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Eduard Zingerman says: ==================== exact states comparison for iterator convergence checks Iterator convergence logic in is_state_visited() uses state_equals() for states with branches counter > 0 to check if iterator based loop converges. This is not fully correct because state_equals() relies on presence of read and precision marks on registers. These marks are not guaranteed to be finalized while state has branches. Commit message for patch #3 describes a program that exhibits such behavior. This patch-set aims to fix iterator convergence logic by adding notion of exact states comparison. Exact comparison does not rely on presence of read or precision marks and thus is more strict. As explained in commit message for patch #3 exact comparisons require addition of speculative register bounds widening. The end result for BPF verifier users could be summarized as follows: (!) After this update verifier would reject programs that conjure an imprecise value on the first loop iteration and use it as precise on the second (for iterator based loops). I urge people to at least skim over the commit message for patch #3. Patches are organized as follows: - patches #1,2: moving/extracting utility functions; - patch #3: introduces exact mode for states comparison and adds widening heuristic; - patch #4: adds test-cases that demonstrate why the series is necessary; - patch #5: extends patch #3 with a notion of state loop entries, these entries have to be tracked to correctly identify that different verifier states belong to the same states loop; - patch #6: adds a test-case that demonstrates a program which requires loop entry tracking for correct verification; - patch #7: just adds a few debug prints. The following actions are planned as a followup for this patch-set: - implementation has to be adapted for callbacks handling logic as a part of a fix for [1]; - it is necessary to explore ways to improve widening heuristic to handle iters_task_vma test w/o need to insert barrier_var() calls; - explored states eviction logic on cache miss has to be extended to either: - allow eviction of checkpoint states -or- - be sped up in case if there are many active checkpoints associated with the same instruction. The patch-set is a followup for mailing list discussion [1]. Changelog: - V2 [3] -> V3: - correct check for stack spills in widen_imprecise_scalars(), added test case progs/iters.c:widen_spill to check the behavior (suggested by Andrii); - allow eviction of checkpoint states in is_state_visited() to avoid pathological verifier performance when iterator based loop does not converge (discussion with Alexei). - V1 [2] -> V2, applied changes suggested by Alexei offlist: - __explored_state() function removed; - same_callsites() function is now used in clean_live_states(); - patches #1,2 are added as preparatory code movement; - in process_iter_next_call() a safeguard is added to verify that cur_st->parent exists and has expected insn index / call sites. [1] https://lore.kernel.org/bpf/97a90da09404c65c8e810cf83c94ac703705dc0e.camel@gmail.com/ [2] https://lore.kernel.org/bpf/20231021005939.1041-1-eddyz87@gmail.com/ [3] https://lore.kernel.org/bpf/20231022010812.9201-1-eddyz87@gmail.com/ ==================== Link: https://lore.kernel.org/r/20231024000917.12153-1-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.