gcc/gcc7-rh1512529-2.patch

commit bed27844b80e17ad786028a0a82c7d47990d15bb
Author: law <law@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed Sep 20 05:05:12 2017 +0000

    2017-09-18  Jeff Law  <law@redhat.com>
    
            * explow.c: Include "params.h".
            (anti_adjust_stack_and_probe_stack_clash): New function.
            (get_stack_check_protect): Likewise.
            (compute_stack_clash_protection_loop_data): Likewise.
            (emit_stack_clash_protection_loop_start): Likewise.
            (emit_stack_clash_protection_loop_end): Likewise.
            (allocate_dynamic_stack_space): Use get_stack_check_protect.
            Use anti_adjust_stack_and_probe_stack_clash.
            * explow.h (compute_stack_clash_protection_loop_data): Prototype.
            (emit_stack_clash_protection_loop_start): Likewise.
            (emit_stack_clash_protection_loop_end): Likewise.
            * rtl.h (get_stack_check_protect): Prototype.
            * target.def (stack_clash_protection_final_dynamic_probe): New hook.
            * targhooks.c (default_stack_clash_protection_final_dynamic_probe): New.
            * targhooks.h (default_stack_clash_protection_final_dynamic_probe):
            Prototype.
            * doc/tm.texi.in (TARGET_STACK_CLASH_PROTECTION_FINAL_DYNAMIC_PROBE):
            Add @hook.
            * doc/tm.texi: Rebuilt.
            * config/aarch64/aarch64.c (aarch64_expand_prologue): Use
            get_stack_check_protect.
            * config/alpha/alpha.c (alpha_expand_prologue): Likewise.
            * config/arm/arm.c (arm_expand_prologue): Likewise.
            (arm_frame_pointer_required): Likewise.
            * config/i386/i386.c (ix86_expand_prologue): Likewise.
            * config/ia64/ia64.c (ia64_expand_prologue): Likewise.
            * config/mips/mips.c (mips_expand_prologue): Likewise.
            * config/powerpcspe/powerpcspe.c (rs6000_emit_prologue): Likewise.
            * config/rs6000/rs6000.c (rs6000_emit_prologue): Likewise.
            * config/sparc/sparc.c (sparc_expand_prologue): Likewise.
            (sparc_flat_expand_prologue): Likewise.
    
            * gcc.dg/stack-check-3.c: New test.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@252995 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 1bd010be756..75c12f1de65 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -3582,12 +3582,14 @@ aarch64_expand_prologue (void)
     {
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (frame_size > PROBE_INTERVAL && frame_size > STACK_CHECK_PROTECT)
-	    aarch64_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					    frame_size - STACK_CHECK_PROTECT);
+	  if (frame_size > PROBE_INTERVAL
+	      && frame_size > get_stack_check_protect ())
+	    aarch64_emit_probe_stack_range (get_stack_check_protect (),
+					    (frame_size
+					     - get_stack_check_protect ()));
 	}
       else if (frame_size > 0)
-	aarch64_emit_probe_stack_range (STACK_CHECK_PROTECT, frame_size);
+	aarch64_emit_probe_stack_range (get_stack_check_protect (), frame_size);
     }
 
   aarch64_sub_sp (IP0_REGNUM, initial_adjust, true);
diff --git a/gcc/config/alpha/alpha.c b/gcc/config/alpha/alpha.c
index 00a69c1a08d..91f3d7cfbeb 100644
--- a/gcc/config/alpha/alpha.c
+++ b/gcc/config/alpha/alpha.c
@@ -7741,7 +7741,7 @@ alpha_expand_prologue (void)
 
   probed_size = frame_size;
   if (flag_stack_check)
-    probed_size += STACK_CHECK_PROTECT;
+    probed_size += get_stack_check_protect ();
 
   if (probed_size <= 32768)
     {
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 1ded0d2a17d..242d7c792fe 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -21604,13 +21604,13 @@ arm_expand_prologue (void)
 
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    arm_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					size - STACK_CHECK_PROTECT,
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    arm_emit_probe_stack_range (get_stack_check_protect (),
+					size - get_stack_check_protect (),
 					regno, live_regs_mask);
 	}
       else if (size > 0)
-	arm_emit_probe_stack_range (STACK_CHECK_PROTECT, size,
+	arm_emit_probe_stack_range (get_stack_check_protect (), size,
 				    regno, live_regs_mask);
     }
 
