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>

3 files changed, 34 insertions, 8 deletions

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 9e7bb316..45cc1b4e 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,11 @@
+2018-10-20  Mark Wielaard  <mark@klomp.org>
+
+	* libdwflP.h (__libdw_open_elf): New internal function declaration.
+	* open.c (what_kind): Rename close_fd to may_close_fd.
+	(__libdw_open_file): Replaced (and renamed) by a call to ...
+	(libdw_open_elf): this. And add never_close_fd argument.
+	(__libdw_open_elf): New function that calls libdw_open_elf.
+
 2018-10-18  Mark Wielaard  <mark@klomp.org>
 
 	* dwfl_segment_report_module.c (consider_note): Take align as new
diff --git a/libdwfl/libdwflP.h b/libdwfl/libdwflP.h
index 31e6e190..941a8b66 100644
--- a/libdwfl/libdwflP.h
+++ b/libdwfl/libdwflP.h
@@ -626,6 +626,10 @@ extern Dwfl_Error __libdw_open_file (int *fdp, Elf **elfp,
 				     bool close_on_fail, bool archive_ok)
   internal_function;
 
+/* Same as __libdw_open_file, but never closes the given file
+   descriptor and ELF_K_AR is always an acceptable type.  */
+extern Dwfl_Error __libdw_open_elf (int fd, Elf **elfp) internal_function;
+
 /* Fetch PT_DYNAMIC P_VADDR from ELF and store it to *VADDRP.  Return success.
    *VADDRP is not modified if the function fails.  */
 extern bool __libdwfl_dynamic_vaddr_get (Elf *elf, GElf_Addr *vaddrp)
diff --git a/libdwfl/open.c b/libdwfl/open.c
index 4e0461bd..74367359 100644
--- a/libdwfl/open.c
+++ b/libdwfl/open.c
@@ -95,7 +95,7 @@ decompress (int fd __attribute__ ((unused)), Elf **elf)
 }
 
 static Dwfl_Error
-what_kind (int fd, Elf **elfp, Elf_Kind *kind, bool *close_fd)
+what_kind (int fd, Elf **elfp, Elf_Kind *kind, bool *may_close_fd)
 {
   Dwfl_Error error = DWFL_E_NOERROR;
   *kind = elf_kind (*elfp);
@@ -108,7 +108,7 @@ what_kind (int fd, Elf **elfp, Elf_Kind *kind, bool *close_fd)
 	  error = decompress (fd, elfp);
 	  if (error == DWFL_E_NOERROR)
 	    {
-	      *close_fd = true;
+	      *may_close_fd = true;
 	      *kind = elf_kind (*elfp);
 	    }
 	}
@@ -116,15 +116,16 @@ what_kind (int fd, Elf **elfp, Elf_Kind *kind, bool *close_fd)
   return error;
 }
 
-Dwfl_Error internal_function
-__libdw_open_file (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok)
+static Dwfl_Error
+libdw_open_elf (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok,
+		bool never_close_fd)
 {
-  bool close_fd = false;
+  bool may_close_fd = false;
 
   Elf *elf = elf_begin (*fdp, ELF_C_READ_MMAP_PRIVATE, NULL);
 
   Elf_Kind kind;
-  Dwfl_Error error = what_kind (*fdp, &elf, &kind, &close_fd);
+  Dwfl_Error error = what_kind (*fdp, &elf, &kind, &may_close_fd);
   if (error == DWFL_E_BADELF)
     {
       /* It's not an ELF file or a compressed file.
@@ -153,7 +154,7 @@ __libdw_open_file (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok)
 	      elf->flags &= ~(ELF_F_MMAPPED | ELF_F_MALLOCED);
 	      elf_end (elf);
 	      elf = subelf;
-	      error = what_kind (*fdp, &elf, &kind, &close_fd);
+	      error = what_kind (*fdp, &elf, &kind, &may_close_fd);
 	    }
 	}
     }
@@ -169,7 +170,8 @@ __libdw_open_file (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok)
       elf = NULL;
     }
 
-  if (error == DWFL_E_NOERROR ? close_fd : close_on_fail)
+  if (! never_close_fd
+      && error == DWFL_E_NOERROR ? may_close_fd : close_on_fail)
     {
       close (*fdp);
       *fdp = -1;
@@ -178,3 +180,15 @@ __libdw_open_file (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok)
   *elfp = elf;
   return error;
 }
+
+Dwfl_Error internal_function
+__libdw_open_file (int *fdp, Elf **elfp, bool close_on_fail, bool archive_ok)
+{
+  return libdw_open_elf (fdp, elfp, close_on_fail, archive_ok, false);
+}
+
+Dwfl_Error internal_function
+__libdw_open_elf (int fd, Elf **elfp)
+{
+  return libdw_open_elf (&fd, elfp, false, true, true);
+}