Skip UID elevation for non-root users to preserve capabilities#495
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February 27, 2026 12:02
The UID round-trip in open_proc_maps() and open_proc_comm() clears all process capabilities when the caller is non-root. The kernel zeroes the permitted, effective, and ambient sets whenever every UID becomes nonzero after at least one was zero (capabilities(7), "Effect of user ID changes on capabilities"). Guard both call sites with is_root(). Fixes DataDog#494 Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
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Sorry for the late reply, I think the fix needs to be slightly more subtle, I'll post something. |
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closing in favour of #509 |
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What does this PR do?
Fixes #494
Guards the UID elevation in
open_proc_maps()andopen_proc_comm()withis_root()so non-root users skip it entirely.Motivation
When ddprof runs as a non-root user with file capabilities (
cap_perfmon,cap_sys_ptrace, etc.), profiling a target that owns different file capabilities (e.g. nginx withcap_net_bind_service) triggers a UID round-trip through 0 inopen_proc_maps(). On the way back (setresuid(1000, 1000, -1)), the kernel permanently clears all capabilities from the permitted, effective, and ambient sets.This is documented in capabilities(7) under "Effect of user ID changes on capabilities":
The rule exists because the kernel treats the transition "was root, now isn't" as a credential downgrade and drops everything. See
cap_emulate_setxuid()inkernel/security/commoncap.c(called fromsetresuid).For root users the round-trip is safe: the saved-set-user-ID stays 0, so the condition never fires. For non-root users the elevation is also pointless: reading a non-dumpable process's
/proc/<pid>/mapsrequiresCAP_SYS_PTRACEregardless of UID.Additional Notes
Rebased on current main. Simplified test compared to v1.
How to test the change?
The new
user_override_caps-uttest verifies that a faileduser_override(0, 0)call (the non-root CI case) does not destroy existing capabilities.To reproduce the full bug path manually: