From 8f6116b5b77b0536d2ad7482ee42bfe58b8fac01 Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Thu, 6 Feb 2025 07:51:52 -0500 Subject: [PATCH] statmount: add a new supported_mask field Some of the fields in the statmount() reply can be optional. If the kernel has nothing to emit in that field, then it doesn't set the flag in the reply. This presents a problem: There is currently no way to know what mask flags the kernel supports since you can't always count on them being in the reply. Add a new STATMOUNT_SUPPORTED_MASK flag and field that the kernel can set in the reply. Userland can use this to determine if the fields it requires from the kernel are supported. This also gives us a way to deprecate fields in the future, if that should become necessary. Reviewed-by: Jan Kara Signed-off-by: Jeff Layton Link: https://lore.kernel.org/r/20250206-statmount-v2-1-6ae70a21c2ab@kernel.org Signed-off-by: Christian Brauner --- fs/namespace.c | 23 +++++++++++++++++++++++ include/uapi/linux/mount.h | 4 +++- 2 files changed, 26 insertions(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index d470a369d42b..c5ada826d488 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -5410,6 +5410,21 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root) return 0; } +/* This must be updated whenever a new flag is added */ +#define STATMOUNT_SUPPORTED (STATMOUNT_SB_BASIC | \ + STATMOUNT_MNT_BASIC | \ + STATMOUNT_PROPAGATE_FROM | \ + STATMOUNT_MNT_ROOT | \ + STATMOUNT_MNT_POINT | \ + STATMOUNT_FS_TYPE | \ + STATMOUNT_MNT_NS_ID | \ + STATMOUNT_MNT_OPTS | \ + STATMOUNT_FS_SUBTYPE | \ + STATMOUNT_SB_SOURCE | \ + STATMOUNT_OPT_ARRAY | \ + STATMOUNT_OPT_SEC_ARRAY | \ + STATMOUNT_SUPPORTED_MASK) + static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id, struct mnt_namespace *ns) { @@ -5479,9 +5494,17 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id, if (!err && s->mask & STATMOUNT_MNT_NS_ID) statmount_mnt_ns_id(s, ns); + if (!err && s->mask & STATMOUNT_SUPPORTED_MASK) { + s->sm.mask |= STATMOUNT_SUPPORTED_MASK; + s->sm.supported_mask = STATMOUNT_SUPPORTED; + } + if (err) return err; + /* Are there bits in the return mask not present in STATMOUNT_SUPPORTED? */ + WARN_ON_ONCE(~STATMOUNT_SUPPORTED & s->sm.mask); + return 0; } diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index c07008816aca..c553dc4ba684 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -179,7 +179,8 @@ struct statmount { __u32 opt_array; /* [str] Array of nul terminated fs options */ __u32 opt_sec_num; /* Number of security options */ __u32 opt_sec_array; /* [str] Array of nul terminated security options */ - __u64 __spare2[46]; + __u64 supported_mask; /* Mask flags that this kernel supports */ + __u64 __spare2[45]; char str[]; /* Variable size part containing strings */ }; @@ -217,6 +218,7 @@ struct mnt_id_req { #define STATMOUNT_SB_SOURCE 0x00000200U /* Want/got sb_source */ #define STATMOUNT_OPT_ARRAY 0x00000400U /* Want/got opt_... */ #define STATMOUNT_OPT_SEC_ARRAY 0x00000800U /* Want/got opt_sec... */ +#define STATMOUNT_SUPPORTED_MASK 0x00001000U /* Want/got supported mask flags */ /* * Special @mnt_id values that can be passed to listmount -- 2.25.1