x32 Fast tracepoints: Customize jump pad address

MAP_32BIT is ignored on x32, meaning the jump pad can end up somewhere
between 2GB and 4GB, too far away from the executable for 5-byte
relative jumps (JMP rel32).  So on x32, try explicitly placing the
jump pad near the middle of the available address space.

gdb/gdbserver/ChangeLog:
2016-08-23  Pedro Alves  <palves@redhat.com>

	* linux-amd64-ipa.c (alloc_jump_pad_buffer) [__ILP32__]: Try
	allocating around 0x80000000.

2 files changed, 53 insertions, 4 deletions

diff --git a/gdb/gdbserver/ChangeLog b/gdb/gdbserver/ChangeLog
index 6b4714a..bed2719 100644
--- a/gdb/gdbserver/ChangeLog
+++ b/gdb/gdbserver/ChangeLog
@@ -1,5 +1,10 @@
 2016-08-23  Pedro Alves  <palves@redhat.com>
 
+	* linux-amd64-ipa.c (alloc_jump_pad_buffer) [__ILP32__]: Try
+	allocating around 0x80000000.
+
+2016-08-23  Pedro Alves  <palves@redhat.com>
+
 	PR gdb/20415
 	* Makefile.in (x32-linux-ipa.o, x32-avx-linux-ipa.o)
 	(x32-avx512-linux-ipa.o): New rules.
diff --git a/gdb/gdbserver/linux-amd64-ipa.c b/gdb/gdbserver/linux-amd64-ipa.c
index 15d08ff..0625b85 100644
--- a/gdb/gdbserver/linux-amd64-ipa.c
+++ b/gdb/gdbserver/linux-amd64-ipa.c
@@ -206,14 +206,57 @@ get_ipa_tdesc (int idx)
 		  "unknown ipa tdesc index: %d", idx);
 }
 
-/* Allocate buffer for the jump pads.  Since we're using 32-bit jumps
-   to reach them, and the executable is at low addresses, MAP_32BIT
-   works just fine.  Shared libraries, being allocated at the top,
-   are unfortunately out of luck.  */
+/* Allocate buffer for the jump pads.  The branch instruction has a
+   reach of +/- 31-bit, and the executable is loaded at low addresses.
+
+   64-bit: Use MAP_32BIT to allocate in the first 2GB.  Shared
+   libraries, being allocated at the top, are unfortunately out of
+   luck.
+
+   x32: Since MAP_32BIT is 64-bit only, do the placement manually.
+   Try allocating at '0x80000000 - SIZE' initially, decreasing until
+   we hit a free area.  This ensures the executable is fully covered,
+   and is as close as possible to the shared libraries, which are
+   usually mapped at the top of the first 4GB of the address space.
+*/
 
 void *
 alloc_jump_pad_buffer (size_t size)
 {
+#if __ILP32__
+  uintptr_t addr;
+  int pagesize;
+
+  pagesize = sysconf (_SC_PAGE_SIZE);
+  if (pagesize == -1)
+    perror_with_name ("sysconf");
+
+  addr = 0x80000000 - size;
+
+  /* size should already be page-aligned, but this can't hurt.  */
+  addr &= ~(pagesize - 1);
+
+  /* Search for a free area.  If we hit 0, we're out of luck.  */
+  for (; addr; addr -= pagesize)
+    {
+      void *res;
+
+      /* No MAP_FIXED - we don't want to zap someone's mapping.  */
+      res = mmap ((void *) addr, size,
+		  PROT_READ | PROT_WRITE | PROT_EXEC,
+		  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+      /* If we got what we wanted, return.  */
+      if ((uintptr_t) res == addr)
+	return res;
+
+      /* If we got a mapping, but at a wrong address, undo it.  */
+      if (res != MAP_FAILED)
+	munmap (res, size);
+    }
+
+  return NULL;
+#else
   void *res = mmap (NULL, size, PROT_READ | PROT_WRITE | PROT_EXEC,
 		    MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
 
@@ -221,6 +264,7 @@ alloc_jump_pad_buffer (size_t size)
     return NULL;
 
   return res;
+#endif
 }
 
 void