@@ -27751,7 +27751,7 @@ arm_frame_pointer_required (void)
 	{
 	  /* We don't have the final size of the frame so adjust.  */
 	  size += 32 * UNITS_PER_WORD;
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
 	    return true;
 	}
       else
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 2d163797ba0..60bab9a26a6 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -14037,7 +14037,7 @@ ix86_expand_prologue (void)
 	  HOST_WIDE_INT size = allocate;
 
 	  if (TARGET_64BIT && size >= HOST_WIDE_INT_C (0x80000000))
-	    size = 0x80000000 - STACK_CHECK_PROTECT - 1;
+	    size = 0x80000000 - get_stack_check_protect () - 1;
 
 	  if (TARGET_STACK_PROBE)
 	    {
@@ -14047,18 +14047,20 @@ ix86_expand_prologue (void)
 		    ix86_emit_probe_stack_range (0, size);
 		}
 	      else
-		ix86_emit_probe_stack_range (0, size + STACK_CHECK_PROTECT);
+		ix86_emit_probe_stack_range (0,
+					     size + get_stack_check_protect ());
 	    }
 	  else
 	    {
 	      if (crtl->is_leaf && !cfun->calls_alloca)
 		{
-		  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-		    ix86_emit_probe_stack_range (STACK_CHECK_PROTECT,
-						 size - STACK_CHECK_PROTECT);
+		  if (size > PROBE_INTERVAL
+		      && size > get_stack_check_protect ())
+		    ix86_emit_probe_stack_range (get_stack_check_protect (),
+						 size - get_stack_check_protect ());
 		}
 	      else
-		ix86_emit_probe_stack_range (STACK_CHECK_PROTECT, size);
+		ix86_emit_probe_stack_range (get_stack_check_protect (), size);
 	    }
 	}
     }
diff --git a/gcc/config/ia64/ia64.c b/gcc/config/ia64/ia64.c
index a9f479b6e5c..17d6b3318a6 100644
--- a/gcc/config/ia64/ia64.c
+++ b/gcc/config/ia64/ia64.c
@@ -3476,15 +3476,16 @@ ia64_expand_prologue (void)
 
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    ia64_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					 size - STACK_CHECK_PROTECT,
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    ia64_emit_probe_stack_range (get_stack_check_protect (),
+					 size - get_stack_check_protect (),
 					 bs_size);
-	  else if (size + bs_size > STACK_CHECK_PROTECT)
-	    ia64_emit_probe_stack_range (STACK_CHECK_PROTECT, 0, bs_size);
+	  else if (size + bs_size > get_stack_check_protect ())
+	    ia64_emit_probe_stack_range (get_stack_check_protect (),
+					 0, bs_size);
 	}
       else if (size + bs_size > 0)
-	ia64_emit_probe_stack_range (STACK_CHECK_PROTECT, size, bs_size);
+	ia64_emit_probe_stack_range (get_stack_check_protect (), size, bs_size);
     }
 
   if (dump_file) 
diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
index 6bfd86a07af..7d85ce7055a 100644
--- a/gcc/config/mips/mips.c
+++ b/gcc/config/mips/mips.c
@@ -12081,12 +12081,12 @@ mips_expand_prologue (void)
     {
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    mips_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					 size - STACK_CHECK_PROTECT);
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    mips_emit_probe_stack_range (get_stack_check_protect (),
+					 size - get_stack_check_protect ());
 	}
       else if (size > 0)
-	mips_emit_probe_stack_range (STACK_CHECK_PROTECT, size);
+	mips_emit_probe_stack_range (get_stack_check_protect (), size);
     }
 
   /* Save the registers.  Allocate up to MIPS_MAX_FIRST_STACK_STEP
diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 66496a1f80f..1836e1d147d 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -29294,12 +29294,12 @@ rs6000_emit_prologue (void)
 
