gdb: select "Cygwin" OS ABI for Cygwin binaries

Before this patch, the "Windows" OS ABI is selected for all Windows
executables, including Cygwin ones.  This patch makes GDB differentiate
Cygwin binaries from non-Cygwin ones, and selects the "Cygwin" OS ABI
for the Cygwin ones.

To check whether a Windows PE executable is a Cygwin one, we check the
library list in the .idata section, see if it contains "cygwin1.dll".

I had to add code to parse the .idata section, because BFD doesn't seem
to expose this information.  BFD does parse this information, but only
to print it in textual form (function pe_print_idata):

  https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/peXXigen.c;h=e42d646552a0ca1e856e082256cd3d943b54ddf0;hb=HEAD#l1261

Here's the relevant portion of the PE format documentation:

  https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#the-idata-section

This page was also useful:

  https://blog.kowalczyk.info/articles/pefileformat.html#9ccef823-67e7-4372-9172-045d7b1fb006

With this patch applied, this is what I get:

    (gdb) file some_mingw_x86_64_binary.exe
    Reading symbols from some_mingw_x86_64_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Windows").
    The default OS ABI is "GNU/Linux".

    (gdb) file some_mingw_i386_binary.exe
    Reading symbols from some_mingw_i386_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Windows").
    The default OS ABI is "GNU/Linux".

    (gdb) file some_cygwin_x86_64_binary.exe
    Reading symbols from some_cygwin_x86_64_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Cygwin").
    The default OS ABI is "GNU/Linux".

gdb/ChangeLog:

	* windows-tdep.h (is_linked_with_cygwin_dll): New declaration.
	* windows-tdep.c (CYGWIN_DLL_NAME): New.
	(pe_import_directory_entry): New struct type.
	(is_linked_with_cygwin_dll): New function.
	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): Select
	GDB_OSABI_CYGWIN if the BFD is linked with the Cygwin DLL.
	* i386-windows-tdep.c (i386_windows_osabi_sniffer): Likewise.

5 files changed, 127 insertions, 6 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 7e844e6..68c8bf7 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,15 @@
 2020-03-16  Simon Marchi  <simon.marchi@efficios.com>
 
+	* windows-tdep.h (is_linked_with_cygwin_dll): New declaration.
+	* windows-tdep.c (CYGWIN_DLL_NAME): New.
+	(pe_import_directory_entry): New struct type.
+	(is_linked_with_cygwin_dll): New function.
+	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): Select
+	GDB_OSABI_CYGWIN if the BFD is linked with the Cygwin DLL.
+	* i386-windows-tdep.c (i386_windows_osabi_sniffer): Likewise.
+
+2020-03-16  Simon Marchi  <simon.marchi@efficios.com>
+
 	* i386-windows-tdep.c: Mass-rename "cygwin" to "windows", except
 	i386_cygwin_core_osabi_sniffer.
 
diff --git a/gdb/amd64-windows-tdep.c b/gdb/amd64-windows-tdep.c
index 88ff794..e0346f8 100644
--- a/gdb/amd64-windows-tdep.c
+++ b/gdb/amd64-windows-tdep.c
@@ -1249,10 +1249,13 @@ amd64_windows_osabi_sniffer (bfd *abfd)
 {
   const char *target_name = bfd_get_target (abfd);
 
-  if (strcmp (target_name, "pei-x86-64") == 0)
-    return GDB_OSABI_WINDOWS;
+  if (!streq (target_name, "pei-x86-64"))
+    return GDB_OSABI_UNKNOWN;
 
-  return GDB_OSABI_UNKNOWN;
+  if (is_linked_with_cygwin_dll (abfd))
+    return GDB_OSABI_CYGWIN;
+
+  return GDB_OSABI_WINDOWS;
 }
 
 void _initialize_amd64_windows_tdep ();
diff --git a/gdb/i386-windows-tdep.c b/gdb/i386-windows-tdep.c
index a71ceda..bd6107b 100644
--- a/gdb/i386-windows-tdep.c
+++ b/gdb/i386-windows-tdep.c
@@ -232,10 +232,13 @@ i386_windows_osabi_sniffer (bfd *abfd)
 {
   const char *target_name = bfd_get_target (abfd);
 
-  if (strcmp (target_name, "pei-i386") == 0)
-    return GDB_OSABI_WINDOWS;
+  if (!streq (target_name, "pei-i386"))
+    return GDB_OSABI_UNKNOWN;
 
