[PATCH 03/25] vfs: Allow fsinfo() to query what's in an fs_context [ver #14]

[David Howells]

Allow fsinfo() to be used to query the filesystem attached to an fs_context
once a superblock has been created or if it comes from fspick().

The caller must specify AT_FSINFO_FROM_FSOPEN in the parameters and must
supply the fd from fsopen() as dfd and must set filename to NULL.

This is done with something like:

        fd = fsopen("ext4", 0);
        ...
        struct fsinfo_params params = {
                .at_flags = AT_FSINFO_FROM_FSOPEN;
                ...
        };
        fsinfo(fd, NULL, &params, ...);

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

 fs/fsinfo.c                |   46 +++++++++++++++++++++++++++++++++++++++++++-
 fs/statfs.c                |    2 +-
 include/uapi/linux/fcntl.h |    2 ++
 3 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/fs/fsinfo.c b/fs/fsinfo.c
index 49b46f96dda3..c24701f994d1 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c
@@ -8,6 +8,7 @@
 #include <linux/security.h>
 #include <linux/uaccess.h>
 #include <linux/fsinfo.h>
+#include <linux/fs_context.h>
 #include <uapi/linux/mount.h>
 #include "internal.h"
 
@@ -340,6 +341,42 @@ static int vfs_fsinfo_fd(unsigned int fd, struct 
fsinfo_kparams *params)
        return ret;
 }
 
+/*
+ * Allow access to an fs_context object as created by fsopen() or fspick().
+ */
+static int vfs_fsinfo_fscontext(int fd, struct fsinfo_kparams *params)
+{
+       struct fs_context *fc;
+       struct fd f = fdget(fd);
+       int ret;
+
+       if (!f.file)
+               return -EBADF;
+
+       ret = -EINVAL;
+       if (f.file->f_op == &fscontext_fops)
+               goto out_f;
+       ret = -EOPNOTSUPP;
+       if (fc->ops == &legacy_fs_context_ops)
+               goto out_f;
+
+       ret = mutex_lock_interruptible(&fc->uapi_mutex);
+       if (ret == 0) {
+               ret = -EIO;
+               if (fc->root) {
+                       struct path path = { .dentry = fc->root };
+
+                       ret = vfs_fsinfo(&path, params);
+               }
+
+               mutex_unlock(&fc->uapi_mutex);
+       }
+
+out_f:
+       fdput(f);
+       return ret;
+}
+
 /*
  * Return buffer information by requestable attribute.
  *
@@ -445,6 +482,9 @@ SYSCALL_DEFINE5(fsinfo,
                params.request = user_params.request;
                params.Nth = user_params.Nth;
                params.Mth = user_params.Mth;
+
+               if ((params.at_flags & AT_FSINFO_FROM_FSOPEN) && filename)
+                       return -EINVAL;
        } else {
                params.request = FSINFO_ATTR_STATFS;
        }
@@ -453,6 +493,8 @@ SYSCALL_DEFINE5(fsinfo,
                user_buf_size = 0;
                user_buffer = NULL;
        }
+       if ((params.at_flags & AT_FSINFO_FROM_FSOPEN) && filename)
+               return -EINVAL;
 
        /* Allocate an appropriately-sized buffer.  We will truncate the
         * contents when we write the contents back to userspace.
@@ -500,7 +542,9 @@ SYSCALL_DEFINE5(fsinfo,
        if (!params.buffer)
                goto error_scratch;
 
-       if (filename)
+       if (params.at_flags & AT_FSINFO_FROM_FSOPEN)
+               ret = vfs_fsinfo_fscontext(dfd, &params);
+       else if (filename)
                ret = vfs_fsinfo_path(dfd, filename, &params);
        else
                ret = vfs_fsinfo_fd(dfd, &params);
diff --git a/fs/statfs.c b/fs/statfs.c
index eea7af6f2f22..b9b63d9f4f24 100644
--- a/fs/statfs.c
+++ b/fs/statfs.c
@@ -86,7 +86,7 @@ int vfs_statfs(const struct path *path, struct kstatfs *buf)
        int error;
 
        error = statfs_by_dentry(path->dentry, buf);
-       if (!error)
+       if (!error && path->mnt)
                buf->f_flags = calculate_f_flags(path->mnt);
        return error;
 }
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 1d338357df8a..6a2402a8fa30 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -91,6 +91,8 @@
 #define AT_STATX_FORCE_SYNC    0x2000  /* - Force the attributes to be sync'd 
with the server */
 #define AT_STATX_DONT_SYNC     0x4000  /* - Don't sync attributes with the 
server */
 
+#define AT_FSINFO_FROM_FSOPEN  0x2000  /* Examine the fs_context attached to 
dfd by fsopen() */
+
 #define AT_RECURSIVE           0x8000  /* Apply to the entire subtree */