d3d88d08b5
Introduction ============ This patch added intial support for bpf program compile once and run everywhere (CO-RE). The main motivation is for bpf program which depends on kernel headers which may vary between different kernel versions. The initial discussion can be found at https://lwn.net/Articles/773198/. Currently, bpf program accesses kernel internal data structure through bpf_probe_read() helper. The idea is to capture the kernel data structure to be accessed through bpf_probe_read() and relocate them on different kernel versions. On each host, right before bpf program load, the bpfloader will look at the types of the native linux through vmlinux BTF, calculates proper access offset and patch the instruction. To accommodate this, three intrinsic functions preserve_{array,union,struct}_access_index are introduced which in clang will preserve the base pointer, struct/union/array access_index and struct/union debuginfo type information. Later, bpf IR pass can reconstruct the whole gep access chains without looking at gep itself. This patch did the following: . An IR pass is added to convert preserve_*_access_index to global variable who name encodes the getelementptr access pattern. The global variable has metadata attached to describe the corresponding struct/union debuginfo type. . An SimplifyPatchable MachineInstruction pass is added to remove unnecessary loads. . The BTF output pass is enhanced to generate relocation records located in .BTF.ext section. Typical CO-RE also needs support of global variables which can be assigned to different values to different hosts. For example, kernel version can be used to guard different versions of codes. This patch added the support for patchable externals as well. Example ======= The following is an example. struct pt_regs { long arg1; long arg2; }; struct sk_buff { int i; struct net_device *dev; }; #define _(x) (__builtin_preserve_access_index(x)) static int (*bpf_probe_read)(void *dst, int size, const void *unsafe_ptr) = (void *) 4; extern __attribute__((section(".BPF.patchable_externs"))) unsigned __kernel_version; int bpf_prog(struct pt_regs *ctx) { struct net_device *dev = 0; // ctx->arg* does not need bpf_probe_read if (__kernel_version >= 41608) bpf_probe_read(&dev, sizeof(dev), _(&((struct sk_buff *)ctx->arg1)->dev)); else bpf_probe_read(&dev, sizeof(dev), _(&((struct sk_buff *)ctx->arg2)->dev)); return dev != 0; } In the above, we want to translate the third argument of bpf_probe_read() as relocations. -bash-4.4$ clang -target bpf -O2 -g -S trace.c The compiler will generate two new subsections in .BTF.ext, OffsetReloc and ExternReloc. OffsetReloc is to record the structure member offset operations, and ExternalReloc is to record the external globals where only u8, u16, u32 and u64 are supported. BPFOffsetReloc Size struct SecLOffsetReloc for ELF section #1 A number of struct BPFOffsetReloc for ELF section #1 struct SecOffsetReloc for ELF section #2 A number of struct BPFOffsetReloc for ELF section #2 ... BPFExternReloc Size struct SecExternReloc for ELF section #1 A number of struct BPFExternReloc for ELF section #1 struct SecExternReloc for ELF section #2 A number of struct BPFExternReloc for ELF section #2 struct BPFOffsetReloc { uint32_t InsnOffset; ///< Byte offset in this section uint32_t TypeID; ///< TypeID for the relocation uint32_t OffsetNameOff; ///< The string to traverse types }; struct BPFExternReloc { uint32_t InsnOffset; ///< Byte offset in this section uint32_t ExternNameOff; ///< The string for external variable }; Note that only externs with attribute section ".BPF.patchable_externs" are considered for Extern Reloc which will be patched by bpf loader right before the load. For the above test case, two offset records and one extern record will be generated: OffsetReloc records: .long .Ltmp12 # Insn Offset .long 7 # TypeId .long 242 # Type Decode String .long .Ltmp18 # Insn Offset .long 7 # TypeId .long 242 # Type Decode String ExternReloc record: .long .Ltmp5 # Insn Offset .long 165 # External Variable In string table: .ascii "0:1" # string offset=242 .ascii "__kernel_version" # string offset=165 The default member offset can be calculated as the 2nd member offset (0 representing the 1st member) of struct "sk_buff". The asm code: .Ltmp5: .Ltmp6: r2 = 0 r3 = 41608 .Ltmp7: .Ltmp8: .loc 1 18 9 is_stmt 0 # t.c:18:9 .Ltmp9: if r3 > r2 goto LBB0_2 .Ltmp10: .Ltmp11: .loc 1 0 9 # t.c:0:9 .Ltmp12: r2 = 8 .Ltmp13: .loc 1 19 66 is_stmt 1 # t.c:19:66 .Ltmp14: .Ltmp15: r3 = *(u64 *)(r1 + 0) goto LBB0_3 .Ltmp16: .Ltmp17: LBB0_2: .loc 1 0 66 is_stmt 0 # t.c:0:66 .Ltmp18: r2 = 8 .loc 1 21 66 is_stmt 1 # t.c:21:66 .Ltmp19: r3 = *(u64 *)(r1 + 8) .Ltmp20: .Ltmp21: LBB0_3: .loc 1 0 66 is_stmt 0 # t.c:0:66 r3 += r2 r1 = r10 .Ltmp22: .Ltmp23: .Ltmp24: r1 += -8 r2 = 8 call 4 For instruction .Ltmp12 and .Ltmp18, "r2 = 8", the number 8 is the structure offset based on the current BTF. Loader needs to adjust it if it changes on the host. For instruction .Ltmp5, "r2 = 0", the external variable got a default value 0, loader needs to supply an appropriate value for the particular host. Compiling to generate object code and disassemble: 0000000000000000 bpf_prog: 0: b7 02 00 00 00 00 00 00 r2 = 0 1: 7b 2a f8 ff 00 00 00 00 *(u64 *)(r10 - 8) = r2 2: b7 02 00 00 00 00 00 00 r2 = 0 3: b7 03 00 00 88 a2 00 00 r3 = 41608 4: 2d 23 03 00 00 00 00 00 if r3 > r2 goto +3 <LBB0_2> 5: b7 02 00 00 08 00 00 00 r2 = 8 6: 79 13 00 00 00 00 00 00 r3 = *(u64 *)(r1 + 0) 7: 05 00 02 00 00 00 00 00 goto +2 <LBB0_3> 0000000000000040 LBB0_2: 8: b7 02 00 00 08 00 00 00 r2 = 8 9: 79 13 08 00 00 00 00 00 r3 = *(u64 *)(r1 + 8) 0000000000000050 LBB0_3: 10: 0f 23 00 00 00 00 00 00 r3 += r2 11: bf a1 00 00 00 00 00 00 r1 = r10 12: 07 01 00 00 f8 ff ff ff r1 += -8 13: b7 02 00 00 08 00 00 00 r2 = 8 14: 85 00 00 00 04 00 00 00 call 4 Instructions #2, #5 and #8 need relocation resoutions from the loader. Signed-off-by: Yonghong Song <yhs@fb.com> Differential Revision: https://reviews.llvm.org/D61524 llvm-svn: 365503
71 lines
3.9 KiB
LLVM
71 lines
3.9 KiB
LLVM
; RUN: llc -march=bpfel -filetype=obj -o - %s | llvm-readelf -x ".BTF" -x ".BTF.ext" | FileCheck -check-prefixes=CHECK,CHECK-EL %s
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; RUN: llc -march=bpfeb -filetype=obj -o - %s | llvm-readelf -x ".BTF" -x ".BTF.ext" | FileCheck -check-prefixes=CHECK,CHECK-EB %s
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; Source code:
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; int f(int a) { return a; }
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; Compilation flag:
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; clang -target bpf -O2 -g -gdwarf-5 -gembed-source -S -emit-llvm t.c
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; Function Attrs: nounwind readnone
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define dso_local i32 @f(i32 returned %a) local_unnamed_addr #0 !dbg !7 {
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entry:
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call void @llvm.dbg.value(metadata i32 %a, metadata !12, metadata !DIExpression()), !dbg !13
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ret i32 %a, !dbg !14
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}
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; CHECK: '.BTF'
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; CHECK-EL: 0x00000000 9feb0100 18000000 00000000 30000000
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; CHECK-EL: 0x00000010 30000000 33000000 01000000 00000001
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; CHECK-EL: 0x00000020 04000000 20000001 00000000 0100000d
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; CHECK-EL: 0x00000030 01000000 05000000 01000000 07000000
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; CHECK-EL: 0x00000040 0000000c 02000000 00696e74 00610066
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; CHECK-EB: 0x00000000 eb9f0100 00000018 00000000 00000030
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; CHECK-EB: 0x00000010 00000030 00000033 00000001 01000000
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; CHECK-EB: 0x00000020 00000004 01000020 00000000 0d000001
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; CHECK-EB: 0x00000030 00000001 00000005 00000001 00000007
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; CHECK-EB: 0x00000040 0c000000 00000002 00696e74 00610066
