[PATCH 3/3] gdbserver: pass osabi to GDB in target description

[Andrew Burgess <aburgess@redhat.com>]

On a Windows machine I built gdbserver, configured for the target
'x86_64-w64-mingw32', then on a GNU/Linux machine I built GDB with
support for all target (--enable-targets=all).

On the Windows machine I start gdbserver with a small test binary:

  $ gdbserver 192.168.129.25:54321 C:\some\directory\executable.exe

On the GNU/Linux machine I start GDB without the test binary, and
connect to gdbserver.

As I have not given GDB the test binary, my expectation is that GDB
would connect to gdbserver and then download the file over the remote
protocol, but instead I was presented with this message:

  (gdb) target remote 192.168.129.25:54321
  Remote debugging using 192.168.129.25:54321
  warning: C:\some\directory\executable.exe: No such file or directory.
  0x00007ffa3e1e1741 in ?? ()
  (gdb)

What I found is that if I told GDB where to find the binary, like
this:

  (gdb) file target:C:/some/directory/executable.exe
  A program is being debugged already.
  Are you sure you want to change the file? (y or n) y
  Reading C:/some/directory/executable.exe from remote target...
  warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
  Reading C:/some/directory/executable.exe from remote target...
  Reading symbols from target:C:/some/directory/executable.exe...
  (gdb)

then GDB would download the executable.

I eventually tracked the problem down to exec_file_find (solib.c).
The remote target was passing an absolute Windows filename (beginning
with "C:/" in this case), but in exec_file_find GDB was failing the
IS_TARGET_ABSOLUTE_PATH call, and so was treating the filename as
relative.

The IS_TARGET_ABSOLUTE_PATH call was failing because GDB thought that
the file system kind was "unix", and as the filename didn't start with
a "/" it assumed the filename was not absolute.

But I'm connecting to a Windows target, my 'target-file-system-kind'
was set to "auto", so should be figuring out that my file-system is
"dos-based".

Looking in effective_target_file_system_kind (filesystem.c), we find
that the logic of "auto" is delegated to the current gdbarch.  However
in windows-tdep.c we see:

  set_gdbarch_has_dos_based_file_system (gdbarch, 1);

So if we are using a Windows gdbarch we should have "dos-based"
filesystems.  What this means is that after connecting to the remote
target GDB has selected the wrong gdbarch.

What's happening is that the target description sent back by the
remote target only includes the x86-64 registers.  There's no
information about which OS we're on.  As a consequence, GDB picks the
first x86-64 gdbarch which can handle the provided register set, which
happens to be a GNU/Linux gdbarch.

And indeed, there doesn't appear to be anywhere in gdbserver that sets
the osabi on the target descriptions, though some target descriptions
do have their osabi set when the description is created, e.g. in:

  gdb/arch/amd64.c	- Sets GNU/Linux osabi when appropriate.
  gdb/arch/i386.c	- Likewise.
  gdb/arch/tic6x.c	- Always set GNU/Linux osabi.

Most target descriptions are created without an osabi, gdbserver does
nothing to fix this, and the description is returned to GDB without an
osabi included.

I propose that we always set the osabi name on the target descriptions
returned from gdbserver.  We could try to do this when the description
is first created, but that would mean passing extra flags into the
tdesc creation code (or just passing the osabi string in), and I don't
think that's really necessary.  If we consider the tdesc creation as
being about figuring out which registers are on the target, then it
makes sense that the osabi information is injected later.

So what I've done is require the osabi name to be passed to the
init_target_desc function.  This is called, I believe, for all
targets, in the gdbserver code.

Now when I connect to the Windows remote the target description
returned includes the osabi name.  With this extra information GDB
selects the correct gdbarch object, which means that GDB understands
the target has a "dos-based" file-system.  With that correct GDB
understands that the filename it was given is absolute, and so fetches
the file from the remote as we'd like.
---
 gdbserver/linux-aarch32-tdesc.cc |  2 +-
 gdbserver/linux-aarch64-tdesc.cc |  3 ++-
 gdbserver/linux-arc-low.cc       |  2 +-
 gdbserver/linux-arm-tdesc.cc     |  2 +-
 gdbserver/linux-csky-low.cc      |  2 +-
 gdbserver/linux-loongarch-low.cc |  2 +-
 gdbserver/linux-riscv-low.cc     |  2 +-
 gdbserver/linux-tic6x-low.cc     |  2 +-
 gdbserver/linux-x86-tdesc.cc     | 14 ++++++++++++--
 gdbserver/netbsd-aarch64-low.cc  |  2 +-
 gdbserver/netbsd-amd64-low.cc    |  2 +-
 gdbserver/netbsd-i386-low.cc     |  2 +-
 gdbserver/tdesc.cc               |  7 ++++++-
 gdbserver/tdesc.h                |  5 +++--
 gdbserver/win32-i386-low.cc      |  4 ++--
 gdbserver/win32-low.h            |  7 +++++++
 16 files changed, 42 insertions(+), 18 deletions(-)

