On Tue, Aug 27, 2024 at 10:32:38AM +0800, Hongbo Li wrote:

On 2024/8/27 10:13, Darrick J. Wong wrote:

quoted

On Tue, Aug 27, 2024 at 01:41:08AM +0000, Hongbo Li wrote:

quoted

Many mainstream file systems already support the GETVERSION ioctl,
and their implementations are completely the same, essentially
just obtain the value of i_generation. We think this ioctl can be
implemented at the VFS layer, so the file systems do not need to
implement it individually.

What if a filesystem never touches i_generation?  Is it ok to advertise
a generation number of zero when that's really meaningless?  Or should
we gate the generic ioctl on (say) whether or not the fs implements file
handles and/or supports nfs?

This ioctl mainly returns the i_generation, and whether it has meaning is up
to the specific file system. Some tools will invoke IOC_GETVERSION, such as
`lsattr -v`(but if it's lattr, it won't), but users may not necessarily
actually use this value.

That's not how that works.  If the kernel starts exporting a datum,
people will start using it, and then the expectation that it will
*continue* to work becomes ingrained in the userspace ABI forever.
Be careful about establishing new behaviors for vfat.

--D

Thanks,
Hongbo

quoted

--D

quoted

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
---
  fs/ioctl.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 64776891120c..dff887ec52c4 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c

@@ -878,6 +878,9 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
  	case FS_IOC_GETFSUUID:
  		return ioctl_getfsuuid(filp, argp);
+	case FS_IOC_GETVERSION:
+		return put_user(inode->i_generation, (int __user *)argp);
+
  	case FS_IOC_GETFSSYSFSPATH:
  		return ioctl_get_fs_sysfs_path(filp, argp);

@@ -992,6 +995,9 @@ COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
  		cmd = (cmd == FS_IOC32_GETFLAGS) ?
  			FS_IOC_GETFLAGS : FS_IOC_SETFLAGS;
  		fallthrough;
+	case FS_IOC32_GETVERSION:
+		cmd = FS_IOC_GETVERSION;
+		fallthrough;
  	/*
  	 * everything else in do_vfs_ioctl() takes either a compatible
  	 * pointer argument or no argument -- call it with a modified

-- 
2.34.1