From 5327ed3d44b754f5cc51d5b3f18e442eaebacff5 Mon Sep 17 00:00:00 2001 From: Jiong Wang Date: Fri, 24 May 2019 23:25:12 +0100 Subject: bpf: verifier: mark verified-insn with sub-register zext flag eBPF ISA specification requires high 32-bit cleared when low 32-bit sub-register is written. This applies to destination register of ALU32 etc. JIT back-ends must guarantee this semantic when doing code-gen. x86_64 and AArch64 ISA has the same semantics, so the corresponding JIT back-end doesn't need to do extra work. However, 32-bit arches (arm, x86, nfp etc.) and some other 64-bit arches (PowerPC, SPARC etc) need to do explicit zero extension to meet this requirement, otherwise code like the following will fail. u64_value = (u64) u32_value ... other uses of u64_value This is because compiler could exploit the semantic described above and save those zero extensions for extending u32_value to u64_value, these JIT back-ends are expected to guarantee this through inserting extra zero extensions which however could be a significant increase on the code size. Some benchmarks show there could be ~40% sub-register writes out of total insns, meaning at least ~40% extra code-gen. One observation is these extra zero extensions are not always necessary. Take above code snippet for example, it is possible u32_value will never be casted into a u64, the value of high 32-bit of u32_value then could be ignored and extra zero extension could be eliminated. This patch implements this idea, insns defining sub-registers will be marked when the high 32-bit of the defined sub-register matters. For those unmarked insns, it is safe to eliminate high 32-bit clearnace for them. Algo: - Split read flags into READ32 and READ64. - Record index of insn that does sub-register write. Keep the index inside reg state and update it during verifier insn walking. - A full register read on a sub-register marks its definition insn as needing zero extension on dst register. A new sub-register write overrides the old one. - When propagating read64 during path pruning, also mark any insn defining a sub-register that is read in the pruned path as full-register. Reviewed-by: Jakub Kicinski Signed-off-by: Jiong Wang Signed-off-by: Alexei Starovoitov --- include/linux/bpf_verifier.h | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) (limited to 'include/linux') diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 405b502283c5..704ed7971472 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -36,9 +36,11 @@ */ enum bpf_reg_liveness { REG_LIVE_NONE = 0, /* reg hasn't been read or written this branch */ - REG_LIVE_READ, /* reg was read, so we're sensitive to initial value */ - REG_LIVE_WRITTEN, /* reg was written first, screening off later reads */ - REG_LIVE_DONE = 4, /* liveness won't be updating this register anymore */ + REG_LIVE_READ32 = 0x1, /* reg was read, so we're sensitive to initial value */ + REG_LIVE_READ64 = 0x2, /* likewise, but full 64-bit content matters */ + REG_LIVE_READ = REG_LIVE_READ32 | REG_LIVE_READ64, + REG_LIVE_WRITTEN = 0x4, /* reg was written first, screening off later reads */ + REG_LIVE_DONE = 0x8, /* liveness won't be updating this register anymore */ }; struct bpf_reg_state { @@ -131,6 +133,11 @@ struct bpf_reg_state { * pointing to bpf_func_state. */ u32 frameno; + /* Tracks subreg definition. The stored value is the insn_idx of the + * writing insn. This is safe because subreg_def is used before any insn + * patching which only happens after main verification finished. + */ + s32 subreg_def; enum bpf_reg_liveness live; }; @@ -233,6 +240,7 @@ struct bpf_insn_aux_data { int ctx_field_size; /* the ctx field size for load insn, maybe 0 */ int sanitize_stack_off; /* stack slot to be cleared */ bool seen; /* this insn was processed by the verifier */ + bool zext_dst; /* this insn zero extends dst reg */ u8 alu_state; /* used in combination with alu_limit */ bool prune_point; unsigned int orig_idx; /* original instruction index */ -- cgit v1.2.3