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 A9596206F3A for ; Tue, 4 Feb 2025 12:38:12 +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=1738672692; cv=none; b=Y7fcixf42z312TgxEY3rFz+FT7TgeHN/PSYHFKO6txlxbq2jDzarsIcqxQ7fLD0BnVjDOHPlge5+RDpQB/P3fKRtq1pBvQJhvyr73wbbpseWPq5QeENZBHPHfA6IztlHsE1jYwiVYn0tT2umCmwDK39Qcu0iu5TZ7RCwjvlGR+0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738672692; c=relaxed/simple; bh=eo0COx2iTEMDcOo0xtXARoiFwSJ7fGBTHN89G0lGvWw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Hx9W5EywRs4xteNyavxeB7XjmPquXpl1SWmvpG9fCOWchU3rk8xTwk019/6RyC80Sm1JSbABi+xxVf2Clkdpty1CdI9IUR0e75254qJR33Bl3cr0UH+8PtT6Sfx8+SwI/RKjGAYgbrTAVF7nE8Y178gGwoVVjvOXNY6AnGr/U3U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=q4fhxhk9; 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="q4fhxhk9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D49C9C4CEDF; Tue, 4 Feb 2025 12:38:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738672692; bh=eo0COx2iTEMDcOo0xtXARoiFwSJ7fGBTHN89G0lGvWw=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=q4fhxhk96Dy0HWNjnX5u/bhpV3lORsxa5dolBbx3pnFCX+70wUx6AJp39p8E2fpQB /x2PayK23dAZ2FLGNGjZIAWzsAFYgOjNIYL2r0ntWP/xi1qVG/S6WwP9zQrfdRLXOE p3y4WMi8+RnNb4sJCv0YTfafTJYxvLi8gbJlGtAfjXn+5gTYiFmm5sO15ItCqrzqh+ FZ2I9TZSnp4pXUDCMNRaS8VzLiYH3txVtv+EW4MNAbO51MYSnz3BrqwYe/0oSDnodj skmwf7DWVh5YbtdS5uscHSKrLpj/Hrwr8vJKLFbngOSMMRdNvzFtnkpdb6ZjnUtTxj eN9sLtHWW8JEw== Message-ID: Subject: Re: [PATCH v2 0/4] 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: Tue, 04 Feb 2025 07:38:10 -0500 In-Reply-To: <20250204-work-mnt_idmap-statmount-v2-0-007720f39f2e@kernel.org> References: <20250204-work-mnt_idmap-statmount-v2-0-007720f39f2e@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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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 resolvable in the caller's user > namespace. For example: >=20 > unshare --user --map-root >=20 > In this case statmount() will skip any mappings that fil to resolve in > the caller's idmapping: >=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/ >=20 > The caller can differentiate between a mount not being idmapped and a > mount that is idmapped but where all mappings fail to resolve in the > caller's idmapping by check for the STATMOUNT_MNT_{G,U}IDMAP flag being > raised but the number of mappings in ->mnt_{g,u}idmap_num being zero. >=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 > --- > Changes in v2: > - EDITME: describe what is new in this series revision. > - EDITME: use bulletpoints and terse descriptions. > - Link to v1: https://lore.kernel.org/r/20250130-work-mnt_idmap-statmount= -v1-0-d4ced5874e14@kernel.org >=20 > --- > Christian Brauner (4): > uidgid: add map_id_range_up() > statmount: allow to retrieve idmappings > samples/vfs: check whether flag was raised > samples/vfs: add STATMOUNT_MNT_{G,U}IDMAP >=20 > fs/internal.h | 1 + > fs/mnt_idmapping.c | 51 ++++++++++++++++++++++++++++++++ > fs/namespace.c | 59 ++++++++++++++++++++++++++++++++= +++++- > include/linux/mnt_idmapping.h | 5 ++++ > include/linux/uidgid.h | 6 ++++ > include/uapi/linux/mount.h | 8 +++++- > kernel/user_namespace.c | 26 +++++++++++------ > samples/vfs/samples-vfs.h | 14 ++++++++- > samples/vfs/test-list-all-mounts.c | 35 ++++++++++++++++++---- > 9 files changed, 187 insertions(+), 18 deletions(-) > --- > base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b > change-id: 20250129-work-mnt_idmap-statmount-e57f258fef8e >=20 This all looks pretty sane to me. Reviewed-by: Jeff Layton