libdwelf: Make dwelf_elf_begin return NULL only when there is an error.

dwelf_elf_begin was slightly different from elf_begin in case the file
turned out to not be an ELF file. elf_begin would return an Elf handle
with ELF_K_NONE. But dwelf_elf_begin would return NULL. This made it
impossible to tell the difference between a file or decompression error
and a (decompressed) file not being an ELF file.

Since dwelf_elf_begin could still return different kinds of ELF files
(ELF_K_ELF or ELF_K_AR - and theoretically ELF_K_COFF) this was not
really useful anyway. So make it so that dwelf_elf_begin always returns
an Elf handle unless there was a real error reading or decompressing
the file. Otherwise return NULL to make clear there was a real error.

Make sure that the decompression function returns DWFL_E_BADELF only
when the file isn't compressed. In which case the Elf handle won't
be replaced and can be returned (as ELF_K_NONE).

Add a new version to dwelf_elf_begin so programs can rely on it
returning NULL only for real errors.

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

3 files changed, 19 insertions, 8 deletions

diff --git a/libdwelf/ChangeLog b/libdwelf/ChangeLog
index 29f9a509..5b48ed8f 100644
--- a/libdwelf/ChangeLog
+++ b/libdwelf/ChangeLog
@@ -1,3 +1,9 @@
+2019-08-12  Mark Wielaard  <mark@klomp.org>
+
+	* libdwelf.h (dwelf_elf_begin): Update documentation.
+	* dwelf_elf_begin.c (dwelf_elf_begin): Don't suppress ELF_K_NONE.
+	Mark old and new version.
+
 2019-06-28  Mark Wielaard  <mark@klomp.org>
 
 	* Makefile.am (libdwelf_a_SOURCES): Add dwelf_elf_e_machine_string.c.
diff --git a/libdwelf/dwelf_elf_begin.c b/libdwelf/dwelf_elf_begin.c
index 79825338..c7d63a1c 100644
--- a/libdwelf/dwelf_elf_begin.c
+++ b/libdwelf/dwelf_elf_begin.c
@@ -41,13 +41,13 @@ 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)
+  if (e == DWFL_E_NOERROR)
     return elf;
 
-  /* Elf wasn't usable.  Make sure there is a proper elf error message.  */
-
-  if (elf != NULL)
-    elf_end (elf);
+  /* Elf wasn't usable.  Make sure there is a proper elf error
+     message.  This is probably not the real error, because there is
+     no good way to propagate errnos or decompression errors, but
+     better than nothing.  */
 
   if (e != DWFL_E_LIBELF)
     {
@@ -60,3 +60,5 @@ dwelf_elf_begin (int fd)
 
   return NULL;
 }
+OLD_VERSION (dwelf_elf_begin, ELFUTILS_0.175)
+NEW_VERSION (dwelf_elf_begin, ELFUTILS_0.177)
diff --git a/libdwelf/libdwelf.h b/libdwelf/libdwelf.h
index cb7ea091..dbb8f08c 100644
--- a/libdwelf/libdwelf.h
+++ b/libdwelf/libdwelf.h
@@ -128,9 +128,12 @@ extern void dwelf_strtab_free (Dwelf_Strtab *st)
 /* 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.  */
+   is created as if created with elf_memory ().  On decompression or
+   file errors NULL is returned (and elf_errno will be set).  If there
+   was no error, but the file is not an ELF file, then an ELF_K_NONE
+   Elf handle is returned (just like with elf_begin).  The Elf handle
+   should be closed with elf_end ().  The file handle will not be
+   closed.  */
 extern Elf *dwelf_elf_begin (int fd);
 
 /* Returns a human readable string for the given ELF header e_machine