mnt: handle EROFS from mkdir/open on read-only mounts#260
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robertswiecki merged 1 commit intogoogle:masterfrom Apr 2, 2026
Merged
mnt: handle EROFS from mkdir/open on read-only mounts#260robertswiecki merged 1 commit intogoogle:masterfrom
robertswiecki merged 1 commit intogoogle:masterfrom
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mkdir() can return EROFS instead of EEXIST for existing entries on read-only mounts (e.g. NFS). Fall back to stat()/fstatat() on EROFS and proceed if the entry exists with the expected type.
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Hi @robertswiecki, just wanted to check if you've had a chance to look at this. Happy to adjust the approach if you'd prefer something different. We're hitting this in production with read-only NFS mounts and the EROFS handling has been solid on our side. Let me know if there's anything I can do to make this easier to review. |
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Thank you! |
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Problem
When using
--chrooton a read-only NFS export, nsjail aborts during mount target creation. Directories like/devand/tmpalready exist in the chroot image, butmkdir()returnsEROFSinstead ofEEXIST.On NFSv3/v4 with a cold dentry cache, the NFS client's exclusive-create optimization (
nfs_is_exclusive_createinfs/nfs/dir.c) skips the on-wire LOOKUP, leaving a negative dentry. The VFS then hits the deferredmnt_want_write()error before discovering the entry already exists.nsjail only tolerates
EEXISTaftermkdir/open(O_CREAT), soEROFSis treated as fatal (exit 255).Fix
On
EROFS, fall back tostat()/fstatat()and proceed if the entry exists with the expected type (S_ISDIR/S_ISREG). All 7 mkdir/mkdirat/open sites patched acrossmnt.cc,mnt_legacy.cc,mnt_newapi.cc, andutil.cc.