feat: use bpf_path_d_path for reading paths - #1570
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Included review availability: Your plan provides up to 12 included reviews per hour; 11 remain after this review. 📝 WalkthroughWalkthroughThe change updates kernel path resolution and makes process lineage and executable-file access reference-safe. Unmount handling now uses ChangesPath and task access safety
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The change adopts trusted argument handling for path reads, but acquisition of the walked parent pointer may still be rejected, putting path-reading correctness at risk. Merge should wait for this concern to be fixed or explicitly accepted by the owner. Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## mauro/feat/disable-preemption #1570 +/- ##
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- Coverage 33.96% 33.20% -0.76%
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Lines 3421 3499 +78
Branches 3421 3499 +78
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- Misses 2254 2332 +78
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In `@fact-ebpf/src/bpf/main.c`:
- Around line 557-559: Check the nullable result of
get_bound_path(BOUND_PATH_MAIN) before dereferencing bound_path in the __d_path
flow; return the existing failure result when the lookup is null, then access
bound_path->path only after validation.
In `@fact-ebpf/src/bpf/process.h`:
- Around line 87-99: Update the parent traversal loop around bpf_task_acquire to
avoid acquiring directly from task->real_parent, which may not satisfy
trusted-argument requirements. Obtain the parent through bpf_task_from_pid or
another supported lookup using the parent identifier, while preserving the
existing NULL, self/root termination checks and task reference release behavior.
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fact-ebpf/src/bpf/d_path.hfact-ebpf/src/bpf/main.cfact-ebpf/src/bpf/process.h
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This is a safer alternative to the bpf_d_path helper that enforces the use of KF_TRUSTED_ARGS semantics, meaning we need to use proper RCU and refcounting to prevent the underlying memory being walked from disappearing from underneath our feet.
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| #include "d_path.h" | ||
| #include "maps.h" | ||
| #include "types.h" | ||
| #include "vmlinux/x86_64.h" |
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My language server is dumb some times, I'll remove it.
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| for (int i = 0; i < LINEAGE_MAX; i++) { | ||
| struct task_struct* parent = task->real_parent; | ||
| bpf_rcu_read_lock(); |
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I wonder if RCU read critical section is needed here. From what I understand it will make synchronize_rcu() and call_rcu() calls to wait for it, but according to the rcu_users implementation [1] call_rcu() is already waiting for rcu_users to become 0. It looks like bpf_task_acquire in this case serves as a way of "pinning" the task to the bpf program [2].
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From the bpf_rcu_read_lock docs, the RCU region is needed so the access to real_parent can be marked as a trusted pointer, in turn bpf_task_acquire requires a trusted pointer in order to return a pinned reference to the task, so my understanding is that we need both.
That said, if I remove the bpf_rcu_read_lock/unlock calls the verifier doesn't complain, so 🤷🏻♂️
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Turns out disabling preemption counts as RCU section [1], thus it ends up having a trusted pointer [2]:
/* By default any pointer obtained from walking a trusted pointer is no
* longer trusted, unless the field being accessed has explicitly been
* marked as inheriting its parent's state of trust (either full or RCU).
* For example:
* 'cgroups' pointer is untrusted if task->cgroups dereference
* happened in a sleepable program outside of bpf_rcu_read_lock()
* section. In a non-sleepable program it's trusted while in RCU CS (aka MEM_RCU).
* Note bpf_rcu_read_unlock() converts MEM_RCU pointers to PTR_UNTRUSTED.
*
* A regular RCU-protected pointer with __rcu tag can also be deemed
* trusted if we are in an RCU CS. Such pointer can be NULL.
*/
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Huh, then I guess I can remove the explicit RCU sections, neat, thanks!
Disabling preemption turns our entire LSM hooks into RCU CS effectively, so there's no need for us to explicitly call `bpf_rcu_read_lock`. More context in #1570 (comment)
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Description
This is a safer alternative to the bpf_d_path helper that enforces the use of KF_TRUSTED_ARGS semantics, meaning we need to use proper RCU and refcounting to prevent the underlying memory being walked from disappearing from underneath our feet.
Checklist
Automated testing
If any of these don't apply, please comment below.
Testing Performed
CI should be enough.
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