proc: implement /proc/[pid]/setgroups#13200
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Without /proc/[pid]/setgroups the canonical rootless user-namespace setup cannot run. The expected sequence starts by writing "deny" to /proc/self/setgroups so that an unprivileged process can then write a non-trivial gid_map, and that file did not exist in gVisor. Tools that take this path, like buildah or podman, fail with ENOENT on /proc/[pid]/setgroups before they can do any work. I followed kernel/user_namespace.c. Each UserNamespace now carries a setgroupsAllowed bit that is initialized true and inherited from the parent at creation. Writing "deny" succeeds while gid_map is still empty and clears the bit, while writing "allow" succeeds only if the bit is still set, which preserves the one-way USERNS_SETGROUPS_ALLOWED transition. The unprivileged branch of SetGIDMap now requires the bit to be clear so that the behavior matches new_idmap_permitted, and setgroups(2) is gated by the conjunction of the bit and a non-empty gid_map, mirroring userns_may_setgroups. Tests in test/syscalls/linux/proc_pid_uid_gid_map.cc cover the file's read and write semantics, the one-way state transition, child-namespace inheritance, the syscall gating before and after deny, and the gid_map interaction. They pass on both runsc and native Linux. Closes #1235 and removes the b/27454212 TODO that has sat in user_namespace.go since the initial commit.
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proc: implement /proc/[pid]/setgroups
Without /proc/[pid]/setgroups the canonical rootless user-namespace setup
cannot run. The expected sequence starts by writing "deny" to
/proc/self/setgroups so that an unprivileged process can then write a
non-trivial gid_map, and that file did not exist in gVisor. Tools that
take this path, like buildah or podman, fail with ENOENT on
/proc/[pid]/setgroups before they can do any work.
I followed kernel/user_namespace.c. Each UserNamespace now carries a
setgroupsAllowed bit that is initialized true and inherited from the
parent at creation. Writing "deny" succeeds while gid_map is still empty
and clears the bit, while writing "allow" succeeds only if the bit is
still set, which preserves the one-way USERNS_SETGROUPS_ALLOWED
transition. The unprivileged branch of SetGIDMap now requires the bit to
be clear so that the behavior matches new_idmap_permitted, and
setgroups(2) is gated by the conjunction of the bit and a non-empty
gid_map, mirroring userns_may_setgroups.
Tests in test/syscalls/linux/proc_pid_uid_gid_map.cc cover the file's
read and write semantics, the one-way state transition, child-namespace
inheritance, the syscall gating before and after deny, and the gid_map
interaction. They pass on both runsc and native Linux.
Closes #1235 and removes the b/27454212 TODO that has sat in user_namespace.go since the initial commit.
FUTURE_COPYBARA_INTEGRATE_REVIEW=#13147 from shayonj:setgroups-impl 373de98