vfs: skip write tracking for special files on open#13149
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Linux skips the mount-level write check when opening a character device, block device, FIFO, or socket, so write-opening /dev/null on a read-only bind mount succeeds. gVisor was running that check unconditionally and returning EROFS, which breaks buildah's mount-bind-then-RO-remount hardening pattern when used under runsc. Add an IsSpecialFile option on FileDescription, paired across the open and close paths so the writer counter stays balanced, and set it on every open path that backs a special-file inode. The socket and host-imported FDs live on internal disconnected mounts that are never remounted RO, so flagging them is a no-op today and exists for parity with the kernel's predicate.
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Linux skips the mount-level write check when opening a character device, block device, FIFO, or socket, so write-opening /dev/null on a read-only bind mount succeeds. gVisor was running that check unconditionally and returning EROFS, which breaks buildah's mount-bind-then-RO-remount hardening pattern when used under runsc.
This adds an
IsSpecialFileoption onFileDescription, paired across the open and close paths so the writer counter stays balanced, and set it on every open path that backs a special-file inode. The socket and host-imported FDs live on internal disconnected mounts that are never remounted RO, so flagging them is a no-op today and exists for parity with the kernel's predicate.Fixes: #13148