[PATCH] gdb: use bfd_get_section_contents to read section contents in, is_linked_with_cygwin_dll (was: Re: [PATCH 6/7] gdb: select "Cygwin" OS ABI for Cygwin binaries)

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2020-04-02 9:22 a.m., Tom Tromey wrote:
> There's no way to ever release this data.  Since it's only used once, it
> may be better to use bfd_get_full_section_contents, so the memory can be
> freed when done.

Good point, I hadn't thought of that.

Here's a patch.  I only tested it on GNU/Linux, making sure GDB is still able
to recognize a mingw binary and a cygwin binary.


From b7f4f941018c7b4a7431837ccf5eee5c992f4111 Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@polymtl.ca>
Date: Thu, 2 Apr 2020 10:34:39 -0400
Subject: [PATCH] gdb: use bfd_get_section_contents to read section contents in
 is_linked_with_cygwin_dll

The function is_linked_with_cygwin_dll currently uses
gdb_bfd_map_section to get some section contents.  This is not ideal
because that memory, which is only used in this function, can't be
released.  Instead, it was suggested to use bfd_get_section_contents,
and this is what this patch does.

I decided to make a small bfd_get_section_contents wrapper in gdb_bfd.c,
which returns the contents in a gdb::byte_vector.  The wrapper also
makes it easy to get the full section contents into a gdb::byte_vector.
Note that BFD provides the bfd_get_full_section_contents function, but
this one returns a newly-allocated buffer.  gdb_bfd_get_section_contents
offers a consistent interface, regardless of whether you need to read
part of a section or the full section.

gdb_bfd_get_section_contents could be used at many places that already
allocate a vector of the size of the section and then call
bfd_get_section_contents.  I think these call sites can be updated over
time.

gdb/ChangeLog:

	* gdb_bfd.h: Include gdbsupport/byte-vector.h and
	gdbsupport/gdb_optional.h.
	(BFD_SIZE_TYPE_MAX): New macro.
	(gdb_bfd_get_section_contents): New declaration.
	* gdb_bfd.c (gdb_bfd_get_section_contents): New function.
	* windows-tdep.c (is_linked_with_cygwin_dll): Use
	gdb_bfd_get_section_contents.
---
 gdb/gdb_bfd.c      | 25 ++++++++++++++++++++++++-
 gdb/gdb_bfd.h      | 15 +++++++++++++++
 gdb/windows-tdep.c | 13 ++++++-------
 3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/gdb/gdb_bfd.c b/gdb/gdb_bfd.c
index 5a6dee2d51a8..2b34e0f3e17c 100644
--- a/gdb/gdb_bfd.c
+++ b/gdb/gdb_bfd.c
@@ -926,7 +926,30 @@ gdb_bfd_requires_relocations (bfd *abfd)
   return gdata->needs_relocations;
 }

-\f
+/* See gdb_bfd.h.  */
+
+bool
+gdb_bfd_get_section_contents (bfd *abfd, asection *section,
+			      gdb::byte_vector *contents, file_ptr offset,
+			      bfd_size_type count)
+{
+  bfd_size_type section_size = bfd_section_size (section);
+
+  /* Allow `offset == section_size`, to allow the corner case of
+     `offset == section_size`, `count = 0`.  */
+  gdb_assert (offset <= section_size);
+
+  /* If COUNT is unspecified, get the contents from OFFSET until the end of the
+     section.  */
+  if (count == BFD_SIZE_TYPE_MAX)
+    count = section_size - offset;
+
+  gdb_assert ((offset + count) <= section_size);
+
+  contents->resize (count);
+
+  return bfd_get_section_contents (abfd, section, contents->data (), offset, count);
+}

 /* A callback for htab_traverse that prints a single BFD.  */

diff --git a/gdb/gdb_bfd.h b/gdb/gdb_bfd.h
index 9b1e292bf18f..ffea4f6b715e 100644
--- a/gdb/gdb_bfd.h
+++ b/gdb/gdb_bfd.h
@@ -21,6 +21,8 @@
 #define GDB_BFD_H

 #include "registry.h"
+#include "gdbsupport/byte-vector.h"
+#include "gdbsupport/gdb_optional.h"
 #include "gdbsupport/gdb_ref_ptr.h"

 DECLARE_REGISTRY (bfd);
@@ -30,6 +32,8 @@ DECLARE_REGISTRY (bfd);

 #define TARGET_SYSROOT_PREFIX "target:"

+#define BFD_SIZE_TYPE_MAX ((bfd_size_type) -1)
+
 /* Returns nonzero if NAME starts with TARGET_SYSROOT_PREFIX, zero
    otherwise.  */

@@ -181,4 +185,15 @@ int gdb_bfd_count_sections (bfd *abfd);

 int gdb_bfd_requires_relocations (bfd *abfd);

+/* Wrapper around bfd_get_section_contents, returning the requested section
+   contents in *CONTENTS.  Return true on success, false otherwise.
+
+   If COUNT is not specified, read from OFFSET until the end of the
+   section.  */
+
+bool
+gdb_bfd_get_section_contents (bfd *abfd, asection *section,
+			      gdb::byte_vector *contents, file_ptr offset = 0,
+			      bfd_size_type count = BFD_SIZE_TYPE_MAX);
+
 #endif /* GDB_BFD_H */
diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
index 9c5dfd183bfa..9b7dc1d12b26 100644
--- a/gdb/windows-tdep.c
+++ b/gdb/windows-tdep.c
@@ -1005,18 +1005,17 @@ is_linked_with_cygwin_dll (bfd *abfd)
   bfd_vma idata_addr
     = pe_data (abfd)->pe_opthdr.DataDirectory[PE_IMPORT_TABLE].VirtualAddress;

-  /* Map the section's data.  */
-  bfd_size_type idata_size;
-  const gdb_byte *const idata_contents
-    = gdb_bfd_map_section (idata_section, &idata_size);
-  if (idata_contents == nullptr)
+  /* Get the section's data.  */
+  gdb::byte_vector idata_contents;
+  if (!gdb_bfd_get_section_contents(abfd, idata_section, &idata_contents))
     {
       warning (_("Failed to get content of .idata section."));
       return false;
     }

-  const gdb_byte *iter = idata_contents;
-  const gdb_byte *end = idata_contents + idata_size;
+  size_t idata_size = idata_contents.size ();
+  const gdb_byte *iter = idata_contents.data ();
+  const gdb_byte *end = idata_contents.data () + idata_size;
   const pe_import_directory_entry null_dir_entry = { 0 };

   /* Iterate through all directory entries.  */
-- 
2.26.0