[PATCH] backends,bpf: add proper relocation support
Yonghong Song
yhs@fb.com
Fri Jun 15 22:40:00 GMT 2018
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
More information about the Elfutils-devel
mailing list