[arch][arm64] Add fault handler table

Allows executing an instruction with a custom fault recovery handler.

Can be used with Load/Store Unprivileged to safely access user-space
memory.

Change-Id: If5390de06182e4d0409b30a8e6a1b93b7acc77b6

arch/arm64/exceptions_c.c

@@ -27,6 +27,14 @@
 
 #define SHUTDOWN_ON_FATAL 1
 
+struct fault_handler_table_entry {
+    uint64_t pc;
+    uint64_t fault_handler;
+};
+
+extern struct fault_handler_table_entry __fault_handler_table_start[];
+extern struct fault_handler_table_entry __fault_handler_table_end[];
+
 static void dump_iframe(const struct arm64_iframe_long *iframe)
 {
     printf("iframe %p:\n", iframe);
@@ -44,6 +52,7 @@ static void dump_iframe(const struct arm64_iframe_long *iframe)
 
 void arm64_sync_exception(struct arm64_iframe_long *iframe)
 {
+    struct fault_handler_table_entry *fault_handler;
     uint32_t esr = ARM64_READ_SYSREG(esr_el1);
     uint32_t ec = esr >> 26;
     uint32_t il = (esr >> 25) & 0x1;
@@ -59,6 +68,13 @@ void arm64_sync_exception(struct arm64_iframe_long *iframe)
     }
 #endif
 
+    for (fault_handler = __fault_handler_table_start; fault_handler < __fault_handler_table_end; fault_handler++) {
+        if (fault_handler->pc == iframe->elr) {
+            iframe->elr = fault_handler->fault_handler;
+            return;
+        }
+    }
+
     printf("sync_exception\n");
     dump_iframe(iframe);
 

arch/arm64/include/arch/asm_macros.h

@@ -85,3 +85,12 @@ ldp \ra, \rb, [sp], #16
     cmp     \tmp, \new_ptr_end
     b.lo    .Lcalloc_bootmem_aligned_clear_loop\@
 .endm
+
+/* Set fault handler for next instruction */
+.macro set_fault_handler, handler
+.Lfault_location\@:
+.pushsection .rodata.fault_handler_table
+    .quad    .Lfault_location\@
+    .quad    \handler
+.popsection
+.endm

arch/arm64/system-onesegment.ld

@@ -48,6 +48,9 @@ SECTIONS
 
     .rodata : ALIGN(8) {
         __rodata_start = .;
+        __fault_handler_table_start = .;
+        KEEP(*(.rodata.fault_handler_table))
+        __fault_handler_table_end = .;
         *(.rodata .rodata.* .gnu.linkonce.r.*)
 INCLUDE "arch/shared_rodata_sections.ld"
         . = ALIGN(8);