From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DF7F31EEE6 for ; Thu, 30 Jan 2025 12:37:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738240658; cv=none; b=RXbI8vnKZ/1vRJQYshJ4hd6Eax1/LvnbJeR8fFs8We/OGddjRBweCp6qdFIX3nTpJLjsxR/jmzFZGad70hs8pM52+v+OiZBfIF/BqkMcirti08gIEeuOZXNo6AoRDcHQ4yBXVQN+lrHDyZM/oOLrSj/Ohib+n8s75ijx/hnZ2Ho= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738240658; c=relaxed/simple; bh=k6R1Q4zpwVXM8eSkbtak8n4hkEL+XBToRJIFriNI0rM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Pu2H8jS0ndWqbzFAj8uwkZKIbFKWDanyaG5YZ3fpWJDnVMYitlcpMSLspaM7qqgdwOS4ljxmJQFucqVDl3LeisZ3LcA8Rtw4eCz94AgCgl+UfNMigwkGoTibH3fJwdR/90mFRVPWo8MSdNHXS6IwZmPYqMkbnJ/PBsi9v/KSQuI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gXi2AH24; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gXi2AH24" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ADC69C4CED2; Thu, 30 Jan 2025 12:37:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738240657; bh=k6R1Q4zpwVXM8eSkbtak8n4hkEL+XBToRJIFriNI0rM=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=gXi2AH24IKpLVOYMuvnKNjxuyOxRQAt/BsnkdprN89kqBSqBhegHrL1fR3Cq5RIDO F69TU3XWfSM6Oke08MWiKH0SynucXJ0sS+de5sx7pSxy25/ObfGIbaU/CfEU9/chEG USOS6dcMSvYb68lvnqJQ0Y8n2iR65QwCdxTVGBQSMl0GJLvEBKCYkNzrCJB7oIhXXx cugUI+uh7PfIPI45ZYN7Sq443c9GHDA530la1xsl7lZTYyrzgXawPZjtdR8dcW49gx pmXLGdrLoqyJx/6sFQFJD7ug67FVvFyZtKHobyYDcq06TUqGJ4hTQw/RBe7Z1X/phV UyDmJQT9k/DLw== Message-ID: Subject: Re: [PATCH 2/4] statmount: allow to retrieve idmappings From: Jeff Layton To: Christian Brauner , linux-fsdevel@vger.kernel.org Cc: Josef Bacik , Lennart Poettering , Daan De Meyer , Seth Forshee , Miklos Szeredi Date: Thu, 30 Jan 2025 07:37:35 -0500 In-Reply-To: <20250130-work-mnt_idmap-statmount-v1-2-d4ced5874e14@kernel.org> References: <20250130-work-mnt_idmap-statmount-v1-0-d4ced5874e14@kernel.org> <20250130-work-mnt_idmap-statmount-v1-2-d4ced5874e14@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Thu, 2025-01-30 at 00:19 +0100, Christian Brauner wrote: > This adds the STATMOUNT_MNT_UIDMAP and STATMOUNT_MNT_GIDMAP options. > It allows the retrieval of idmappings via statmount(). >=20 > Currently it isn't possible to figure out what idmappings are applied to > an idmapped mount. This information is often crucial. Before statmount() > the only realistic options for an interface like this would have been to > add it to /proc//fdinfo/ or to expose it in > /proc//mountinfo. Both solution would have been pretty ugly and > would've shown information that is of strong interest to some > application but not all. statmount() is perfect for this. >=20 > The idmappings applied to an idmapped mount are shown relative to the > caller's user namespace. This is the most useful solution that doesn't > risk leaking information or confuse the caller. >=20 > For example, an idmapped mount might have been created with the > following idmappings: >=20 > mount --bind -o X-mount.idmap=3D"0:10000:1000 2000:2000:1 3000:3000:1= " /srv /opt >=20 > Listing the idmappings through statmount() in the same context shows: >=20 > mnt_id: 2147485088 > mnt_parent_id: 2147484816 > fs_type: btrfs > mnt_root: /srv > mnt_point: /opt > mnt_opts: ssd,discard=3Dasync,space_cache=3Dv2,subvolid=3D5,subv= ol=3D/ > mnt_uidmap[0]: 0 10000 1000 > mnt_uidmap[1]: 2000 2000 1 > mnt_uidmap[2]: 3000 3000 1 > mnt_gidmap[0]: 0 10000 1000 > mnt_gidmap[1]: 2000 2000 1 > mnt_gidmap[2]: 3000 3000 1 >=20 > But the idmappings might not always be resolvablein the caller's user > namespace. For example: >=20 > unshare --user --map-root >=20 > In this case statmount() will indicate the failure to resolve the idmappi= ngs > in the caller's user namespace by listing 4294967295 aka (uid_t) -1 as > the target of the mapping while still showing the source and range of > the mapping: >=20 > mnt_id: 2147485087 > mnt_parent_id: 2147484016 > fs_type: btrfs > mnt_root: /srv > mnt_point: /opt > mnt_opts: ssd,discard=3Dasync,space_cache=3Dv2,subvolid=3D5,subv= ol=3D/ > mnt_uidmap[0]: 0 4294967295 1000 > mnt_uidmap[1]: 2000 4294967295 1 > mnt_uidmap[2]: 3000 4294967295 1 > mnt_gidmap[0]: 0 4294967295 1000 > mnt_gidmap[1]: 2000 4294967295 1 > mnt_gidmap[2]: 3000 4294967295 1 >=20 > Note that statmount() requires that the whole range must be resolvable > in the caller's user namespace. If a subrange fails to map it will still > list the map as not resolvable. This is a practical compromise to avoid > having to find which subranges are resovable and wich aren't. >=20 > Idmappings are listed as a string array with each mapping separated by > zero bytes. This allows to retrieve the idmappings and immediately use > them for writing to e.g., /proc//{g,u}id_map and it also allow for > simple iteration like: >=20 > if (stmnt->mask & STATMOUNT_MNT_UIDMAP) { > const char *idmap =3D stmnt->str + stmnt->mnt_uidmap; >=20 > for (size_t idx =3D 0; idx < stmnt->mnt_uidmap_nr; idx++) { > printf("mnt_uidmap[%lu]: %s\n", idx, idmap); > idmap +=3D strlen(idmap) + 1; > } > } >=20 > Signed-off-by: Christian Brauner > --- > fs/internal.h | 1 + > fs/mnt_idmapping.c | 49 ++++++++++++++++++++++++++++++++++++++++= ++++++ > fs/namespace.c | 43 +++++++++++++++++++++++++++++++++++++++- > include/uapi/linux/mount.h | 8 +++++++- > 4 files changed, 99 insertions(+), 2 deletions(-) >=20 > diff --git a/fs/internal.h b/fs/internal.h > index e7f02ae1e098..db6094d5cb0b 100644 > --- a/fs/internal.h > +++ b/fs/internal.h > @@ -338,3 +338,4 @@ static inline bool path_mounted(const struct path *pa= th) > return path->mnt->mnt_root =3D=3D path->dentry; > } > void file_f_owner_release(struct file *file); > +int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, b= ool uid_map); > diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c > index 7b1df8cc2821..4aca8e3ba97e 100644 > --- a/fs/mnt_idmapping.c > +++ b/fs/mnt_idmapping.c > @@ -6,6 +6,7 @@ > #include > #include > #include > +#include > =20 > #include "internal.h" > =20 > @@ -334,3 +335,51 @@ void mnt_idmap_put(struct mnt_idmap *idmap) > free_mnt_idmap(idmap); > } > EXPORT_SYMBOL_GPL(mnt_idmap_put); > + > +int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, b= ool uid_map) > +{ > + struct uid_gid_map *map, *map_up; > + > + if (idmap =3D=3D &nop_mnt_idmap || idmap =3D=3D &invalid_mnt_idmap) > + return 0; > + > + /* > + * Idmappings are shown relative to the caller's idmapping. > + * This is both the most intuitive and most useful solution. > + */ > + if (uid_map) { > + map =3D &idmap->uid_map; > + map_up =3D ¤t_user_ns()->uid_map; > + } else { > + map =3D &idmap->gid_map; > + map_up =3D ¤t_user_ns()->gid_map; > + } > + > + for (u32 idx =3D 0; idx < map->nr_extents; idx++) { > + uid_t lower; > + struct uid_gid_extent *extent; > + > + if (map->nr_extents <=3D UID_GID_MAP_MAX_BASE_EXTENTS) > + extent =3D &map->extent[idx]; > + else > + extent =3D &map->forward[idx]; > + > + /* > + * Verify that the whole range of the mapping can be > + * resolved in the caller's idmapping. If it cannot be > + * resolved 1/4294967295 will be shown as the target of nit: I think you mean '-1/4294967295'. > + * the mapping. The source and range are shown as a hint > + * to the caller. > + */ > + lower =3D map_id_range_up(map_up, extent->lower_first, extent->count); > + if (lower =3D=3D (uid_t) -1) > + seq_printf(seq, "%u %u %u", extent->first, -1, extent->count); > + else > + seq_printf(seq, "%u %u %u", extent->first, lower, extent->count); Again, I think a different syntax for an unresolveable range would be better. Another idea -- if you separate the fields by ':', you could just leave out the middle field when it can't be resolved -- e.g. "1000::1000". > + seq->count++; /* mappings