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[BPF] Support for compile once and run everywhere

Authored by yonghong-song on Jul 9 2019, 8:28 AM.

Description

[BPF] Support for compile once and run everywhere

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