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; CHECK: 0x00000050 002e7465 7874002f 746d702f 742e6300
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; CHECK: 0x00000060 696e7420 6628696e 74206129 207b2072
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; CHECK: 0x00000070 65747572 6e20613b 207d00
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; CHECK: '.BTF.ext'
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; CHECK-EL: 0x00000000 9feb0100 28000000 00000000 14000000
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; CHECK-EL: 0x00000010 14000000 2c000000 40000000 00000000
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; CHECK-EL: 0x00000020 40000000 00000000 08000000 09000000
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; CHECK-EL: 0x00000030 01000000 00000000 03000000 10000000
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; CHECK-EL: 0x00000040 09000000 02000000 00000000 0f000000
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; CHECK-EL: 0x00000050 18000000 00040000 08000000 0f000000
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; CHECK-EL: 0x00000060 18000000 10040000
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; CHECK-EB: 0x00000000 eb9f0100 00000028 00000000 00000014
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; CHECK-EB: 0x00000010 00000014 0000002c 00000040 00000000
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; CHECK-EB: 0x00000020 00000040 00000000 00000008 00000009
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; CHECK-EB: 0x00000030 00000001 00000000 00000003 00000010
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; CHECK-EB: 0x00000040 00000009 00000002 00000000 0000000f
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; CHECK-EB: 0x00000050 00000018 00000400 00000008 0000000f
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; CHECK-EB: 0x00000060 00000018 00000410
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; Function Attrs: nounwind readnone speculatable
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declare void @llvm.dbg.value(metadata, metadata, metadata) #1
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attributes #0 = { nounwind readnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
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attributes #1 = { nounwind readnone speculatable }
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!llvm.dbg.cu = !{!0}
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!llvm.module.flags = !{!3, !4, !5}
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!llvm.ident = !{!6}
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!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 8.0.0 (trunk 344789) (llvm/trunk 344782)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
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!1 = !DIFile(filename: "t.c", directory: "/tmp", checksumkind: CSK_MD5, checksum: "1924f0d78deb326ceb76cd8e9f450775", source: "int f(int a) { return a; }\0A")
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!2 = !{}
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!3 = !{i32 2, !"Dwarf Version", i32 5}
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!4 = !{i32 2, !"Debug Info Version", i32 3}
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!5 = !{i32 1, !"wchar_size", i32 4}
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!6 = !{!"clang version 8.0.0 (trunk 344789) (llvm/trunk 344782)"}
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!7 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, isOptimized: true, unit: !0, retainedNodes: !11)
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!8 = !DISubroutineType(types: !9)
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!9 = !{!10, !10}
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!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!11 = !{!12}
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!12 = !DILocalVariable(name: "a", arg: 1, scope: !7, file: !1, line: 1, type: !10)
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!13 = !DILocation(line: 1, column: 11, scope: !7)
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!14 = !DILocation(line: 1, column: 16, scope: !7)
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