libdwelf: New function dwelf_elf_begin.

This introduces a new function dwelf_elf_begin which creates a (read-only)
ELF handle from a possibly compressed file handle or a file that start
with a linux kernel header. This can be used in eu-readelf to (re)open a
(pure) ELF.

eu-readelf uses libdwfl to relocate addresses in the original file in
case it is ET_REL. But to show the "raw" data it might need to (re)open
the file. Which could fail if the file was compressed. And produced an
obscure error message: "cannot create EBL handle".

This rewrites __libdw_open_file a little so that the given file handle
will never be closed (whether on success or failure) and introduces a
new internal function __libdw_open_elf that dwelf_elf_begin wraps.

Signed-off-by: Mark Wielaard <mark@klomp.org>

4 files changed, 79 insertions, 3 deletions

diff --git a/libdwelf/ChangeLog b/libdwelf/ChangeLog
index ba921347..88be3421 100644
--- a/libdwelf/ChangeLog
+++ b/libdwelf/ChangeLog
@@ -1,9 +1,15 @@
+2018-10-21  Mark Wielaard  <mark@klomp.org>
+
+	* libdwelf.h (dwelf_elf_begin): Add function declaration.
+	* dwelf_elf_begin.c: New file.
+	* Makefile.am (libdwelf_a_SOURCES): Add dwelf_elf_begin.c.
+
 2018-10-18  Mark Wielaard  <mark@klomp.org>
 
 	* dwelf_elf_gnu_build_id.c (find_elf_build_id): Check p_align to
 	set ELF type.
 
-2015-10-11  Akihiko Odaki  <akihiko.odaki.4i@stu.hosei.ac.jp>
+2016-10-11  Akihiko Odaki  <akihiko.odaki.4i@stu.hosei.ac.jp>
 
 	* dwelf_strtab.c: Remove sys/param.h include.
 	(MIN): Remove definition.
diff --git a/libdwelf/Makefile.am b/libdwelf/Makefile.am
index 7ca767a9..a7933fda 100644
--- a/libdwelf/Makefile.am
+++ b/libdwelf/Makefile.am
@@ -41,7 +41,7 @@ noinst_HEADERS = libdwelfP.h
 
 libdwelf_a_SOURCES = dwelf_elf_gnu_debuglink.c dwelf_dwarf_gnu_debugaltlink.c \
 		     dwelf_elf_gnu_build_id.c dwelf_scn_gnu_compressed_size.c \
-		     dwelf_strtab.c
+		     dwelf_strtab.c dwelf_elf_begin.c
 
 libdwelf = $(libdw)
 
diff --git a/libdwelf/dwelf_elf_begin.c b/libdwelf/dwelf_elf_begin.c
new file mode 100644
index 00000000..79825338
--- /dev/null
+++ b/libdwelf/dwelf_elf_begin.c
@@ -0,0 +1,62 @@
+/* Creates an ELF handle from a possibly compressed file descriptor.
+   Copyright (C) 2018 Red Hat, Inc.
+   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 "libdwelfP.h"
+#include "libdwflP.h"
+#include "libelfP.h"
+
+#include <unistd.h>
+
+Elf *
+dwelf_elf_begin (int fd)
+{
+  Elf *elf = NULL;
+  Dwfl_Error e = __libdw_open_elf (fd, &elf);
+  if (elf != NULL && elf_kind (elf) != ELF_K_NONE)
+    return elf;
+
+  /* Elf wasn't usable.  Make sure there is a proper elf error message.  */
+
+  if (elf != NULL)
+    elf_end (elf);
+
+  if (e != DWFL_E_LIBELF)
+    {
+      /* Force a bad ELF error.  */
+      char badelf[EI_NIDENT] = { };
+      Elf *belf = elf_memory (badelf, EI_NIDENT);
+      elf32_getehdr (belf);
+      elf_end (belf);
+    }
+
+  return NULL;
+}
diff --git a/libdwelf/libdwelf.h b/libdwelf/libdwelf.h
index 72089dbf..6d491847 100644
--- a/libdwelf/libdwelf.h
+++ b/libdwelf/libdwelf.h
@@ -1,5 +1,5 @@
 /* Interfaces for libdwelf. DWARF ELF Low-level Functions.
-   Copyright (C) 2014, 2015, 2016 Red Hat, Inc.
+   Copyright (C) 2014, 2015, 2016, 2018 Red Hat, Inc.
    This file is part of elfutils.
 
    This file is free software; you can redistribute it and/or modify
@@ -125,6 +125,14 @@ extern const char *dwelf_strent_str (Dwelf_Strent *se)
 extern void dwelf_strtab_free (Dwelf_Strtab *st)
   __nonnull_attribute__ (1);
 
+/* Creates a read-only Elf handle from the given file handle.  The
+   file may be compressed and/or contain a linux kernel image header,
+   in which case it is eagerly decompressed in full and the Elf handle
+   is created as if created with elf_memory ().  On error NULL is
+   returned.  The Elf handle should be closed with elf_end ().  The
+   file handle will not be closed.  Does not return ELF_K_NONE handles.  */
+extern Elf *dwelf_elf_begin (int fd);
+
 #ifdef __cplusplus
 }
 #endif