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Andrii Nakryiko says: ==================== Complete BPF verifier precision tracking support for register spills Add support to BPF verifier to track and support register spill/fill to/from stack regardless if it was done through read-only R10 register (which is the only form supported today), or through a general register after copying R10 into it, while also potentially modifying offset. Once we add register this generic spill/fill support to precision backtracking, we can take advantage of it to stop doing eager STACK_ZERO conversion on register spill. Instead we can rely on (im)precision of spilled const zero register to improve verifier state pruning efficiency. This situation of using const zero register to initialize stack slots is very common with __builtin_memset() usage or just zero-initializing variables on the stack, and it causes unnecessary state duplication, as that STACK_ZERO knowledge is often not necessary for correctness, as those zero values are never used in precise context. Thus, relying on register imprecision helps tremendously, especially in real-world BPF programs. To make spilled const zero register behave completely equivalently to STACK_ZERO, we need to improve few other small pieces, which is done in the second part of the patch set. See individual patches for details. There are also two small bug fixes spotted during STACK_ZERO debugging. The patch set consists of logically three changes: - patch #1 (and corresponding tests in patch #2) is fixing/impoving precision propagation for stack spills/fills. This can be landed as a stand-alone improvement; - patches #3 through #9 is improving verification scalability by utilizing register (im)precision instead of eager STACK_ZERO. These changes depend on patch #1. - patch #10 is a memory efficiency improvement to how instruction/jump history is tracked and maintained. It depends on patch #1, but is not strictly speaking required, even though I believe it's a good long-term solution to have a path-dependent per-instruction information. Kind of like a path-dependent counterpart to path-agnostic insn_aux array. v3->v3: - fixed up Fixes tag (Alexei); - fixed few more selftests to not use BPF_ST instruction in inline asm directly, checked with CI, it was happy (CI); v2->v3: - BPF_ST instruction workaround (Eduard); - force dereference in added tests to catch problems (Eduard); - some commit message massaging (Alexei); v1->v2: - clean ups, WARN_ONCE(), insn_flags helpers added (Eduard); - added more selftests for STACK_ZERO/STACK_MISC cases (Eduard); - a bit more detailed explanation of effect of avoiding STACK_ZERO in favor of register spill in patch #8 commit (Alexei); - global shared instruction history refactoring moved to be the last patch in the series to make it easier to revert it, if applied (Alexei). ==================== Link: https://lore.kernel.org/r/20231205184248.1502704-1-andrii@kernel.org 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.