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    rs6000_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					   size - STACK_CHECK_PROTECT);
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    rs6000_emit_probe_stack_range (get_stack_check_protect (),
+					   size - get_stack_check_protect ());
 	}
       else if (size > 0)
-	rs6000_emit_probe_stack_range (STACK_CHECK_PROTECT, size);
+	rs6000_emit_probe_stack_range (get_stack_check_protect (), size);
     }
 
   if (TARGET_FIX_AND_CONTINUE)
diff --git a/gcc/config/sparc/sparc.c b/gcc/config/sparc/sparc.c
index 83ca1dcc617..b32bb9859e1 100644
--- a/gcc/config/sparc/sparc.c
+++ b/gcc/config/sparc/sparc.c
@@ -5695,12 +5695,12 @@ sparc_expand_prologue (void)
     {
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    sparc_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					  size - STACK_CHECK_PROTECT);
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    sparc_emit_probe_stack_range (get_stack_check_protect (),
+					  size - get_stack_check_protect ());
 	}
       else if (size > 0)
-	sparc_emit_probe_stack_range (STACK_CHECK_PROTECT, size);
+	sparc_emit_probe_stack_range (get_stack_check_protect (), size);
     }
 
   if (size == 0)
@@ -5806,12 +5806,12 @@ sparc_flat_expand_prologue (void)
     {
       if (crtl->is_leaf && !cfun->calls_alloca)
 	{
-	  if (size > PROBE_INTERVAL && size > STACK_CHECK_PROTECT)
-	    sparc_emit_probe_stack_range (STACK_CHECK_PROTECT,
-					  size - STACK_CHECK_PROTECT);
+	  if (size > PROBE_INTERVAL && size > get_stack_check_protect ())
+	    sparc_emit_probe_stack_range (get_stack_check_protect (),
+					  size - get_stack_check_protect ());
 	}
       else if (size > 0)
-	sparc_emit_probe_stack_range (STACK_CHECK_PROTECT, size);
+	sparc_emit_probe_stack_range (get_stack_check_protect (), size);
     }
 
   if (sparc_save_local_in_regs_p)
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index c4f2c893c8e..0ba2925a1bd 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -3419,6 +3419,10 @@ GCC computed the default from the values of the above macros and you will
 normally not need to override that default.
 @end defmac
 
+@deftypefn {Target Hook} bool TARGET_STACK_CLASH_PROTECTION_FINAL_DYNAMIC_PROBE (rtx @var{residual})
+Some targets make optimistic assumptions about the state of stack probing when they emit their prologues.  On such targets a probe into the end of any dynamically allocated space is likely required for safety against stack clash style attacks.  Define this variable to return nonzero if such a probe is required or zero otherwise.  You need not define this macro if it would always have the value zero.
+@end deftypefn
+
 @need 2000
 @node Frame Registers
 @subsection Registers That Address the Stack Frame
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 1c471d8da35..d31255e86cb 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -2999,6 +2999,8 @@ GCC computed the default from the values of the above macros and you will
 normally not need to override that default.
 @end defmac
 
+@hook TARGET_STACK_CLASH_PROTECTION_FINAL_DYNAMIC_PROBE
+
 @need 2000
 @node Frame Registers
 @subsection Registers That Address the Stack Frame
diff --git a/gcc/explow.c b/gcc/explow.c
index 67cb6ff1513..e7db438b1ec 100644
--- a/gcc/explow.c
+++ b/gcc/explow.c
@@ -39,8 +39,11 @@ along with GCC; see the file COPYING3.  If not see
 #include "expr.h"
 #include "common/common-target.h"
 #include "output.h"
+#include "params.h"
+#include "dumpfile.h"
 
 static rtx break_out_memory_refs (rtx);
+static void anti_adjust_stack_and_probe_stack_clash (rtx);
 
 
 /* Truncate and perhaps sign-extend C as appropriate for MODE.  */
@@ -1271,6 +1274,29 @@ get_dynamic_stack_size (rtx *psize, unsigned size_align,
   *psize = size;
 }
 
+/* Return the number of bytes to "protect" on the stack for -fstack-check.
+
+   "protect" in the context of -fstack-check means how many bytes we
+   should always ensure are available on the stack.  More importantly
+   this is how many bytes are skipped when probing the stack.
+
+   On some targets we want to reuse the -fstack-check prologue support
+   to give a degree of protection against stack clashing style attacks.
+
+   In that scenario we do not want to skip bytes before probing as that
+   would render the stack clash protections useless.
+
+   So we never use STACK_CHECK_PROTECT directly.  Instead we indirect though
+   this helper which allows us to provide different values for
+   -fstack-check and -fstack-clash-protection.  */
+HOST_WIDE_INT
+get_stack_check_protect (void)
+{
+  if (flag_stack_clash_protection)
+    return 0;
+ return STACK_CHECK_PROTECT;
+}
+
 /* Return an rtx representing the address of an area of memory dynamically
    pushed on the stack.
 
@@ -1429,7 +1455,7 @@ allocate_dynamic_stack_space (rtx size, unsigned size_align,
     probe_stack_range (STACK_OLD_CHECK_PROTECT + STACK_CHECK_MAX_FRAME_SIZE,
 		       size);
   else if (flag_stack_check == STATIC_BUILTIN_STACK_CHECK)
-    probe_stack_range (STACK_CHECK_PROTECT, size);
+    probe_stack_range (get_stack_check_protect (), size);
 
   /* Don't let anti_adjust_stack emit notes.  */
   suppress_reg_args_size = true;
@@ -1482,6 +1508,8 @@ allocate_dynamic_stack_space (rtx size, unsigned size_align,
 
       if (flag_stack_check && STACK_CHECK_MOVING_SP)
 	anti_adjust_stack_and_probe (size, false);
+      else if (flag_stack_clash_protection)
+	anti_adjust_stack_and_probe_stack_clash (size);
       else
 	anti_adjust_stack (size);
 
@@ -1757,6 +1785,219 @@ probe_stack_range (HOST_WIDE_INT first, rtx size)
   emit_insn (gen_blockage ());
 }
 
+/* Compute parameters for stack clash probing a dynamic stack
+   allocation of SIZE bytes.
+
+   We compute ROUNDED_SIZE, LAST_ADDR, RESIDUAL and PROBE_INTERVAL.
+
+   Additionally we conditionally dump the type of probing that will
+   be needed given the values computed.  */
+
+void
+compute_stack_clash_protection_loop_data (rtx *rounded_size, rtx *last_addr,
+					  rtx *residual,
+					  HOST_WIDE_INT *probe_interval,
+					  rtx size)
+{
+  /* Round SIZE down to STACK_CLASH_PROTECTION_PROBE_INTERVAL */
+  *probe_interval
+    = 1 << PARAM_VALUE (PARAM_STACK_CLASH_PROTECTION_PROBE_INTERVAL);
+  *rounded_size = simplify_gen_binary (AND, Pmode, size,
+				        GEN_INT (-*probe_interval));
+
+  /* Compute the value of the stack pointer for the last iteration.
+     It's just SP + ROUNDED_SIZE.  */
+  rtx rounded_size_op = force_operand (*rounded_size, NULL_RTX);
+  *last_addr = force_operand (gen_rtx_fmt_ee (STACK_GROW_OP, Pmode,
+					      stack_pointer_rtx,
+					      rounded_size_op),
+			      NULL_RTX);
+
+  /* Compute any residuals not allocated by the loop above.  Residuals
+     are just the ROUNDED_SIZE - SIZE.  */
+  *residual = simplify_gen_binary (MINUS, Pmode, size, *rounded_size);
+
+  /* Dump key information to make writing tests easy.  */
+  if (dump_file)
+    {
+      if (*rounded_size == CONST0_RTX (Pmode))
+	fprintf (dump_file,
+		 "Stack clash skipped dynamic allocation and probing loop.\n");
+      else if (GET_CODE (*rounded_size) == CONST_INT
+	       && INTVAL (*rounded_size) <= 4 * *probe_interval)
+	fprintf (dump_file,
+		 "Stack clash dynamic allocation and probing inline.\n");
+      else if (GET_CODE (*rounded_size) == CONST_INT)
+	fprintf (dump_file,
+		 "Stack clash dynamic allocation and probing in "
+		 "rotated loop.\n");
+      else
+	fprintf (dump_file,
+		 "Stack clash dynamic allocation and probing in loop.\n");
+
+      if (*residual != CONST0_RTX (Pmode))
+	fprintf (dump_file,
+		 "Stack clash dynamic allocation and probing residuals.\n");
+      else
+	fprintf (dump_file,
+		 "Stack clash skipped dynamic allocation and "
+		 "probing residuals.\n");
+    }
+}
+
+/* Emit the start of an allocate/probe loop for stack
+   clash protection.
+
+   LOOP_LAB and END_LAB are returned for use when we emit the
+   end of the loop.
+
+   LAST addr is the value for SP which stops the loop.  */
+void
+emit_stack_clash_protection_probe_loop_start (rtx *loop_lab,
+					      rtx *end_lab,
+					      rtx last_addr,
+					      bool rotated)
+{
+  /* Essentially we want to emit any setup code, the top of loop
+     label and the comparison at the top of the loop.  */
+  *loop_lab = gen_label_rtx ();
+  *end_lab = gen_label_rtx ();
+
+  emit_label (*loop_lab);
+  if (!rotated)
+    emit_cmp_and_jump_insns (stack_pointer_rtx, last_addr, EQ, NULL_RTX,
+			     Pmode, 1, *end_lab);
+}
+
+/* Emit the end of a stack clash probing loop.
+
+   This consists of just the jump back to LOOP_LAB and
+   emitting END_LOOP after the loop.  */
+
+void
+emit_stack_clash_protection_probe_loop_end (rtx loop_lab, rtx end_loop,
+					    rtx last_addr, bool rotated)
+{
+  if (rotated)
+    emit_cmp_and_jump_insns (stack_pointer_rtx, last_addr, NE, NULL_RTX,
+			     Pmode, 1, loop_lab);
+  else
+    emit_jump (loop_lab);
+
+  emit_label (end_loop);
+
+}
+
+/* Adjust the stack pointer by minus SIZE (an rtx for a number of bytes)
+   while probing it.  This pushes when SIZE is positive.  SIZE need not
+   be constant.
+
+   This is subtly different than anti_adjust_stack_and_probe to try and
+   prevent stack-clash attacks
+
+     1. It must assume no knowledge of the probing state, any allocation
+	must probe.
+
+	Consider the case of a 1 byte alloca in a loop.  If the sum of the
+	allocations is large, then this could be used to jump the guard if
+	probes were not emitted.
+
+     2. It never skips probes, whereas anti_adjust_stack_and_probe will
+	skip probes on the first couple PROBE_INTERVALs on the assumption
+	they're done elsewhere.
+
+     3. It only allocates and probes SIZE bytes, it does not need to
+	allocate/probe beyond that because this probing style does not
+	guarantee signal handling capability if the guard is hit.  */
+
+static void
+anti_adjust_stack_and_probe_stack_clash (rtx size)
+{
+  /* First ensure SIZE is Pmode.  */
+  if (GET_MODE (size) != VOIDmode && GET_MODE (size) != Pmode)
+    size = convert_to_mode (Pmode, size, 1);
+
+  /* We can get here with a constant size on some targets.  */
+  rtx rounded_size, last_addr, residual;
+  HOST_WIDE_INT probe_interval;
+  compute_stack_clash_protection_loop_data (&rounded_size, &last_addr,
+					    &residual, &probe_interval, size);
+
+  if (rounded_size != CONST0_RTX (Pmode))
+    {
+      if (INTVAL (rounded_size) <= 4 * probe_interval)
+	{
+	  for (HOST_WIDE_INT i = 0;
+	       i < INTVAL (rounded_size);
+	       i += probe_interval)
+	    {
+	      anti_adjust_stack (GEN_INT (probe_interval));
+
+	      /* The prologue does not probe residuals.  Thus the offset
+		 here to probe just beyond what the prologue had already
+		 allocated.  */
+	      emit_stack_probe (plus_constant (Pmode, stack_pointer_rtx,
+					       (probe_interval
+						- GET_MODE_SIZE (word_mode))));
+	      emit_insn (gen_blockage ());
+	    }
+	}
+      else
+	{
+	  rtx loop_lab, end_loop;
+	  bool rotate_loop = GET_CODE (rounded_size) == CONST_INT;
+	  emit_stack_clash_protection_probe_loop_start (&loop_lab, &end_loop,
+							last_addr, rotate_loop);
+
+	  anti_adjust_stack (GEN_INT (probe_interval));
+
+	  /* The prologue does not probe residuals.  Thus the offset here
+	     to probe just beyond what the prologue had already allocated.  */
+	  emit_stack_probe (plus_constant (Pmode, stack_pointer_rtx,
+					   (probe_interval
+					    - GET_MODE_SIZE (word_mode))));
+
+	  emit_stack_clash_protection_probe_loop_end (loop_lab, end_loop,
+						      last_addr, rotate_loop);
+	  emit_insn (gen_blockage ());
+	}
+    }
+
+  if (residual != CONST0_RTX (Pmode))
+    {
+      rtx x = force_reg (Pmode, plus_constant (Pmode, residual,
+					       -GET_MODE_SIZE (word_mode)));
+      anti_adjust_stack (residual);
+      emit_stack_probe (gen_rtx_PLUS (Pmode, stack_pointer_rtx, x));
+      emit_insn (gen_blockage ());
+    }
+
+  /* Some targets make optimistic assumptions in their prologues about
+     how the caller may have probed the stack.  Make sure we honor
+     those assumptions when needed.  */
+  if (size != CONST0_RTX (Pmode)
+      && targetm.stack_clash_protection_final_dynamic_probe (residual))
+    {
+      /* Ideally we would just probe at *sp.  However, if SIZE is not
+	 a compile-time constant, but is zero at runtime, then *sp
+	 might hold live data.  So probe at *sp if we know that
+	 an allocation was made, otherwise probe into the red zone
+	 which is obviously undesirable.  */
+      if (GET_CODE (size) == CONST_INT)
+	{
+	  emit_stack_probe (stack_pointer_rtx);
+	  emit_insn (gen_blockage ());
+	}
+      else
+	{
+	  emit_stack_probe (plus_constant (Pmode, stack_pointer_rtx,
+					   -GET_MODE_SIZE (word_mode)));
+	  emit_insn (gen_blockage ());
+	}
+    }
+}
+
+
 /* Adjust the stack pointer by minus SIZE (an rtx for a number of bytes)
    while probing it.  This pushes when SIZE is positive.  SIZE need not
    be constant.  If ADJUST_BACK is true, adjust back the stack pointer
diff --git a/gcc/explow.h b/gcc/explow.h
index 217a3226adb..b85c051e8ce 100644
--- a/gcc/explow.h
+++ b/gcc/explow.h
@@ -69,6 +69,15 @@ extern void anti_adjust_stack (rtx);
 /* Add some bytes to the stack while probing it.  An rtx says how many. */
 extern void anti_adjust_stack_and_probe (rtx, bool);
 
+/* Support for building allocation/probing loops for stack-clash
+   protection of dyamically allocated stack space.  */
+extern void compute_stack_clash_protection_loop_data (rtx *, rtx *, rtx *,
+						      HOST_WIDE_INT *, rtx);
+extern void emit_stack_clash_protection_probe_loop_start (rtx *, rtx *,
+							  rtx, bool);
+extern void emit_stack_clash_protection_probe_loop_end (rtx, rtx,
+							rtx, bool);
+
 /* This enum is used for the following two functions.  */
 enum save_level {SAVE_BLOCK, SAVE_FUNCTION, SAVE_NONLOCAL};
 
diff --git a/gcc/rtl.h b/gcc/rtl.h
index 93330425c00..98f993a95a5 100644
--- a/gcc/rtl.h
+++ b/gcc/rtl.h
@@ -2707,6 +2707,7 @@ get_full_set_src_cost (rtx x, machine_mode mode, struct full_rtx_costs *c)
 /* In explow.c */
 extern HOST_WIDE_INT trunc_int_for_mode	(HOST_WIDE_INT, machine_mode);
 extern rtx plus_constant (machine_mode, rtx, HOST_WIDE_INT, bool = false);
+extern HOST_WIDE_INT get_stack_check_protect (void);
 
 /* In rtl.c */
 extern rtx rtx_alloc_stat (RTX_CODE MEM_STAT_DECL);
diff --git a/gcc/target.def b/gcc/target.def
index 6bebfd5b9d6..8b8ad1f4938 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -5473,6 +5473,13 @@ these registers when the target switches are opposed to them.)",
  void, (void),
  hook_void_void)
 
+DEFHOOK
+(stack_clash_protection_final_dynamic_probe,
+ "Some targets make optimistic assumptions about the state of stack probing when they emit their prologues.  On such targets a probe into the end of any dynamically allocated space is likely required for safety against stack clash style attacks.  Define this variable to return nonzero if such a probe is required or zero otherwise.  You need not define this macro if it would always have the value zero.",
+ bool, (rtx residual),
+ default_stack_clash_protection_final_dynamic_probe)
+
+
 /* Functions specific to the C family of frontends.  */
 #undef HOOK_PREFIX
 #define HOOK_PREFIX "TARGET_C_"
diff --git a/gcc/targhooks.c b/gcc/targhooks.c
index 1cdec068ed8..225048e7518 100644
--- a/gcc/targhooks.c
+++ b/gcc/targhooks.c
@@ -2107,4 +2107,10 @@ default_excess_precision (enum excess_precision_type ATTRIBUTE_UNUSED)
   return FLT_EVAL_METHOD_PROMOTE_TO_FLOAT;
 }
 
+HOST_WIDE_INT
+default_stack_clash_protection_final_dynamic_probe (rtx residual ATTRIBUTE_UNUSED)
+{
+  return 0;
+}
+
 #include "gt-targhooks.h"
diff --git a/gcc/targhooks.h b/gcc/targhooks.h
index 18070df7839..32ae9d88050 100644
--- a/gcc/targhooks.h
+++ b/gcc/targhooks.h
@@ -263,5 +263,6 @@ extern unsigned int default_min_arithmetic_precision (void);
 
 extern enum flt_eval_method
 default_excess_precision (enum excess_precision_type ATTRIBUTE_UNUSED);
+extern bool default_stack_clash_protection_final_dynamic_probe (rtx);
 
 #endif /* GCC_TARGHOOKS_H */
diff --git a/gcc/testsuite/gcc.dg/stack-check-3.c b/gcc/testsuite/gcc.dg/stack-check-3.c
new file mode 100644
index 00000000000..58fb65649ee
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/stack-check-3.c
@@ -0,0 +1,86 @@
+/* The goal here is to ensure that dynamic allocations via vlas or
+   alloca calls receive probing.
+
+   Scanning the RTL or assembly code seems like insanity here as does
+   checking for particular allocation sizes and probe offsets.  For
+   now we just verify that there's an allocation + probe loop and
+   residual allocation + probe for f?.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -fstack-clash-protection -fdump-rtl-expand -fno-optimize-sibling-calls --param stack-clash-protection-probe-interval=4096 --param stack-clash-protection-guard-size=4096" } */
+/* { dg-require-effective-target supports_stack_clash_protection } */
+
+__attribute__((noinline, noclone)) void
+foo (char *p)
+{
+  asm volatile ("" : : "r" (p) : "memory");
+}
+
+/* Simple VLA, no other locals. */
+__attribute__((noinline, noclone)) void
+f0 (int x)
+{
+  char vla[x];
+  foo (vla);
+}
+
+/* Simple VLA, small local frame.  */
+__attribute__((noinline, noclone)) void
+f1 (int x)
+{
+  char locals[128];
+  char vla[x];
+  foo (vla);
+}
+
+/* Small constant alloca, no other locals. */
+__attribute__((noinline, noclone)) void
+f2 (int x)
+{
+  char *vla = __builtin_alloca (128);
+  foo (vla);
+}
+
+/* Big constant alloca, small local frame.  */
+__attribute__((noinline, noclone)) void
+f3 (int x)
+{
+  char locals[128];
+  char *vla = __builtin_alloca (16384);
+  foo (vla);
+}
+
+/* Big constant alloca, small local frame.  */
+__attribute__((noinline, noclone)) void
+f3a (int x)
+{
+  char locals[128];
+  char *vla = __builtin_alloca (32768);
+  foo (vla);
+}
+
+/* Nonconstant alloca, no other locals. */
+__attribute__((noinline, noclone)) void
+f4 (int x)
+{
+  char *vla = __builtin_alloca (x);
+  foo (vla);
+}
+
+/* Nonconstant alloca, small local frame.  */
+__attribute__((noinline, noclone)) void
+f5 (int x)
+{
+  char locals[128];
+  char *vla = __builtin_alloca (x);
+  foo (vla);
+}
+
+/* { dg-final { scan-rtl-dump-times "allocation and probing residuals" 7 "expand" } } */
+
+
+/* { dg-final { scan-rtl-dump-times "allocation and probing in loop" 7 "expand" { target callee_realigns_stack } } } */
+/* { dg-final { scan-rtl-dump-times "allocation and probing in loop" 4 "expand" { target { ! callee_realigns_stack } } } } */
+/* { dg-final { scan-rtl-dump-times "allocation and probing in rotated loop" 1 "expand" { target { ! callee_realigns_stack } } } } */
+/* { dg-final { scan-rtl-dump-times "allocation and probing inline" 1 "expand" { target { ! callee_realigns_stack } } } } */
+/* { dg-final { scan-rtl-dump-times "skipped dynamic allocation and probing loop" 1 "expand" { target { ! callee_realigns_stack } } } } */