common-trampoline: Ensure omission of frame-pointer

Some architecture-specific implementations of the FETCH_LINKPTR
(e.g. the one for x86) attempt to find the linkptr relative to the
stack pointer. Therefore, they make assumption about the value of
the SP at the point where the FETCH_LINKPTR code is executed. This
means that we don't want the C compiler to mess with our SP.

For this purpose, local variables in this function are currently
declared using the register storage specifier. Additionally, we
need to make sure that a frame pointer is /always/ omitted for
this function. On x86 Alpine, the frame pointer is presently not
omitted with GCC 13 when using -Os, therefore the tests segfault
with -Os on Alpine presently.

Fixes #52

arch/common/common-trampoline.c

@@ -14,7 +14,19 @@
 #include <stdlib.h>
 #include <stdio.h>
 
+/* We need to make sure that the C compiler doesn't push any
+ * additional data to the stack frame. Otherwise, assumptions
+ * made by the architecture-specific implementation of the
+ * FETCH_LINKPTR() macro about the location of the linkptr,
+ * relative to the stack pointer, will not hold.
+ *
+ * Hence, we compile this function with -fomit-frame-pointer
+ * and use the register storage-class specifier for all local
+ * vars. Note that the latter is just a "suggestion" (see C99).
+ */
+
 __attribute__ ((visibility ("hidden")))
+__attribute__ ((optimize ("omit-frame-pointer")))
 void
 libucontext_trampoline(void)
 {