[PATCH] D59441: [BPF] Add BTF Var and DataSec Support

[Yonghong Song via Phabricator via llvm-commits]

yonghong-song created this revision.
yonghong-song added a reviewer: ast.
Herald added subscribers: llvm-commits, jdoerfert, aprantl.
Herald added a project: LLVM.

  Two new kinds, BTF_KIND_VAR and BTF_KIND_DATASEC, are added.
  
  BTF_KIND_VAR has the following specification:
     btf_type.name: var name
     btf_type.info: type kind
     btf_type.type: var type
     // btf_type is followed by one u32
     u32: varinfo (currently, only 0 - static, 1 - global allocated in elf sections)
  
  Not all globals are supported in this patch. The following globals are supported:
    . static variables with or without section attributes
    . global variables with section attributes
  
  The inclusion of globals with section attributes
  is for future potential extraction of key/value
  type id's from map definition.
  
  BTF_KIND_DATASEC has the following specification:
    btf_type.name: section name associated with variable or
                   one of .data/.bss/.readonly
    btf_type.info: type kind and vlen for # of variables
    btf_type.size: 0
    #vlen number of the following:
      u32: id of corresponding BTF_KIND_VAR
      u32: in-session offset of the var
      u32: the size of memory var occupied
  
  At the time of debug info emission, the data section
  size is unknown, so the btf_type.size = 0 for
  BTF_KIND_DATASEC. The loader can patch it during
  loading time.
  
  The in-session offseet of the var is only available
  for static variables. For global variables, the
  loader neeeds to assign the global variable symbol value in
  symbol table to in-section offset.
  
  The size of memory is used to specify the amount of the
  memory a variable occupies. Typically, it equals to
  the type size, but for certain structures, e.g.,

  struct tt {
    int a;
    int b;
    char c[];
   };
   static volatile struct tt s2 = {3, 4, "abcdefghi"};

  The static variable s2 has size of 20.
  
  Note that for BTF_KIND_DATASEC name, the section name
  does not contain object name. The compiler does have
  input module name. For example, two cases below:
     . clang -target bpf -O2 -g -c test.c
       The compiler knows the input file (module) is test.c
       and can generate sec name like test.data/test.bss etc.
     . clang -target bpf -O2 -g -emit-llvm -c test.c -o - |
       llc -march=bpf -filetype=obj -o test.o
       The llc compiler has the input file as stdin, and
       would generate something like stdin.data/stdin.bss etc.
       which does not really make sense.
  
  For any user specificed section name, e.g.,

  static volatile int a __attribute__((section("id1")));
  static volatile const int b __attribute__((section("id2")));

  The DataSec with name "id1" and "id2" does not contain
  information whether the section is readonly or not.
  The loader needs to check the corresponding elf section
  flags for such information.
  
  A simple example:

  -bash-4.4$ cat t.c
  int g1;
  int g2 = 3;
  const int g3 = 4;
  static volatile int s1;
  struct tt {
   int a;
   int b;
   char c[];
  };
  static volatile struct tt s2 = {3, 4, "abcdefghi"};
  static volatile const int s3 = 4;
  int m __attribute__((section("maps"), used)) = 4;
  int test() { return g1 + g2 + g3 + s1 + s2.a + s3 + m; }
  -bash-4.4$ clang -target bpf -O2 -g -S t.c

  Checking t.s, 4 BTF_KIND_VAR's are generated (s1, s2, s3 and m).
  4 BTF_KIND_DATASEC's are generated with names
  ".data", ".bss", ".rodata" and "maps".


Repository:
  rL LLVM

https://reviews.llvm.org/D59441

Files:
  lib/Target/BPF/BTF.def
  lib/Target/BPF/BTF.h
  lib/Target/BPF/BTFDebug.cpp
  lib/Target/BPF/BTFDebug.h
  test/CodeGen/BPF/BTF/global-var-sec-readonly.ll
  test/CodeGen/BPF/BTF/global-var-sec.ll
  test/CodeGen/BPF/BTF/static-var-inited-sec.ll
  test/CodeGen/BPF/BTF/static-var-inited.ll
  test/CodeGen/BPF/BTF/static-var-readonly-sec.ll
  test/CodeGen/BPF/BTF/static-var-readonly.ll
  test/CodeGen/BPF/BTF/static-var-sec.ll
  test/CodeGen/BPF/BTF/static-var-zerolen-array.ll
  test/CodeGen/BPF/BTF/static-var.ll

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