diff --git a/gdbserver/linux-aarch32-tdesc.cc b/gdbserver/linux-aarch32-tdesc.cc
index b8987752b9f..bff671b4f1e 100644
--- a/gdbserver/linux-aarch32-tdesc.cc
+++ b/gdbserver/linux-aarch32-tdesc.cc
@@ -34,7 +34,7 @@ aarch32_linux_read_description ()
       tdesc_aarch32 = aarch32_create_target_description (false);
 
       static const char *expedite_regs[] = { "r11", "sp", "pc", 0 };
-      init_target_desc (tdesc_aarch32, expedite_regs);
+      init_target_desc (tdesc_aarch32, expedite_regs, "GNU/Linux");
     }
   return tdesc_aarch32;
 }
diff --git a/gdbserver/linux-aarch64-tdesc.cc b/gdbserver/linux-aarch64-tdesc.cc
index 31ec7854cc0..f6c258f5162 100644
--- a/gdbserver/linux-aarch64-tdesc.cc
+++ b/gdbserver/linux-aarch64-tdesc.cc
@@ -67,7 +67,8 @@ aarch64_linux_read_description (const aarch64_features &features)
 
       expedited_registers.push_back (nullptr);
 
-      init_target_desc (tdesc, (const char **) expedited_registers.data ());
+      init_target_desc (tdesc, (const char **) expedited_registers.data (),
+			"GNU/Linux");
 
       tdesc_aarch64_map[features] = tdesc;
     }
diff --git a/gdbserver/linux-arc-low.cc b/gdbserver/linux-arc-low.cc
index dda224f0a74..d382791c4bb 100644
--- a/gdbserver/linux-arc-low.cc
+++ b/gdbserver/linux-arc-low.cc
@@ -114,7 +114,7 @@ arc_linux_read_description (void)
   target_desc_up tdesc = arc_create_target_description (features);
 
   static const char *expedite_regs[] = { "sp", "status32", nullptr };
-  init_target_desc (tdesc.get (), expedite_regs);
+  init_target_desc (tdesc.get (), expedite_regs, "GNU/Linux");
 
   return tdesc.release ();
 }
diff --git a/gdbserver/linux-arm-tdesc.cc b/gdbserver/linux-arm-tdesc.cc
index 559f9b0f3dc..e6bb12bad79 100644
--- a/gdbserver/linux-arm-tdesc.cc
+++ b/gdbserver/linux-arm-tdesc.cc
@@ -37,7 +37,7 @@ arm_linux_read_description (arm_fp_type fp_type)
       tdesc = arm_create_target_description (fp_type, false);
 
       static const char *expedite_regs[] = { "r11", "sp", "pc", 0 };
-      init_target_desc (tdesc, expedite_regs);
+      init_target_desc (tdesc, expedite_regs, "GNU/Linux");
 
       tdesc_arm_list[fp_type] = tdesc;
     }
diff --git a/gdbserver/linux-csky-low.cc b/gdbserver/linux-csky-low.cc
index 2eb5a2df17b..0154516f196 100644
--- a/gdbserver/linux-csky-low.cc
+++ b/gdbserver/linux-csky-low.cc
@@ -133,7 +133,7 @@ csky_target::low_arch_setup ()
 
   if (tdesc->expedite_regs.empty ())
     {
-      init_target_desc (tdesc.get (), expedite_regs);
+      init_target_desc (tdesc.get (), expedite_regs, "GNU/Linux");
       gdb_assert (!tdesc->expedite_regs.empty ());
     }
 
diff --git a/gdbserver/linux-loongarch-low.cc b/gdbserver/linux-loongarch-low.cc
index 584ea64a7d9..3161d72b733 100644
--- a/gdbserver/linux-loongarch-low.cc
+++ b/gdbserver/linux-loongarch-low.cc
@@ -85,7 +85,7 @@ loongarch_target::low_arch_setup ()
 
