vclog: Use volatile to avoid memcpy optimisation

vclog works fine with -O1 but gives a bus error with -O2.
If I can trust the backtrace it looks like gcc turned the
copy-loop into a memcpy call...

Program received signal SIGBUS, Bus error.
__memcpy_generic () at ../sysdeps/aarch64/multiarch/../memcpy.S:96
96		ldr	A_l, [src]
(gdb) where
    src=0x7fb6dc4164 "\247\200 \001",
    src@entry=0x1 <error: Cannot access memory at address 0x1>, n=376432, n@entry=376444)
    at /home/hias/libreelec/libreelec-master/build.LibreELEC-RPi4.aarch64-12.0-devel/build/bcm2835-utils-a843bed/vclog/vcdbg.c:731
    at /home/hias/libreelec/libreelec-master/build.LibreELEC-RPi4.aarch64-12.0-devel/build/bcm2835-utils-a843bed/vclog/vcdbg.c:267
(gdb) up
    src=0x7fb6dc4164 "\247\200 \001",
    src@entry=0x1 <error: Cannot access memory at address 0x1>, n=376432, n@entry=376444)
    at /home/hias/libreelec/libreelec-master/build.LibreELEC-RPi4.aarch64-12.0-devel/build/bcm2835-utils-a843bed/vclog/vcdbg.c:731
731	    *dest = *src++;
(gdb) list
726	  // Manually copy bytes before first boundary
727	  size_t bytes_to_manually_copy =
728	      n < bytes_until_boundary ? n : bytes_until_boundary;
729	  n -= bytes_to_manually_copy;
730	  while (bytes_to_manually_copy--) {
731	    *dest = *src++;
732	    dest++;
733	  }
734
735	  // return if we copied all of n
(gdb) p bytes_to_manually_copy
$1 = 11
(gdb) p src
$2 = 0x7fb6dc4164 "\247\200 \001"

Author: popcornmix

vclog/vcdbg.c

@@ -164,7 +164,7 @@ static bool parse_and_print_assert_msg(char *txt, size_t size_of_text,
 src address is not a aligned to a boundary at the start of memcpy (and at the
 end of the region copied)
 dest & src MUST have the same alignment */
-static void memcpy_vc_memory(char *restrict dest, const char *restrict src,
+static void memcpy_vc_memory(char *restrict dest, const volatile char *restrict src,
                              size_t n);
 
 /******************************************************************************
@@ -715,8 +715,12 @@ static bool vc_write_ptr_within_current_msg(
   return true;
 }
 
-/*****************************************************************************/
-static void memcpy_vc_memory(char *restrict dest, const char *restrict src,
+/********************************************************************************/
+/* Note: gcc with -O2 or higher may replace most of this code with memcpy       */
+/* which causes a bus error when given an insufficiently aligned mmap-ed buffer */
+/* Using volatile disables that optimisation                                    */
+/********************************************************************************/
+static void memcpy_vc_memory(char *restrict dest, const volatile char *restrict src,
                              size_t n) {
   // Calculate non-boundary aligned bytes at start/end of region
   size_t src_offset = (uintptr_t)src % boundary;
@@ -739,7 +743,7 @@ static void memcpy_vc_memory(char *restrict dest, const char *restrict src,
   // memcpy centre region starting/ending on boundaries
   int bytes_to_memcpy = n - bytes_over_end_boundary;
   if (bytes_to_memcpy) {
-    memcpy(dest, src, bytes_to_memcpy);
+    memcpy(dest, (const void *)src, bytes_to_memcpy);
     dest += bytes_to_memcpy;
     src += bytes_to_memcpy;
     n -= bytes_to_memcpy;