[PATCH v4 6/6] GDB: aarch64-linux: Fix build failure on musl systems

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

When building against musl (e.g. on Alpine Linux), the following error
happens:

    CXX    linux-aarch64-low.o
  In file included from /home/bauermann/src/binutils-gdb/gdbserver/linux-aarch64-low.cc:48:
  /home/bauermann/src/binutils-gdb/gdbserver/../gdb/nat/aarch64-gcs-linux.h:36:8: error: redefinition of 'struct user_gcs'
     36 | struct user_gcs
        |        ^~~~~~~~
  In file included from /home/bauermann/src/binutils-gdb/gdbserver/linux-aarch64-low.cc:36:
  /usr/include/asm/ptrace.h:329:8: note: previous definition of 'struct user_gcs'
    329 | struct user_gcs {
        |        ^~~~~~~~
  make[2]: *** [Makefile:565: linux-aarch64-low.o] Error 1

aarch64-linux-tdep.c fails to build in the same way.  This happens because
aarch64-gcs-linux-ptrace.h uses GCS_MAGIC to see whether the system headers
have GCS-related definitions.  The problem is that GCS_MAGIC is defined in
<asm/sigcontext.h> while struct gcs_user is defined in <asm/ptrace.h>.
It's fine on glibc systems because in the set of system headers that
linux-aarch64-low.cc and aarch64-linux-tdep.c include, <asm/sigcontext.h>
ends up being included implicitly as well.  This doesn't happen when using
musl's headers though.

There isn't a macro in <asm/ptrace.h> whose presence is correlated with
the presence of the struct user_gcs definition, so a configure check is
needed to detect it and conditionally define the struct.

Note that there's another build issue with musl, described in
PR gdb/33747 affecting compilation of gdb/ser-unix.c.  In order to be
able to test this patch, I applied the patch in comment 11 there.

Tested with a native build on an Alpine Linux aarch64 system, and also
verified that all gdb.arch/aarch64-gcs*.exp tests pass on it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33926
Co-authored-by: Chris Packham <judge.packham@gmail.com>
---

As I mentioned in the cover letter, if the series is approved I will
propose a version of this patch for the GDB 17 branch which doesn't depend
on the previous patches.

Changes since v3:
- Renamed gdb/nat/aarch64-gcs-linux-ptrace.h to gdb/nat/aarch64-gcs-linux.h.

Changes since v2:
- It's the same patch except that the header changed is
  gdb/nat/aarch64-gcs-linux-ptrace.h rather than gdb/nat/aarch64-linux.h,
  which doesn't exist in this version of the patch series.

 gdb/nat/aarch64-gcs-linux.h |  5 ++---
 gdbsupport/config.in        |  3 +++
 gdbsupport/configure        | 36 ++++++++++++++++++++++++++++++++++++
 gdbsupport/configure.ac     | 19 +++++++++++++++++++
 4 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/gdb/nat/aarch64-gcs-linux.h b/gdb/nat/aarch64-gcs-linux.h
index ddf8342a4284..3ce7b6486785 100644
--- a/gdb/nat/aarch64-gcs-linux.h
+++ b/gdb/nat/aarch64-gcs-linux.h
@@ -28,8 +28,7 @@
 #define HWCAP_GCS (1ULL << 32)
 #endif
 
-/* Make sure we only define these if the kernel header doesn't.  */
-#ifndef GCS_MAGIC
+#ifndef HAVE_STRUCT_USER_GCS
 
 /* GCS state (NT_ARM_GCS).  */
 
@@ -40,6 +39,6 @@ struct user_gcs
   uint64_t gcspr_el0;
 };
 
-#endif /* GCS_MAGIC */
+#endif /* HAVE_STRUCT_USER_GCS */
 
 #endif /* GDB_NAT_AARCH64_GCS_LINUX_H */
diff --git a/gdbsupport/config.in b/gdbsupport/config.in
index 0beacf22c057..2957ee0f0301 100644
--- a/gdbsupport/config.in
+++ b/gdbsupport/config.in
@@ -271,6 +271,9 @@
 /* Define to 1 if `st_blocks' is a member of `struct stat'. */
 #undef HAVE_STRUCT_STAT_ST_BLOCKS
 
+/* Define to 1 if your system has struct user_gcs. */
+#undef HAVE_STRUCT_USER_GCS
+
 /* Define to 1 if you have the <sys/param.h> header file. */
 #undef HAVE_SYS_PARAM_H
 
diff --git a/gdbsupport/configure b/gdbsupport/configure
index 05ad00f48c4f..584a80ef8255 100755
--- a/gdbsupport/configure
+++ b/gdbsupport/configure
@@ -14307,6 +14307,42 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
+# Check for `struct user_gcs`
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for struct user_gcs" >&5
+$as_echo_n "checking for struct user_gcs... " >&6; }
+if ${gdb_cv_struct_user_gcs+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <sys/ptrace.h>
+	 #include <asm/ptrace.h>
+int
+main ()
+{
+struct user_gcs u;
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_struct_user_gcs=yes
+else
+  gdb_cv_struct_user_gcs=no
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_struct_user_gcs" >&5
+$as_echo "$gdb_cv_struct_user_gcs" >&6; }
+if test "$gdb_cv_struct_user_gcs" = yes; then
+
+$as_echo "#define HAVE_STRUCT_USER_GCS 1" >>confdefs.h
+
+fi
+
 # Set the 'development' global.
 . $srcdir/../bfd/development.sh
 
diff --git a/gdbsupport/configure.ac b/gdbsupport/configure.ac
index 5491b8f3f10a..6c06832f0913 100644
--- a/gdbsupport/configure.ac
+++ b/gdbsupport/configure.ac
@@ -68,6 +68,25 @@ GDB_AC_PTRACE
 AM_GDB_COMPILER_TYPE
 AM_GDB_WARNINGS
 
+# Check for `struct user_gcs`
+AC_CACHE_CHECK(
+  [for struct user_gcs],
+  [gdb_cv_struct_user_gcs],
+  [AC_COMPILE_IFELSE(
+     [AC_LANG_PROGRAM(
+	[#include <sys/ptrace.h>
+	 #include <asm/ptrace.h>],
+	[struct user_gcs u;]
+      )],
+     [gdb_cv_struct_user_gcs=yes],
+     [gdb_cv_struct_user_gcs=no]
+  )]
+)
+if test "$gdb_cv_struct_user_gcs" = yes; then
+  AC_DEFINE(HAVE_STRUCT_USER_GCS, 1,
+	    [Define to 1 if your system has struct user_gcs.])
+fi
+
 # Set the 'development' global.
 . $srcdir/../bfd/development.sh