[PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic

Alexander Popov
2020-06-04T13:50:47Z

Some time ago Variable Length Arrays (VLA) were removed from the kernel.
The kernel is built with '-Wvla'. Let's exclude alloca() from the
instrumentation logic and make it simpler. The build-time assertion
against alloca() is added instead.

Unfortunately, for that assertion we can't simply check cfun->calls_alloca
during RTL phase. It turned out that gcc before version 7 called
allocate_dynamic_stack_space() from expand_stack_vars() for runtime
alignment of constant-sized stack variables. That caused cfun->calls_alloca
to be set for functions that don't use alloca().

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
scripts/gcc-plugins/stackleak_plugin.c | 51 +++++++++++---------------
1 file changed, 21 insertions(+), 30 deletions(-)

diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
index cc75eeba0be1..1ecfe50d0bf5 100644
--- a/scripts/gcc-plugins/stackleak_plugin.c
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -9,10 +9,9 @@
* any of the gcc libraries
*
* This gcc plugin is needed for tracking the lowest border of the kernel stack.
- * It instruments the kernel code inserting stackleak_track_stack() calls:
- * - after alloca();
- * - for the functions with a stack frame size greater than or equal
- * to the "track-min-size" plugin parameter.
+ * It instruments the kernel code inserting stackleak_track_stack() calls
+ * for the functions with a stack frame size greater than or equal to
+ * the "track-min-size" plugin parameter.
*
* This plugin is ported from grsecurity/PaX. For more information see:
* https://grsecurity.net/
@@ -46,7 +45,7 @@ static struct plugin_info stackleak_plugin_info = {
"disable\t\tdo not activate the plugin\n"
};

-static void stackleak_add_track_stack(gimple_stmt_iterator *gsi, bool after)
+static void stackleak_add_track_stack(gimple_stmt_iterator *gsi)
{
gimple stmt;
gcall *stackleak_track_stack;
@@ -56,12 +55,7 @@ static void stackleak_add_track_stack(gimple_stmt_iterator *gsi, bool after)
/* Insert call to void stackleak_track_stack(void) */
stmt = gimple_build_call(track_function_decl, 0);
stackleak_track_stack = as_a_gcall(stmt);
- if (after) {
- gsi_insert_after(gsi, stackleak_track_stack,
- GSI_CONTINUE_LINKING);
- } else {
- gsi_insert_before(gsi, stackleak_track_stack, GSI_SAME_STMT);
- }
+ gsi_insert_before(gsi, stackleak_track_stack, GSI_SAME_STMT);

/* Update the cgraph */
bb = gimple_bb(stackleak_track_stack);
@@ -87,14 +81,13 @@ static bool is_alloca(gimple stmt)

/*
* Work with the GIMPLE representation of the code. Insert the
- * stackleak_track_stack() call after alloca() and into the beginning
- * of the function if it is not instrumented.
+ * stackleak_track_stack() call into the beginning of the function.
*/
static unsigned int stackleak_instrument_execute(void)
{
basic_block bb, entry_bb;
- bool prologue_instrumented = false, is_leaf = true;
- gimple_stmt_iterator gsi;
+ bool is_leaf = true;
+ gimple_stmt_iterator gsi = { 0 };

/*
* ENTRY_BLOCK_PTR is a basic block which represents possible entry
@@ -111,27 +104,17 @@ static unsigned int stackleak_instrument_execute(void)
*/
FOR_EACH_BB_FN(bb, cfun) {
for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
- gimple stmt;
-
- stmt = gsi_stmt(gsi);
+ gimple stmt = gsi_stmt(gsi);

/* Leaf function is a function which makes no calls */
if (is_gimple_call(stmt))
is_leaf = false;

- if (!is_alloca(stmt))
- continue;
-
- /* Insert stackleak_track_stack() call after alloca() */
- stackleak_add_track_stack(&gsi, true);
- if (bb == entry_bb)
- prologue_instrumented = true;
+ /* Variable Length Arrays are forbidden in the kernel */
+ gcc_assert(!is_alloca(stmt));
}
}

- if (prologue_instrumented)
- return 0;
-
/*
* Special cases to skip the instrumentation.
*
@@ -168,7 +151,7 @@ static unsigned int stackleak_instrument_execute(void)
bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
}
gsi = gsi_after_labels(bb);
- stackleak_add_track_stack(&gsi, false);
+ stackleak_add_track_stack(&gsi);

return 0;
}
@@ -185,12 +168,20 @@ static bool large_stack_frame(void)
/*
* Work with the RTL representation of the code.
* Remove the unneeded stackleak_track_stack() calls from the functions
- * which don't call alloca() and don't have a large enough stack frame size.
+ * that don't have a large enough stack frame size.
*/
static unsigned int stackleak_cleanup_execute(void)
{
rtx_insn *insn, *next;

+ /*
+ * gcc before version 7 called allocate_dynamic_stack_space() from
+ * expand_stack_vars() for runtime alignment of constant-sized stack
+ * variables. That caused cfun->calls_alloca to be set for functions
+ * that don't use alloca().
+ * For more info see gcc commit 7072df0aae0c59ae437e.
+ * Let's leave such functions instrumented.
+ */
if (cfun->calls_alloca)
return 0;

--
2.25.2