i#5463: Avoid globals pre-relocation

Fixes errors that can lead to crashes in relocate_dynamorio, where its
access to a function pointer and to the page size can both return
garbage pre-relocation.  We solve this with assembly to get the
current PC, and just using a 4K minimum page size for the backward
walk.

Tested with the PR #5462 branch where the new client.attach_blocking
test crashes without this fix on AArch64.
Manually tested on arm as well.

Fixes #5463

core/arch/arch_exports.h

@@ -302,9 +302,19 @@ get_stack_ptr(void);
                     __asm__ __volatile__("rdtsc" : "=a"(low), "=d"(high)); \
                     (llval) = ((uint64)high << 32) | low;                  \
                 } while (0)
+#            define GET_CUR_PC(var)                                         \
+                __asm__ __volatile__("call 1f; 1: pop %%rax; mov %%rax, %0" \
+                                     : "=m"(var)                            \
+                                     :                                      \
+                                     : "rax", "memory")
 #        else
 /* We define RDTSC_LL differently on 32-bit for better performance */
 #            define RDTSC_LL(llval) __asm__ __volatile__("rdtsc" : "=A"(llval))
+#            define GET_CUR_PC(var)                                         \
+                __asm__ __volatile__("call 1f; 1: pop %%rax; mov %%eax, %0" \
+                                     : "=m"(var)                            \
+                                     :                                      \
+                                     : "eax", "memory")
 #        endif /* 64/32 */
 #        define GET_FRAME_PTR(var) \
             asm("mov %%" IF_X64_ELSE("rbp", "ebp") ", %0" : "=m"(var))
@@ -318,12 +328,16 @@ get_stack_ptr(void);
 
 #        define GET_FRAME_PTR(var) __asm__ __volatile__("mov %0, x29" : "=r"(var))
 #        define GET_STACK_PTR(var) __asm__ __volatile__("mov %0, sp" : "=r"(var))
+#        define GET_CUR_PC(var) \
+            __asm__ __volatile__("bl 1f; 1: str x30, %0" : "=m"(var) : : "x30")
 #    elif defined(DR_HOST_ARM)
 #        define RDTSC_LL(llval) (llval) = proc_get_timestamp()
 /* FIXME i#1551: frame pointer is r7 in thumb mode */
 #        define GET_FRAME_PTR(var) \
             __asm__ __volatile__("str " IF_X64_ELSE("x29", "r11") ", %0" : "=m"(var))
 #        define GET_STACK_PTR(var) __asm__ __volatile__("str sp, %0" : "=m"(var))
+#        define GET_CUR_PC(var) \
+            __asm__ __volatile__("bl 1f; 1: str lr, %0" : "=m"(var) : : "lr")
 #    endif /* X86/ARM */
 #endif     /* UNIX */
 

core/unix/loader.c

@@ -1819,13 +1819,18 @@ relocate_dynamorio(byte *dr_map, size_t dr_size, byte *sp)
     const char **env = (const char **)sp + argc + 2;
     os_privmod_data_t opd = { { 0 } };
 
-    os_page_size_init(env, true);
+    /* We can't use PAGE_SIZE as that may require relocations to access. */
+    const int min_page_size = 4096;
 
     if (dr_map == NULL) {
+        /* We can't start with the address of relocate_dynamorio or something as that
+         * may require relocations to access!
+         */
+        GET_CUR_PC(dr_map);
         /* we do not know where dynamorio is, so check backward page by page */
-        dr_map = (app_pc)ALIGN_BACKWARD((ptr_uint_t)relocate_dynamorio, PAGE_SIZE);
+        dr_map = (app_pc)ALIGN_BACKWARD(dr_map, min_page_size);
         while (dr_map != NULL && !privload_mem_is_elf_so_header(dr_map)) {
-            dr_map -= PAGE_SIZE;
+            dr_map -= min_page_size;
         }
     }
     if (dr_map == NULL)
@@ -1834,6 +1839,8 @@ relocate_dynamorio(byte *dr_map, size_t dr_size, byte *sp)
     /* Relocate it */
     if (privload_get_os_privmod_data(dr_map, &opd))
         privload_early_relocate_os_privmod_data(&opd, dr_map);
+
+    os_page_size_init(env, true);
 }
 
 /* i#1227: on a conflict with the app we reload ourselves.