->buf_size - 1,
    + };
    + int ret = 0;
    +
    + switch (ctx->requested_attr) {
    + case FSINFO_ATTR_CONFIGURATION:
    + seq_puts(&m, sb_rdonly(sb) ? "ro" : "rw");
    + ret = security_sb_show_options(&m, sb);
    + if (!ret && sb->s_op->show_options)
    + ret = sb->s_op->show_options(&m, path->mnt->mnt_root);
    + break;
    +
    + case FSINFO_ATTR_FS_STATISTICS:
    + if (sb->s_op->show_stats)
    + ret = sb->s_op->show_stats(&m, path->mnt->mnt_root);
    + break;
    + }
    +
    + if (ret < 0)
    + return ret;
    + if (seq_has_overflowed(&m))
    + return ctx->buf_size + PAGE_SIZE;
    +
    + ((char *)ctx->buffer)[ctx->skip + m.count] = 0;
    + return m.count + 1;
    +}
    +
    static const struct fsinfo_attribute fsinfo_common_attributes[] = {
    FSINFO_VSTRUCT (FSINFO_ATTR_STATFS, fsinfo_generic_statfs),
    FSINFO_VSTRUCT (FSINFO_ATTR_IDS, fsinfo_generic_ids),
    @@ -240,6 +276,9 @@ static const struct fsinfo_attribute fsinfo_common_attributes[] = {
    FSINFO_STRING (FSINFO_ATTR_VOLUME_ID, fsinfo_generic_volume_id),
    FSINFO_VSTRUCT (FSINFO_ATTR_VOLUME_UUID, fsinfo_generic_volume_uuid),
    FSINFO_VSTRUCT (FSINFO_ATTR_FEATURES, fsinfo_generic_features),
    + FSINFO_STRING (FSINFO_ATTR_SOURCE, fsinfo_generic_mount_source),
    + FSINFO_STRING (FSINFO_ATTR_CONFIGURATION, fsinfo_generic_seq_read),
    + FSINFO_STRING (FSINFO_ATTR_FS_STATISTICS, fsinfo_generic_seq_read),

    FSINFO_LIST (FSINFO_ATTR_FSINFO_ATTRIBUTES, (void *)123UL),
    FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, (void *)123UL),
    diff --git a/fs/internal.h b/fs/internal.h
    index ea60d864a8cb..0b57da498f06 100644
    --- a/fs/internal.h
    +++ b/fs/internal.h
    @@ -89,6 +89,8 @@ extern int __mnt_want_write_file(struct file *);
    extern void __mnt_drop_write_file(struct file *);

    extern void dissolve_on_fput(struct vfsmount *);
    +extern int fsinfo_generic_mount_source(struct path *, struct fsinfo_context *);
    +
    /*
    * fs_struct.c
    */
    diff --git a/fs/namespace.c b/fs/namespace.c
    index 73ff5bf0c9af..ead8d1a16610 100644
    --- a/fs/namespace.c
    +++ b/fs/namespace.c
    @@ -30,6 +30,7 @@
    #include <uapi/linux/mount.h>
    #include <linux/fs_context.h>
    #include <linux/shmem_fs.h>
    +#include <linux/fsinfo.h>

    #include "pnode.h"
    #include "internal.h"
    @@ -4111,3 +4112,43 @@ const struct proc_ns_operations mntns_operations = {
    .install = mntns_install,
    .owner = mntns_owner,
    };
    +
    +#ifdef CONFIG_FSINFO
    +static inline void mangle(struct seq_file *m, const char *s)
    +{
    + seq_escape(m, s, " \t\n\\");
    +}
    +
    +/*
    + * Return the mount source/device name as seen from this mountpoint. Shared
    + * mounts may vary here and the filesystem is permitted to substitute its own
    + * rendering.
    + */
    +int fsinfo_generic_mount_source(struct path *path, struct fsinfo_context *ctx)
    +{
    + struct super_block *sb = path->mnt->mnt_sb;
    + struct mount *mnt = real_mount(path->mnt);
    + struct seq_file m = {
    + .buf = ctx->buffer,
    + .size = ctx->buf_size - 1,
    + };
    + int ret;
    +
    + if (sb->s_op->show_devname) {
    + ret = sb->s_op->show_devname(&m, mnt->mnt.mnt_root);
    + if (ret < 0)
    + return ret;
    + } else {
    + if (!mnt->mnt_devname)
    + return fsinfo_string("none", ctx);
    + mangle(&m, mnt->mnt_devname);
    + }
    +
    + if (seq_has_overflowed(&m))
    + return ctx->buf_size + PAGE_SIZE;
    +
    + ((char *)ctx->buffer)[m.count] = 0;
    + return m.count + 1;
    +}
    +
    +#endif /* CONFIG_FSINFO */
    diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
    index b8b2c836267b..a27e92b68266 100644
    --- a/include/uapi/linux/fsinfo.h
    +++ b/include/uapi/linux/fsinfo.h
    @@ -24,6 +24,9 @@
    #define FSINFO_ATTR_VOLUME_UUID 0x06 /* Volume UUID (LE uuid) */
    #define FSINFO_ATTR_VOLUME_NAME 0x07 /* Volume name (string) */
    #define FSINFO_ATTR_FEATURES 0x08 /* Filesystem features (bits) */
    +#define FSINFO_ATTR_SOURCE 0x09 /* Superblock source/device name (string) */
    +#define FSINFO_ATTR_CONFIGURATION 0x0a /* Superblock configuration/options (string) */
    +#define FSINFO_ATTR_FS_STATISTICS 0x0b /* Superblock filesystem statistics (string) */

    #define FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO 0x100 /* Information about attr N (for path) */
    #define FSINFO_ATTR_FSINFO_ATTRIBUTES 0x101 /* List of supported attrs (for path) */
    diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
    index c5932109f683..634f30b7e67f 100644
    --- a/samples/vfs/test-fsinfo.c
    +++ b/samples/vfs/test-fsinfo.c
    @@ -364,6 +364,10 @@ static const struct fsinfo_attribute fsinfo_attributes[] = {
    FSINFO_STRING (FSINFO_ATTR_VOLUME_ID, string),
    FSINFO_VSTRUCT (FSINFO_ATTR_VOLUME_UUID, fsinfo_generic_volume_uuid),
    FSINFO_STRING (FSINFO_ATTR_VOLUME_NAME, string),
    + FSINFO_STRING (FSINFO_ATTR_SOURCE, string),
    + FSINFO_STRING (FSINFO_ATTR_CONFIGURATION, string),
    + FSINFO_STRING (FSINFO_ATTR_FS_STATISTICS, string),
    +
    FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, fsinfo_meta_attribute_info),
    FSINFO_LIST (FSINFO_ATTR_FSINFO_ATTRIBUTES, fsinfo_meta_attributes),
    {}

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