[PATCH] backends,bpf: add proper relocation support

[Yonghong Song <yhs at fb dot com>]

Due to libdw does not have proper BPF relocation support,
the pahole cannot display filenames correctly for objects
with default llvm options. So we have to invent
a special option "llc -march=bpf -mattr=dwarfris" to
prevent llvm from generating cross-section dwarf relocation
records (https://reviews.llvm.org/rL326505).
The pahole related discussion is in linux netdev
mailing list (http://lists.openwall.net/netdev/2018/06/15/38, etc.)

We would like to add proper BPF relocation support
to libdw so eventually we could retire the special llc bpf
flag "-mattr=dwarfris".

The bpf relocations are defined in
llvm_repo:include/llvm/BinaryFormat/ELFRelocs/BPF.def:
  ELF_RELOC(R_BPF_NONE,        0)
  ELF_RELOC(R_BPF_64_64,       1)
  ELF_RELOC(R_BPF_64_32,      10)

Removed the relocation type R_BPF_MAP_FD whoes name does not
confirm to llvm definition and replaced it with R_BPF_64_64.
The BPF object is just a relocatible object, not an executable or
a shared library, so assign ELF type to REL only in bpf_reloc.def.

Tested locally with building pahole with this patch and
pahole is able to display structures in bpf object file properly.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 backends/Makefile.am   |  2 +-
 backends/bpf_init.c    |  1 +
 backends/bpf_reloc.def |  3 ++-
 backends/bpf_symbol.c  | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++
 libelf/elf.h           |  3 ++-
 5 files changed, 60 insertions(+), 3 deletions(-)
 create mode 100644 backends/bpf_symbol.c

diff --git a/backends/Makefile.am b/backends/Makefile.am
index 80aa00e7..b5e721d8 100644
--- a/backends/Makefile.am
+++ b/backends/Makefile.am
@@ -126,7 +126,7 @@ am_libebl_m68k_pic_a_OBJECTS = $(m68k_SRCS:.c=.os)
 # an issue.
 m68k_corenote_no_Wpacked_not_aligned = yes
 
-bpf_SRCS = bpf_init.c bpf_regs.c
+bpf_SRCS = bpf_init.c bpf_regs.c bpf_symbol.c
 cpu_bpf = ../libcpu/libcpu_bpf.a
 libebl_bpf_pic_a_SOURCES = $(bpf_SRCS)
 am_libebl_bpf_pic_a_OBJECTS = $(bpf_SRCS:.c=.os)
diff --git a/backends/bpf_init.c b/backends/bpf_init.c
index 8ea1bc1a..a046e069 100644
--- a/backends/bpf_init.c
+++ b/backends/bpf_init.c
@@ -53,6 +53,7 @@ bpf_init (Elf *elf __attribute__ ((unused)),
   bpf_init_reloc (eh);
   HOOK (eh, register_info);
   HOOK (eh, disasm);
+  HOOK (eh, reloc_simple_type);
 
   return MODVERSION;
 }
diff --git a/backends/bpf_reloc.def b/backends/bpf_reloc.def
index a410da97..59f519b5 100644
--- a/backends/bpf_reloc.def
+++ b/backends/bpf_reloc.def
@@ -28,4 +28,5 @@
 /*	    NAME,		REL|EXEC|DYN	*/
 
 RELOC_TYPE (NONE,		EXEC|DYN)
-RELOC_TYPE (MAP_FD,		REL|EXEC|DYN)
+RELOC_TYPE (64_64,		REL)
+RELOC_TYPE (64_32,		REL)
diff --git a/backends/bpf_symbol.c b/backends/bpf_symbol.c
new file mode 100644
index 00000000..c9856f26
--- /dev/null
+++ b/backends/bpf_symbol.c
@@ -0,0 +1,54 @@
+/* BPF specific symbolic name handling.
+   This file is part of elfutils.
+
+   This file is free software; you can redistribute it and/or modify
+   it under the terms of either
+
+     * the GNU Lesser General Public License as published by the Free
+       Software Foundation; either version 3 of the License, or (at
+       your option) any later version
+
+   or
+
+     * the GNU General Public License as published by the Free
+       Software Foundation; either version 2 of the License, or (at
+       your option) any later version
+
+   or both in parallel, as here.
+
+   elfutils is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received copies of the GNU General Public License and
+   the GNU Lesser General Public License along with this program.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <assert.h>
+#include <elf.h>
+#include <stddef.h>
+#include <string.h>
+
+#define BACKEND bpf_
+#include "libebl_CPU.h"
+
+
+/* Check for the simple reloc types.  */
+Elf_Type
+bpf_reloc_simple_type (Ebl *ebl __attribute__ ((unused)), int type)
+{
+  switch (type)
+    {
+    case R_BPF_64_64:
+      return ELF_T_XWORD;
+    case R_BPF_64_32:
+      return ELF_T_WORD;
+    default:
+      return ELF_T_NUM;
+    }
+}
diff --git a/libelf/elf.h b/libelf/elf.h
index f7748983..940e88dd 100644
--- a/libelf/elf.h
+++ b/libelf/elf.h
@@ -3848,7 +3848,8 @@ enum
 /* BPF specific declarations.  */
 
 #define R_BPF_NONE		0	/* No reloc */
-#define R_BPF_MAP_FD		1	/* Map fd to pointer */
+#define R_BPF_64_64		1
+#define R_BPF_64_32		10
 
 /* Imagination Meta specific relocations. */
 
-- 
2.14.3