Re: [PATCH v2 1/5] userns: record how far up CAP_SETFCAP reaches

create_user_ns() records in ns->parent_could_setfcap whether the creator
had CAP_SETFCAP in its effective set, and verify_root_map() trusts that
when a task in the new namespace maps the parent's uid 0.  A bool only
describes the parent, though.  File capabilities belong to the kuid of a
root user, and a namespace that maps the parent's uid 0 shares that kuid
with the parent and possibly with ancestors further up, so what matters
is how far up the creator's CAP_SETFCAP counted.

Replace the bool with ns->setfcap_level: CAP_SETFCAP held in the
namespace counts there and in its ancestors up to the one at that
->level.  cap_setfcap_level() computes it from the creator's cred.  A
creator that holds CAP_SETFCAP passes on the setfcap_level of its own
namespace, one that doesn't gets the new namespace's own level, and the
initial namespace has 0.

verify_root_map() keeps checking the same condition: the bool was false
exactly when the level is the namespace's own.

No functional change.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261014-b4-setfcap-userns-v2-0-fac4d19281d7@toxicpanda.com?part=1