-  return GDB_OSABI_UNKNOWN;
+  if (is_linked_with_cygwin_dll (abfd))
+    return GDB_OSABI_CYGWIN;
+
+  return GDB_OSABI_WINDOWS;
 }
 
 static enum gdb_osabi
diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
index e02b1ce..31b7b57 100644
--- a/gdb/windows-tdep.c
+++ b/gdb/windows-tdep.c
@@ -38,6 +38,8 @@
 #include "libcoff.h"
 #include "solist.h"
 
+#define CYGWIN_DLL_NAME "cygwin1.dll"
+
 /* Windows signal numbers differ between MinGW flavors and between
    those and Cygwin.  The below enumeration was gleaned from the
    respective headers; the ones marked with MinGW64/Cygwin are defined
@@ -898,6 +900,103 @@ static const struct internalvar_funcs tlb_funcs =
   NULL
 };
 
+/* Layout of an element of a PE's Import Directory Table.  Based on:
+
+     https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#import-directory-table
+ */
+
+struct pe_import_directory_entry
+{
+  uint32_t import_lookup_table_rva;
+  uint32_t timestamp;
+  uint32_t forwarder_chain;
+  uint32_t name_rva;
+  uint32_t import_address_table_rva;
+};
+
+gdb_static_assert (sizeof (pe_import_directory_entry) == 20);
+
+/* See windows-tdep.h.  */
+
+bool
+is_linked_with_cygwin_dll (bfd *abfd)
+{
+  /* The list of DLLs a PE is linked to is in the .idata section.  See:
+
+     https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#the-idata-section
+   */
+  asection *idata_section = bfd_get_section_by_name (abfd, ".idata");
+  if (idata_section == nullptr)
+    return false;
+
+  /* Find the virtual address of the .idata section.  We must subtract this
+     from the RVAs (relative virtual addresses) to obtain an offset in the
+     section. */
+  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)
+    {
+      warning (_("Failed to get content of .idata section."));
+      return false;
+    }
+
+  const gdb_byte *iter = idata_contents;
+  const gdb_byte *end = idata_contents + idata_size;
+  const pe_import_directory_entry null_dir_entry = { 0 };
+
+  /* Iterate through all directory entries.  */
+  while (true)
+    {
+      /* Is there enough space left in the section for another entry?  */
+      if (iter + sizeof (pe_import_directory_entry) > end)
+	{
+	  warning (_("Failed to parse .idata section: unexpected end of "
+		     ".idata section."));
+	  break;
+	}
+
+      pe_import_directory_entry *dir_entry = (pe_import_directory_entry *) iter;
+
+      /* Is it the end of list marker?  */
+      if (memcmp (dir_entry, &null_dir_entry,
+		  sizeof (pe_import_directory_entry)) == 0)
+	break;
+
+      bfd_vma name_addr = dir_entry->name_rva;
+
+      /* If the name's virtual address is smaller than the section's virtual
+         address, there's a problem.  */
+      if (name_addr < idata_addr
+	  || name_addr >= (idata_addr + idata_size))
+	{
+	  warning (_("\
+Failed to parse .idata section: name's virtual address (0x%" BFD_VMA_FMT "x) \
+is outside .idata section's range [0x%" BFD_VMA_FMT "x, 0x%" BFD_VMA_FMT "x[."),
+		   name_addr, idata_addr, idata_addr + idata_size);
+	  break;
+	}
+
+      const gdb_byte *name = &idata_contents[name_addr - idata_addr];
+
+      /* Make sure we don't overshoot the end of the section with the streq.  */
+      if (name + sizeof(CYGWIN_DLL_NAME) > end)
+	continue;
+
+      /* Finally, check if this is the dll name we are looking for.  */
+      if (streq ((const char *) name, CYGWIN_DLL_NAME))
+	return true;
+
+      iter += sizeof(pe_import_directory_entry);
+    }
+
+    return false;
+}
+
 void _initialize_windows_tdep ();
 void
 _initialize_windows_tdep ()
diff --git a/gdb/windows-tdep.h b/gdb/windows-tdep.h
index 34474f2..f2dc426 100644
--- a/gdb/windows-tdep.h
+++ b/gdb/windows-tdep.h
@@ -33,4 +33,10 @@ extern void windows_xfer_shared_library (const char* so_name,
 
 extern void windows_init_abi (struct gdbarch_info info,
 			      struct gdbarch *gdbarch);
+
+/* Return true if the Portable Executable behind ABFD uses the Cygwin dll
+   (cygwin1.dll).  */
+
+extern bool is_linked_with_cygwin_dll (bfd *abfd);
+
 #endif