hurd: Add enough auxv support for AT_ENTRY for PIE binaries

Add PIE support for hurd, by faking an AT_ENTRY auxv entry.  That value
is expected to be read by svr4_exec_displacement, which will propagate
the executable displacement.

gdb/ChangeLog:

	* gdb/gnu-nat.c: Include <elf.h> and <link.h>.
	(gnu_xfer_auxv): New function.
	(gnu_xfer_partial): Call gnu_xfer_auxv when `object' is
	TARGET_OBJECT_AUXV.

2 files changed, 65 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index cf93d22..238bcba 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2018-01-08  Samuel Thibault  <samuel.thibault@ens-lyon.org>
+
+	* gdb/gnu-nat.c: Include <elf.h> and <link.h>.
+	(gnu_xfer_auxv): New function.
+	(gnu_xfer_partial): Call gnu_xfer_auxv when `object' is
+	TARGET_OBJECT_AUXV.
+
 2018-01-08  Yao Qi  <yao.qi@linaro.org>
 	    Simon Marchi  <simon.marchi@ericsson.com>
 
diff --git a/gdb/gnu-nat.c b/gdb/gnu-nat.c
index 3972b22..f2d5fca 100644
--- a/gdb/gnu-nat.c
+++ b/gdb/gnu-nat.c
@@ -52,6 +52,8 @@ extern "C"
 #include <setjmp.h>
 #include <signal.h>
 #include <sys/ptrace.h>
+#include <elf.h>
+#include <link.h>
 
 #include "inferior.h"
 #include "symtab.h"
@@ -2541,6 +2543,60 @@ gnu_xfer_memory (gdb_byte *readbuf, const gdb_byte *writebuf,
     }
 }
 
+/* GNU does not have auxv, but we can at least fake the AT_ENTRY entry for PIE
+   binaries.  */
+static enum target_xfer_status
+gnu_xfer_auxv (gdb_byte *readbuf, const gdb_byte *writebuf,
+	       CORE_ADDR memaddr, ULONGEST len, ULONGEST *xfered_len)
+{
+  task_t task = (gnu_current_inf
+		 ? (gnu_current_inf->task
+		    ? gnu_current_inf->task->port : 0)
+		 : 0);
+  process_t proc;
+  int res;
+  kern_return_t err;
+  vm_address_t entry;
+  ElfW(auxv_t) auxv[2];
+
+  if (task == MACH_PORT_NULL)
+    return TARGET_XFER_E_IO;
+  if (writebuf != NULL)
+    return TARGET_XFER_E_IO;
+
+  if (memaddr == sizeof (auxv))
+    return TARGET_XFER_EOF;
+  if (memaddr > sizeof (auxv))
+    return TARGET_XFER_E_IO;
+
+  err = proc_task2proc (proc_server, task, &proc);
+  if (err != 0)
+    return TARGET_XFER_E_IO;
+
+  /* Get entry from proc server.  */
+  err = proc_get_entry (proc, &entry);
+  if (err != 0)
+    return TARGET_XFER_E_IO;
+
+  /* Fake auxv entry.  */
+  auxv[0].a_type = AT_ENTRY;
+  auxv[0].a_un.a_val = entry;
+  auxv[1].a_type = AT_NULL;
+  auxv[1].a_un.a_val = 0;
+
+  inf_debug (gnu_current_inf, "reading auxv %s[%s] --> %s",
+	     paddress (target_gdbarch (), memaddr), pulongest (len),
+	     host_address_to_string (readbuf));
+
+  if (memaddr + len > sizeof (auxv))
+    len = sizeof (auxv) - memaddr;
+
+  memcpy (readbuf, (gdb_byte *) &auxv + memaddr, len);
+  *xfered_len = len;
+
+  return TARGET_XFER_OK;
+}
+
 /* Target to_xfer_partial implementation.  */
 
 static enum target_xfer_status
@@ -2553,6 +2609,8 @@ gnu_xfer_partial (struct target_ops *ops, enum target_object object,
     {
     case TARGET_OBJECT_MEMORY:
       return gnu_xfer_memory (readbuf, writebuf, offset, len, xfered_len);
+    case TARGET_OBJECT_AUXV:
+      return gnu_xfer_auxv (readbuf, writebuf, offset, len, xfered_len);
     default:
       return TARGET_XFER_E_IO;
     }