Hi.
As requested in PR81715, GCC emits bigger middle redzones for small variables.
It's analyzed in following comment:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715#c28
For now I'm suggesting to shrink shadow memory for variables <= 16B to 32B
(including variable storage).
LLVM is more aggressive as they allocate just 16B of shadow memory for
variables <= 4B. That would
require bigger code refactoring in asan.c and I would like to avoid that.
For detailed statistics of Linux Kernel, please take a look here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715#c29
I'm testing the patch now.
Thoughts?
gcc/ChangeLog:
2018-09-25 Martin Liska <mli...@suse.cz>
* cfgexpand.c (expand_stack_vars): Smaller middle redzones
if requested.
* doc/invoke.texi: New param.
* params.def (PARAM_ASAN_STACK_SMALL_REDZONE): Likewise.
* params.h (ASAN_STACK_SMALL_REDZONE): Likewise.
gcc/testsuite/ChangeLog:
2018-09-25 Martin Liska <mli...@suse.cz>
* c-c++-common/asan/asan-stack-small.c: New test.
---
gcc/cfgexpand.c | 13 +++++++--
gcc/doc/invoke.texi | 3 ++
gcc/params.def | 6 ++++
gcc/params.h | 2 ++
.../c-c++-common/asan/asan-stack-small.c | 29 +++++++++++++++++++
5 files changed, 51 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/c-c++-common/asan/asan-stack-small.c
diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 35ca276e4ad..c735e9ca2a3 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -1128,9 +1128,18 @@ expand_stack_vars (bool (*pred) (size_t), struct stack_vars_data *data)
MAX (alignb, ASAN_RED_ZONE_SIZE),
!FRAME_GROWS_DOWNWARD);
tree repr_decl = NULL_TREE;
+ poly_uint64 size = stack_vars[i].size;
+ /* For small variables shrink middle redzone (including
+ * variable store) just to ASAN_RED_ZONE_SIZE. */
+ if (ASAN_STACK_SMALL_REDZONE
+ && i != n - 1
+ && (stack_vars[i].size.to_constant ()
+ <= (ASAN_RED_ZONE_SIZE / 2)))
+ ;
+ else
+ size += ASAN_RED_ZONE_SIZE;
offset
- = alloc_stack_frame_space (stack_vars[i].size
- + ASAN_RED_ZONE_SIZE,
+ = alloc_stack_frame_space (size,
MAX (alignb, ASAN_RED_ZONE_SIZE));
data->asan_vec.safe_push (prev_offset);
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 7ef4e7a449b..1c55887b9cd 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -11313,6 +11313,9 @@ Enable buffer overflow detection for stack objects. This kind of
protection is enabled by default when using @option{-fsanitize=address}.
To disable stack protection use @option{--param asan-stack=0} option.
+@item asan-stack-small-redzone
+Emit smaller middle redzone for variables smaller or equal to 16 bytes.
+
@item asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of
protection is enabled by default when using @option{-fsanitize=address}.
diff --git a/gcc/params.def b/gcc/params.def
index 9f0697327d4..f1fafb4002d 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -1201,6 +1201,12 @@ DEFPARAM (PARAM_ASAN_STACK,
"Enable asan stack protection.",
1, 0, 1)
+DEFPARAM (PARAM_ASAN_STACK_SMALL_REDZONE,
+ "asan-stack-small-redzone",
+ "Emit smaller middle redzone for variables smaller "
+ "or equal to 16 bytes.",
+ 0, 0, 1)
+
DEFPARAM (PARAM_ASAN_PROTECT_ALLOCAS,
"asan-instrument-allocas",
"Enable asan allocas/VLAs protection.",
diff --git a/gcc/params.h b/gcc/params.h
index 8aa960a904e..42df40e66fd 100644
--- a/gcc/params.h
+++ b/gcc/params.h
@@ -236,6 +236,8 @@ extern void init_param_values (int *params);
PARAM_VALUE (PARAM_ALLOW_PACKED_STORE_DATA_RACES)
#define ASAN_STACK \
PARAM_VALUE (PARAM_ASAN_STACK)
+#define ASAN_STACK_SMALL_REDZONE \
+ PARAM_VALUE (PARAM_ASAN_STACK_SMALL_REDZONE)
#define ASAN_PROTECT_ALLOCAS \
PARAM_VALUE (PARAM_ASAN_PROTECT_ALLOCAS)
#define ASAN_GLOBALS \
diff --git a/gcc/testsuite/c-c++-common/asan/asan-stack-small.c b/gcc/testsuite/c-c++-common/asan/asan-stack-small.c
new file mode 100644
index 00000000000..dc08edaad71
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/asan/asan-stack-small.c
@@ -0,0 +1,29 @@
+/* { dg-do run } */
+/* { dg-options "--param asan-stack-small-redzone=1" } */
+
+char *pa;
+char *pb;
+char *pc;
+
+void access (volatile char *ptr)
+{
+ *ptr = 'x';
+}
+
+int main (int argc, char **argv)
+{
+ char a;
+ char b;
+ char c;
+
+ pa = &a;
+ pb = &b;
+ pc = &c;
+
+ access (pb);
+ access (pc);
+ // access 'b' here
+ access (pa + 32);
+
+ return 0;
+}
