[PATCH] [aarch64] tpidr2: Fix erroneous detection logic for TPIDR2

[Luis Machado <luis.machado@arm.com>]

The detection logic for TPIDR2 was implemented incorrectly.  Originally
the detection was supposed to be through a ptrace error code, but in reality,
for backwards compatibility, the detection should be based on the size of
the returned iovec.

For instance, if a target supports both TPIDR and TPIDR2, ptrace will return a
iovec size of 16.  If a target only supports TPIDR and not TPIDR2, it will
return a iovec size of 8, even if we asked for 16 bytes.

This patch fixes this issue in code that is shared between gdb and gdbserver,
therefore both gdb and gdbserver are fixed.

Tested on AArch64/Linux Ubuntu 20.04.
---
 gdb/nat/aarch64-linux.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/gdb/nat/aarch64-linux.c b/gdb/nat/aarch64-linux.c
index 6c8a78f1729..a84faf1cfbf 100644
--- a/gdb/nat/aarch64-linux.c
+++ b/gdb/nat/aarch64-linux.c
@@ -261,13 +261,13 @@ aarch64_tls_register_count (int tid)
   iovec.iov_base = tls_regs;
   iovec.iov_len = sizeof (tls_regs);
 
-  /* Attempt to read both TPIDR and TPIDR2.  If the request fails, it means
-     the Linux Kernel does not support TPIDR2.
-
-     Otherwise the Linux Kernel supports both TPIDR and TPIDR2.  */
+  /* Attempt to read both TPIDR and TPIDR2.  If ptrace returns less data than
+     we are expecting, that means it doesn't support all the registers.  From
+     the iovec length, figure out how many TPIDR registers the target actually
+     supports.  */
   if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_TLS, &iovec) != 0)
-    return 1;
+    return 0;
 
-  /* TPIDR2 is available as well.  */
-  return 2;
+  /* Calculate how many TPIDR registers we have.  */
+  return iovec.iov_len / sizeof (uint64_t);
 }
-- 
2.25.1