are separated by \0 */ > + if (seq_has_overflowed(seq)) > + return -EAGAIN; > + } > + > + return map->nr_extents; > +} > diff --git a/fs/namespace.c b/fs/namespace.c > index 4013fbac354a..535e4829061f 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -4915,6 +4915,7 @@ struct kstatmount { > struct statmount __user *buf; > size_t bufsize; > struct vfsmount *mnt; > + struct mnt_idmap *idmap; > u64 mask; > struct path root; > struct statmount sm; > @@ -5185,6 +5186,30 @@ static int statmount_opt_sec_array(struct kstatmou= nt *s, struct seq_file *seq) > return 0; > } > =20 > +static inline int statmount_mnt_uidmap(struct kstatmount *s, struct seq_= file *seq) > +{ > + int ret; > + > + ret =3D statmount_mnt_idmap(s->idmap, seq, true); > + if (ret < 0) > + return ret; > + > + s->sm.mnt_uidmap_num =3D ret; > + return 0; > +} > + > +static inline int statmount_mnt_gidmap(struct kstatmount *s, struct seq_= file *seq) > +{ > + int ret; > + > + ret =3D statmount_mnt_idmap(s->idmap, seq, false); > + if (ret < 0) > + return ret; > + > + s->sm.mnt_gidmap_num =3D ret; > + return 0; > +} > + > static int statmount_string(struct kstatmount *s, u64 flag) > { > int ret =3D 0; > @@ -5226,6 +5251,14 @@ static int statmount_string(struct kstatmount *s, = u64 flag) > sm->sb_source =3D start; > ret =3D statmount_sb_source(s, seq); > break; > + case STATMOUNT_MNT_UIDMAP: > + sm->mnt_uidmap =3D start; > + ret =3D statmount_mnt_uidmap(s, seq); > + break; > + case STATMOUNT_MNT_GIDMAP: > + sm->mnt_gidmap =3D start; > + ret =3D statmount_mnt_gidmap(s, seq); > + break; > default: > WARN_ON_ONCE(true); > return -EINVAL; > @@ -5350,6 +5383,7 @@ static int do_statmount(struct kstatmount *s, u64 m= nt_id, u64 mnt_ns_id, > return err; > =20 > s->root =3D root; > + s->idmap =3D mnt_idmap(s->mnt); > if (s->mask & STATMOUNT_SB_BASIC) > statmount_sb_basic(s); > =20 > @@ -5383,6 +5417,12 @@ static int do_statmount(struct kstatmount *s, u64 = mnt_id, u64 mnt_ns_id, > if (!err && s->mask & STATMOUNT_SB_SOURCE) > err =3D statmount_string(s, STATMOUNT_SB_SOURCE); > =20 > + if (!err && s->mask & STATMOUNT_MNT_UIDMAP) > + err =3D statmount_string(s, STATMOUNT_MNT_UIDMAP); > + > + if (!err && s->mask & STATMOUNT_MNT_GIDMAP) > + err =3D statmount_string(s, STATMOUNT_MNT_GIDMAP); > + > if (!err && s->mask & STATMOUNT_MNT_NS_ID) > statmount_mnt_ns_id(s, ns); > =20 > @@ -5406,7 +5446,8 @@ static inline bool retry_statmount(const long ret, = size_t *seq_size) > #define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT |= \ > STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | \ > STATMOUNT_FS_SUBTYPE | STATMOUNT_SB_SOURCE | \ > - STATMOUNT_OPT_ARRAY | STATMOUNT_OPT_SEC_ARRAY) > + STATMOUNT_OPT_ARRAY | STATMOUNT_OPT_SEC_ARRAY | \ > + STATMOUNT_MNT_UIDMAP | STATMOUNT_MNT_GIDMAP) > =20 > static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *= kreq, > struct statmount __user *buf, size_t bufsize, > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h > index c07008816aca..0be6ac4c1624 100644 > --- a/include/uapi/linux/mount.h > +++ b/include/uapi/linux/mount.h > @@ -179,7 +179,11 @@ 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]; > + __u32 mnt_uidmap_num; /* Number of uid mappings */ > + __u32 mnt_uidmap; /* [str] Array of uid mappings (as seen from callers = namespace) */ > + __u32 mnt_gidmap_num; /* Number of gid mappings */ > + __u32 mnt_gidmap; /* [str] Array of gid mappings (as seen from callers = namespace) */ > + __u64 __spare2[44]; > char str[]; /* Variable size part containing strings */ > }; > =20 > @@ -217,6 +221,8 @@ 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_MNT_UIDMAP 0x00001000U /* Want/got uidmap... */ > +#define STATMOUNT_MNT_GIDMAP 0x00002000U /* Want/got gidmap... */ > =20 > /* > * Special @mnt_id values that can be passed to listmount >=20 --=20 Jeff Layton