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bpf: add syscall side map freeze support
This patch adds a new BPF_MAP_FREEZE command which allows to "freeze" the map globally as read-only / immutable from syscall side. Map permission handling has been refactored into map_get_sys_perms() and drops FMODE_CAN_WRITE in case of locked map. Main use case is to allow for setting up .rodata sections from the BPF ELF which are loaded into the kernel, meaning BPF loader first allocates map, sets up map value by copying .rodata section into it and once complete, it calls BPF_MAP_FREEZE on the map fd to prevent further modifications. Right now BPF_MAP_FREEZE only takes map fd as argument while remaining bpf_attr members are required to be zero. I didn't add write-only locking here as counterpart since I don't have a concrete use-case for it on my side, and I think it makes probably more sense to wait once there is actually one. In that case bpf_attr can be extended as usual with a flag field and/or others where flag 0 means that we lock the map read-only hence this doesn't prevent to add further extensions to BPF_MAP_FREEZE upon need. A map creation flag like BPF_F_WRONCE was not considered for couple of reasons: i) in case of a generic implementation, a map can consist of more than just one element, thus there could be multiple map updates needed to set the map into a state where it can then be made immutable, ii) WRONCE indicates exact one-time write before it is then set immutable. A generic implementation would set a bit atomically on map update entry (if unset), indicating that every subsequent update from then onwards will need to bail out there. However, map updates can fail, so upon failure that flag would need to be unset again and the update attempt would need to be repeated for it to be eventually made immutable. While this can be made race-free, this approach feels less clean and in combination with reason i), it's not generic enough. A dedicated BPF_MAP_FREEZE command directly sets the flag and caller has the guarantee that map is immutable from syscall side upon successful return for any future syscall invocations that would alter the map state, which is also more intuitive from an API point of view. A command name such as BPF_MAP_LOCK has been avoided as it's too close with BPF map spin locks (which already has BPF_F_LOCK flag). BPF_MAP_FREEZE is so far only enabled for privileged users. Signed-off-by:Daniel Borkmann <daniel@iogearbox.net> Acked-by: