From 41a82fb711f3637b4b7f57756492b628058f9d5f Mon Sep 17 00:00:00 2001
From: Harshit Sharma <harshitsharmajs@gmail.com>
Date: Fri, 10 Jul 2020 13:06:08 -0700
Subject: [PATCH] crossgcc: Enable GCC to get asan shadow offset at runtime

Unlike Linux kernel which has a static shadow region layout, we have multiple stages in
coreboot and thus require a different shadow offset address. Unfortunately, GCC currently
only supports adding a static shadow offset at compile time using -fasan-shadow-offset flag.

For this reason, we enable GCC to determine asan shadow offset address at runtime using a
callback function named __asan_shadow_offset(). This supersedes the need to specify this
address at compile time. GCC then makes use of this shadow offset to protect stack buffers
by inserting red zones around them.

Some other benefits of having this GCC patch are:
a. We can place the shadow region in a separate linker section with all its advantages like
   automatic fit insurance. This ensures if a platform doesn't have enough memory space to
   hold shadow region, the build will fail. (However, if we use a fixed shadow offset on a
   platform that actually doesn't have enough memory, it may still build without any errors.)
b. We don't modify the memory layout compared to the current one, as we are placing the
   shadow region at the end of the space already occupied by the program.
c. We can be much more flexible later if needed (thinking of other stages like bootblock).
d. Since we are appending the shadow buffer to the region already occupied, we make efficient
   use of the limited memory available which is highly beneficial when using cache as ram.

Further, we have made sure that if you compile you tree with ASan enabled but missed this
patch, it will end up in the following compilation error:
"invalid --param name 'asan-use-shadow-offset-callback'"
So, you cannot accidentally enable the feature without having your compiler patched.

[pgeorgi: Updated for gcc 11.1]

Signed-off-by: Harshit Sharma <harshitsharmajs@gmail.com>
Signed-off-by: Patrick Georgi <pgeorgi@google.com>

diff --git a/gcc/asan.c b/gcc/asan.c
index 235e21947..713bf994d 100644
--- a/gcc/asan.c
+++ b/gcc/asan.c
@@ -1389,13 +1389,28 @@ asan_emit_stack_protection (rtx base, rtx pbase, unsigned int alignb,
   TREE_ASM_WRITTEN (decl) = 1;
   TREE_ASM_WRITTEN (id) = 1;
   emit_move_insn (mem, expand_normal (build_fold_addr_expr (decl)));
-  shadow_base = expand_binop (Pmode, lshr_optab, base,
-			      gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
-			      NULL_RTX, 1, OPTAB_DIRECT);
-  shadow_base
-    = plus_constant (Pmode, shadow_base,
-		     asan_shadow_offset ()
-		     + (base_align_bias >> ASAN_SHADOW_SHIFT));
+  if (param_asan_use_shadow_offset_callback) {
+    rtx addr, shadow_offset_rtx;
+    ret = init_one_libfunc ("__asan_shadow_offset");
+    addr = convert_memory_address (ptr_mode, base);
+    ret = emit_library_call_value (ret, NULL_RTX, LCT_NORMAL, ptr_mode,
+                                  addr, ptr_mode);
+    shadow_offset_rtx = convert_memory_address (Pmode, ret);
+    shadow_base = expand_binop (Pmode, lshr_optab, base,
+			                          gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
+			                          NULL_RTX, 1, OPTAB_DIRECT);
+    shadow_base = expand_binop (Pmode, add_optab, shadow_base,
+			                          shadow_offset_rtx, NULL_RTX, 1, OPTAB_LIB_WIDEN);
+    shadow_base = plus_constant (Pmode, shadow_base,
+		                            (base_align_bias >> ASAN_SHADOW_SHIFT));
+  } else {
+    shadow_base = expand_binop (Pmode, lshr_optab, base,
+			                          gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
+			                          NULL_RTX, 1, OPTAB_DIRECT);
+    shadow_base = plus_constant (Pmode, shadow_base,
+		                            asan_shadow_offset ()
+		                            + (base_align_bias >> ASAN_SHADOW_SHIFT));
+  }
   gcc_assert (asan_shadow_set != -1
 	      && (ASAN_RED_ZONE_SIZE >> ASAN_SHADOW_SHIFT) == 4);
   shadow_mem = gen_rtx_MEM (SImode, shadow_base);
--- gcc-11.1.0/gcc/params.opt~	2021-05-11 09:02:51.897508677 +0200
+++ gcc-11.1.0/gcc/params.opt	2021-05-11 09:10:43.692610696 +0200
@@ -50,6 +50,10 @@
 Common Joined UInteger Var(param_asan_instrumentation_with_call_threshold) Init(7000) Param Optimization
 Use callbacks instead of inline code if number of accesses in function becomes greater or equal to this number.

+-param=asan-use-shadow-offset-callback=
+Common Joined UInteger Var(param_asan_use_shadow_offset_callback) Init(0) IntegerRange(0, 1) Param Optimization
+Use shadow offset callback function at runtime instead of fixed value at compile time at the cost of runtime overhead.
+
 -param=asan-memintrin=
 Common Joined UInteger Var(param_asan_memintrin) Init(1) IntegerRange(0, 1) Param Optimization
 Enable asan builtin functions protection.