   if (tdesc->expedite_regs.empty ())
     {
-      init_target_desc (tdesc.get (), expedite_regs);
+      init_target_desc (tdesc.get (), expedite_regs, "GNU/Linux");
       gdb_assert (!tdesc->expedite_regs.empty ());
     }
   current_process ()->tdesc = tdesc.release ();
diff --git a/gdbserver/linux-riscv-low.cc b/gdbserver/linux-riscv-low.cc
index c4554c507a8..a02a2bef35d 100644
--- a/gdbserver/linux-riscv-low.cc
+++ b/gdbserver/linux-riscv-low.cc
@@ -91,7 +91,7 @@ riscv_target::low_arch_setup ()
 
   if (tdesc->expedite_regs.empty ())
     {
-      init_target_desc (tdesc.get (), expedite_regs);
+      init_target_desc (tdesc.get (), expedite_regs, "GNU/Linux");
       gdb_assert (!tdesc->expedite_regs.empty ());
     }
 
diff --git a/gdbserver/linux-tic6x-low.cc b/gdbserver/linux-tic6x-low.cc
index 707be2e7b0f..a6764149454 100644
--- a/gdbserver/linux-tic6x-low.cc
+++ b/gdbserver/linux-tic6x-low.cc
@@ -228,7 +228,7 @@ tic6x_read_description (enum c6x_feature feature)
     {
       *tdesc = tic6x_create_target_description (feature);
       static const char *expedite_regs[] = { "A15", "PC", NULL };
-      init_target_desc (*tdesc, expedite_regs);
+      init_target_desc (*tdesc, expedite_regs, "GNU/Linux");
     }
 
   return *tdesc;
diff --git a/gdbserver/linux-x86-tdesc.cc b/gdbserver/linux-x86-tdesc.cc
index 13c80762605..f6051a6729f 100644
--- a/gdbserver/linux-x86-tdesc.cc
+++ b/gdbserver/linux-x86-tdesc.cc
@@ -26,10 +26,20 @@
 void
 x86_linux_post_init_tdesc (target_desc *tdesc, bool is_64bit)
 {
+  constexpr const char *osabi_name = "GNU/Linux";
+
+#ifndef IN_PROCESS_AGENT
+  /* x86 target descriptions are created with the osabi already set.
+     However, init_target_desc requires us to override the already set
+     value.  That's fine, out new string should match the old one.  */
+  gdb_assert (tdesc_osabi_name (tdesc) != nullptr);
+  gdb_assert (strcmp (tdesc_osabi_name (tdesc), osabi_name) == 0);
+#endif /* ! IN_PROCESS_AGENT */
+
 #ifdef __x86_64__
   if (is_64bit)
-    init_target_desc (tdesc, amd64_expedite_regs);
+    init_target_desc (tdesc, amd64_expedite_regs, osabi_name);
   else
 #endif
-    init_target_desc (tdesc, i386_expedite_regs);
+    init_target_desc (tdesc, i386_expedite_regs, osabi_name);
 }
diff --git a/gdbserver/netbsd-aarch64-low.cc b/gdbserver/netbsd-aarch64-low.cc
index f20a1a71773..c86c46335e0 100644
--- a/gdbserver/netbsd-aarch64-low.cc
+++ b/gdbserver/netbsd-aarch64-low.cc
@@ -98,7 +98,7 @@ netbsd_aarch64_target::low_arch_setup ()
     = aarch64_create_target_description ({});
 
   static const char *expedite_regs_aarch64[] = { "x29", "sp", "pc", NULL };
-  init_target_desc (tdesc, expedite_regs_aarch64);
+  init_target_desc (tdesc, expedite_regs_aarch64, "NetBSD");
 
   current_process ()->tdesc = tdesc;
 }
diff --git a/gdbserver/netbsd-amd64-low.cc b/gdbserver/netbsd-amd64-low.cc
index b3f3aab5ec3..c8c41ae1bc8 100644
--- a/gdbserver/netbsd-amd64-low.cc
+++ b/gdbserver/netbsd-amd64-low.cc
@@ -193,7 +193,7 @@ netbsd_amd64_target::low_arch_setup ()
   target_desc *tdesc
     = amd64_create_target_description (X86_XSTATE_SSE_MASK, false, false, false);
 
