[binutils-gdb] Fix step-over-syscall.exp failure

[Luis Machado]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=0c27188999bfc5bf03536bf44593c4ed8df296c3

commit 0c27188999bfc5bf03536bf44593c4ed8df296c3
Author: Luis Machado <luis.machado@linaro.org>
Date:   Thu Jan 9 16:04:36 2020 -0300

    Fix step-over-syscall.exp failure
    
    In particular, this one:
    
    FAIL: gdb.base/step-over-syscall.exp: fork: displaced=on: check_pc_after_cross_syscall: single step over fork final pc
    
    When ptrace fork event reporting is enabled, GDB gets a PTRACE_EVENT_FORK
    event whenever the inferior executes the fork syscall.
    
    Then the logic is that GDB needs to step the inferior yet again in order to
    receive a predetermined SIGTRAP, but no execution takes place because the
    signal was already queued for delivery. That means the PC should stay the same.
    
    I noticed the aarch64 code is currently adjusting the PC in this situation,
    making the inferior skip an instruction without executing it.
    
    The following change checks if we did not execute the instruction
    (pc - to == 0), making proper adjustments for such case.
    
    Regression tested on aarch64-linux-gnu on the tryserver.
    
    gdb/ChangeLog:
    
    2020-01-21  Luis Machado  <luis.machado@linaro.org>
    
    	* aarch64-tdep.c (struct aarch64_displaced_step_closure )
    	<pc_adjust>: Adjust the documentation.
    	(aarch64_displaced_step_fixup): Check if PC really moved before
    	adjusting it.

Diff:
---
 gdb/ChangeLog      |  7 +++++++
 gdb/aarch64-tdep.c | 17 +++++++++++++----
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 93a125b..146767f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2020-01-21  Luis Machado  <luis.machado@linaro.org>
+
+	* aarch64-tdep.c (struct aarch64_displaced_step_closure )
+	<pc_adjust>: Adjust the documentation.
+	(aarch64_displaced_step_fixup): Check if PC really moved before
+	adjusting it.
+
 2020-01-19  Tom Tromey  <tom@tromey.com>
 
 	* disasm.c (~gdb_disassembler): New destructor.
diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 8451a91..fb7b862 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -2737,7 +2737,8 @@ struct aarch64_displaced_step_closure : public displaced_step_closure
      is being displaced stepping.  */
   int cond = 0;
 
-  /* PC adjustment offset after displaced stepping.  */
+  /* PC adjustment offset after displaced stepping.  If 0, then we don't
+     write the PC back, assuming the PC is already the right address.  */
   int32_t pc_adjust = 0;
 };
 
@@ -3032,11 +3033,12 @@ aarch64_displaced_step_fixup (struct gdbarch *gdbarch,
 {
   aarch64_displaced_step_closure *dsc = (aarch64_displaced_step_closure *) dsc_;
 
+  ULONGEST pc;
+
+  regcache_cooked_read_unsigned (regs, AARCH64_PC_REGNUM, &pc);
+
   if (dsc->cond)
     {
-      ULONGEST pc;
-
-      regcache_cooked_read_unsigned (regs, AARCH64_PC_REGNUM, &pc);
       if (pc - to == 8)
 	{
 	  /* Condition is true.  */
@@ -3052,6 +3054,13 @@ aarch64_displaced_step_fixup (struct gdbarch *gdbarch,
 
   if (dsc->pc_adjust != 0)
     {
+      /* Make sure the previous instruction was executed (that is, the PC
+	 has changed).  If the PC didn't change, then discard the adjustment
+	 offset.  Otherwise we may skip an instruction before its execution
+	 took place.  */
+      if ((pc - to) == 0)
+	dsc->pc_adjust = 0;
+
       if (debug_displaced)
 	{
 	  debug_printf ("displaced: fixup: set PC to %s:%d\n",