watch_mount(), watch_sb(), and fsinfo() (again) [LWN.net]

watch_mount(), watch_sb(), and fsinfo() (again)

Posted Feb 24, 2020 23:09 UTC (Mon) by Cyberax (✭ supporter ✭, #52523)
Parent article: watch_mount(), watch_sb(), and fsinfo() (again)

> struct fsinfo_params {
> __u32 at_flags;
Sigh.

You know what's missing here? Yes, it's the explicit structure length.

to post comments

watch_mount(), watch_sb(), and fsinfo() (again)

Posted Feb 25, 2020 8:00 UTC (Tue) by geert (subscriber, #98403) [Link] (1 responses)

And the explicit padding between an odd number of 32-bit fields, and the 64-bit fields.
Fortunately the latter is present in the real declaration:

__u32 __reserved32[1]; /* Reserved params; all must be 0 */

Perhaps that can be traded for an explicit structure length?

watch_mount(), watch_sb(), and fsinfo() (again)

Posted Feb 26, 2020 2:19 UTC (Wed) by cyphar (subscriber, #110703) [Link]

Indeed, that was the entire point of copy_struct_from_user() -- so that new syscalls can be extensible and avoid the need to predict how many future fields you might want (I gave a talk about this at LCA this year). I've sent a suggestion to the ML, though there is some discussion on the latest thread to remove the need for fsinfo(2) entirely.