-  init_target_desc (tdesc, amd64_expedite_regs);
+  init_target_desc (tdesc, amd64_expedite_regs, "NetBSD");
 
   current_process ()->tdesc = tdesc;
 }
diff --git a/gdbserver/netbsd-i386-low.cc b/gdbserver/netbsd-i386-low.cc
index 247a39797c4..f5da9dc449b 100644
--- a/gdbserver/netbsd-i386-low.cc
+++ b/gdbserver/netbsd-i386-low.cc
@@ -142,7 +142,7 @@ netbsd_i386_target::low_arch_setup ()
   target_desc *tdesc
     = i386_create_target_description (X86_XSTATE_SSE_MASK, false, false);
 
-  init_target_desc (tdesc, i386_expedite_regs);
+  init_target_desc (tdesc, i386_expedite_regs, "NetBSD");
 
   current_process ()->tdesc = tdesc;
 }
diff --git a/gdbserver/tdesc.cc b/gdbserver/tdesc.cc
index 7f11120c318..94ec46deb22 100644
--- a/gdbserver/tdesc.cc
+++ b/gdbserver/tdesc.cc
@@ -53,7 +53,8 @@ void target_desc::accept (tdesc_element_visitor &v) const
 
 void
 init_target_desc (struct target_desc *tdesc,
-		  const char **expedite_regs)
+		  const char **expedite_regs,
+		  const char *osabi)
 {
   int offset = 0;
 
@@ -88,6 +89,10 @@ init_target_desc (struct target_desc *tdesc,
   int expedite_count = 0;
   while (expedite_regs[expedite_count] != nullptr)
     tdesc->expedite_regs.push_back (expedite_regs[expedite_count++]);
+
+  /* If TDESC doesn't already have an osabi set, then set it now.  */
+  gdb_assert (osabi != nullptr);
+  set_tdesc_osabi (tdesc, osabi);
 #endif
 }
 
diff --git a/gdbserver/tdesc.h b/gdbserver/tdesc.h
index 6fc37d038bc..05c48f7c989 100644
--- a/gdbserver/tdesc.h
+++ b/gdbserver/tdesc.h
@@ -81,10 +81,11 @@ void copy_target_description (struct target_desc *dest,
 			      const struct target_desc *src);
 
 /* Initialize TDESC, and then set its expedite_regs field to
-   EXPEDITE_REGS.  */
+   EXPEDITE_REGS.  The osabi name of TDESC is set to OSABI.  */
 
 void init_target_desc (struct target_desc *tdesc,
-		       const char **expedite_regs);
+		       const char **expedite_regs,
+		       const char *osabi);
 
 /* Return the current inferior's target description.  Never returns
    NULL.  */
diff --git a/gdbserver/win32-i386-low.cc b/gdbserver/win32-i386-low.cc
index 0a761ca58ef..b6cd0429557 100644
--- a/gdbserver/win32-i386-low.cc
+++ b/gdbserver/win32-i386-low.cc
@@ -596,12 +596,12 @@ i386_arch_setup (void)
 #ifdef __x86_64__
   tdesc = amd64_create_target_description (X86_XSTATE_SSE_MASK, false,
 					   false, false);
-  init_target_desc (tdesc, amd64_expedite_regs);
+  init_target_desc (tdesc, amd64_expedite_regs, OSABI_NAME);
   win32_tdesc = tdesc;
 #endif
 
   tdesc = i386_create_target_description (X86_XSTATE_SSE_MASK, false, false);
-  init_target_desc (tdesc, i386_expedite_regs);
+  init_target_desc (tdesc, i386_expedite_regs, OSABI_NAME);
 #ifdef __x86_64__
   wow64_win32_tdesc = tdesc;
 #else
diff --git a/gdbserver/win32-low.h b/gdbserver/win32-low.h
index ff997df0a66..e447cc5f9b2 100644
--- a/gdbserver/win32-low.h
+++ b/gdbserver/win32-low.h
@@ -31,6 +31,13 @@ extern const struct target_desc *win32_tdesc;
 extern const struct target_desc *wow64_win32_tdesc;
 #endif
 
+#ifdef __CYGWIN__
+#define OSABI_NAME "Cygwin"
+constexpr const char *OSABI_NAME = "Cygwin";
+#else
+constexpr const char *OSABI_NAME = "Windows";
+#endif
+
 struct win32_target_ops
 {
   /* Architecture-specific setup.  */
-- 
2.25.4