[hwasan] Fix and re-enable deep-recursion.c (#69265)

deep-recursion.c was disabled

(c007e0f)
because the test may get unlucky and end up with a zero-tagged variable,
leading to a false negative
(#69221).

This patch re-enables the test and adds a workaround: it checks
if the variable is zero-tagged, and if so, it will instead use the
neighboring variable, which must have a different (hence non-zero)
tag.

Fixing the stack allocation tagging is left as an exercise for the
reader. It is non-trivial because, even if the stackTagBase is
non-zero, tags for subsequent allocations in the stack frame may wrap
around to zero; working around this would require adding multiple
instructions to each alloca.

---------

Co-authored-by: Thurston Dang <thurston@google.com>

Author: thurstond

compiler-rt/test/hwasan/TestCases/deep-recursion.c

@@ -17,9 +17,6 @@
 // Stack histories are currently not recorded on x86.
 // XFAIL: target=x86_64{{.*}}
 
-// Flaky on AArch64 Linux, see https://github.com/llvm/llvm-project/issues/69221.
-// UNSUPPORTED: target=aarch64{{.*}}
-
 #include <stdlib.h>
 // At least -O1 is needed for this function to not have a stack frame on
 // AArch64.
@@ -29,7 +26,23 @@ void USE(void *x) { // pretend_to_do_something(void *x)
 
 volatile int four = 4;
 
-__attribute__((noinline)) void OOB() { int x[4]; x[four] = 0; USE(&x[0]); }
+__attribute__((noinline)) void OOB() {
+  int x[4];
+  int y[4];
+
+  // Tags for stack-allocated variables can occasionally be zero, resulting in
+  // a false negative for this test. This is not easy to fix, hence we work
+  // around it: if the tag is zero, we use the neighboring variable instead,
+  // which must have a different (hence non-zero) tag.
+  // This tag check assumes aarch64.
+  if (((uintptr_t)&x) >> 56 == 0) {
+    y[four] = 0;
+  } else {
+    x[four] = 0;
+  }
+  USE(&x[0]);
+  USE(&y[0]);
+}
 __attribute__((noinline)) void FUNC1() { int x; USE(&x); OOB(); }
 __attribute__((noinline)) void FUNC2() { int x; USE(&x); FUNC1(); }
 __attribute__((noinline)) void FUNC3() { int x; USE(&x); FUNC2(); }