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Thunderbird Subject: Re: [PATCH] btrfs: use mount idmap for defrag permission check To: Seth Forshee , Tao Cui , Qu Wenruo Cc: clm@fb.com, dsterba@suse.com, josef@toxicpanda.com, brauner@kernel.org, linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Tao Cui References: <20260813034146.1207640-1-cui.tao@linux.dev> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNIlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT7CwJQEEwEIAD4CGwMFCwkI BwIGFQgJCgsCBBYCAwECHgECF4AWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCZxF1YAUJEP5a 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y1L6pc3Emwqdx8IlnwH5slGmvLzzxv8nQ9hG1U/Ubnvqv9W99myGUVfixpKXiI6k1oy62EZ+V p1nt2Ja6YOO+hphhc7AwyOOB2bQ8Iw1g9wbsRVPIMILlD/7Zyq1n7KNu9c/EILa5kgodM14Po 22aXJwh7RDPO/q/SEhCocQg0Y7NE= =E5=9C=A8 2026/8/14 02:41, Seth Forshee =E5=86=99=E9=81=93: > On Thu, Aug 13, 2026 at 04:04:05PM +0930, Qu Wenruo wrote: >> >> >> =E5=9C=A8 2026/8/13 13:11, Tao Cui =E5=86=99=E9=81=93: >>> From: Tao Cui >>> >>> btrfs_ioctl_defrag() checks MAY_WRITE with nop_mnt_idmap, which skips >>> the mount idmap. On an idmapped mount the owner comparison then uses >>> the caller's fsuid against the raw on-disk uid, dropping the mapping. >>> Every other permission/owner check in btrfs ioctl uses >>> file_mnt_idmap(file) (e.g. :1152, :1310, :1946); this one missed it. >>> >>> Switch to file_mnt_idmap(file). It equals nop_mnt_idmap on a normal >>> mount, and the check stays behind !capable(CAP_SYS_ADMIN), so only >>> unprivileged callers on idmapped btrfs change. The RO-fd note in the >>> comment above is about the file descriptor, not this inode check, and >>> is unaffected. >>> >>> Signed-off-by: Tao Cui >> >> Fixes: 4609e1f18e19 ("fs: port ->permission() to pass mnt_idmap") >=20 > This is a misattribution. Using nop_mnt_idmap there keeps the behavior > the same as it was before the commit. Switching to the mount idmap opens > up BTRFS_IOC_DEFRAG* to idmapped users when they weren't before, which > is a separate policy decision that didn't belong in that change. OK, removed from for-next, and will not add the fixes tag. >=20 > Saying that these ioctls should be allowed for idmapped users just > because others are isn't a good justification. Why are these ioctls safe > for idmapped users? Do they actually require this capability? I'll let the author to do the explanation. Thanks, Qu >=20 > Thanks, > Seth >=20