mesa: fix display list 8-byte alignment issue

The _mesa_dlist_alloc() function is only guaranteed to return a pointer
with 4-byte alignment.  On 64-bit systems which don't support unaligned
loads (e.g. SPARC or MIPS) this could lead to a bus error in the VBO code.

The solution is to add a new  _mesa_dlist_alloc_aligned() function which
will return a pointer to an 8-byte aligned address on 64-bit systems.
This is accomplished by inserting a 4-byte NOP instruction in the display
list when needed.

The only place this actually matters is the VBO code where we need to
allocate a 'struct vbo_save_vertex_list' which needs to be 8-byte
aligned (just as if it were malloc'd).

The gears demo and others hit this bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88662
Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

1 files changed, 67 insertions, 6 deletions

diff --git a/src/mesa/main/dlist.c b/src/mesa/main/dlist.c
index 138d360..7e5b83c 100644
--- a/src/mesa/main/dlist.c
+++ b/src/mesa/main/dlist.c
@@ -487,6 +487,7 @@ typedef enum
    /* The following three are meta instructions */
    OPCODE_ERROR,                /* raise compiled-in error */
    OPCODE_CONTINUE,
+   OPCODE_NOP,                  /* No-op (used for 8-byte alignment */
    OPCODE_END_OF_LIST,
    OPCODE_EXT_0
 } OpCode;
@@ -1012,13 +1013,16 @@ memdup(const void *src, GLsizei bytes)
  * Allocate space for a display list instruction (opcode + payload space).
  * \param opcode  the instruction opcode (OPCODE_* value)
  * \param bytes   instruction payload size (not counting opcode)
- * \return pointer to allocated memory (the opcode space)
+ * \param align8  does the payload need to be 8-byte aligned?
+ *                This is only relevant in 64-bit environments.
+ * \return pointer to allocated memory (the payload will be at pointer+1)
  */
 static Node *
-dlist_alloc(struct gl_context *ctx, OpCode opcode, GLuint bytes)
+dlist_alloc(struct gl_context *ctx, OpCode opcode, GLuint bytes, bool align8)
 {
    const GLuint numNodes = 1 + (bytes + sizeof(Node) - 1) / sizeof(Node);
    const GLuint contNodes = 1 + POINTER_DWORDS;  /* size of continue info */
+   GLuint nopNode;
    Node *n;
 
    if (opcode < OPCODE_EXT_0) {
@@ -1032,7 +1036,20 @@ dlist_alloc(struct gl_context *ctx, OpCode opcode, GLuint bytes)
       }
    }
 
-   if (ctx->ListState.CurrentPos + numNodes + contNodes > BLOCK_SIZE) {
+   if (sizeof(void *) > sizeof(Node) && align8
+       && ctx->ListState.CurrentPos % 2 == 0) {
+      /* The opcode would get placed at node[0] and the payload would start
+       * at node[1].  But the payload needs to be at an even offset (8-byte
+       * multiple).
+       */
+      nopNode = 1;
+   }
+   else {
+      nopNode = 0;
+   }
+
+   if (ctx->ListState.CurrentPos + nopNode + numNodes + contNodes
+       > BLOCK_SIZE) {
       /* This block is full.  Allocate a new block and chain to it */
       Node *newblock;
       n = ctx->ListState.CurrentBlock + ctx->ListState.CurrentPos;
@@ -1042,13 +1059,34 @@ dlist_alloc(struct gl_context *ctx, OpCode opcode, GLuint bytes)
          _mesa_error(ctx, GL_OUT_OF_MEMORY, "Building display list");
          return NULL;
       }
+
+      /* a fresh block should be 8-byte aligned on 64-bit systems */
+      assert(((GLintptr) newblock) % sizeof(void *) == 0);
+
       save_pointer(&n[1], newblock);
       ctx->ListState.CurrentBlock = newblock;
       ctx->ListState.CurrentPos = 0;
+
+      /* Display list nodes are always 4 bytes.  If we need 8-byte alignment
+       * we have to insert a NOP so that the payload of the real opcode lands
+       * on an even location:
+       *   node[0] = OPCODE_NOP
+       *   node[1] = OPCODE_x;
+       *   node[2] = start of payload
+       */
+      nopNode = sizeof(void *) > sizeof(Node) && align8;
    }
 
    n = ctx->ListState.CurrentBlock + ctx->ListState.CurrentPos;
-   ctx->ListState.CurrentPos += numNodes;
+   if (nopNode) {
+      assert(ctx->ListState.CurrentPos % 2 == 0); /* even value */
+      n[0].opcode = OPCODE_NOP;
+      n++;
+      /* The "real" opcode will now be at an odd location and the payload
+       * will be at an even location.
+       */
+   }
+   ctx->ListState.CurrentPos += nopNode + numNodes;
 
    n[0].opcode = opcode;
 
@@ -1069,7 +1107,22 @@ dlist_alloc(struct gl_context *ctx, OpCode opcode, GLuint bytes)
 void *
 _mesa_dlist_alloc(struct gl_context *ctx, GLuint opcode, GLuint bytes)
 {
-   Node *n = dlist_alloc(ctx, (OpCode) opcode, bytes);
+   Node *n = dlist_alloc(ctx, (OpCode) opcode, bytes, false);
+   if (n)
+      return n + 1;  /* return pointer to payload area, after opcode */
+   else
+      return NULL;
+}
+
+
+/**
+ * Same as _mesa_dlist_alloc(), but return a pointer which is 8-byte
+ * aligned in 64-bit environments, 4-byte aligned otherwise.
+ */
+void *
+_mesa_dlist_alloc_aligned(struct gl_context *ctx, GLuint opcode, GLuint bytes)
+{
+   Node *n = dlist_alloc(ctx, (OpCode) opcode, bytes, true);
    if (n)
       return n + 1;  /* return pointer to payload area, after opcode */
    else
@@ -1119,7 +1172,7 @@ _mesa_dlist_alloc_opcode(struct gl_context *ctx,
 static inline Node *
 alloc_instruction(struct gl_context *ctx, OpCode opcode, GLuint nparams)
 {
-   return dlist_alloc(ctx, opcode, nparams * sizeof(Node));
+   return dlist_alloc(ctx, opcode, nparams * sizeof(Node), false);
 }
 
 
@@ -8897,6 +8950,9 @@ execute_list(struct gl_context *ctx, GLuint list)
          case OPCODE_CONTINUE:
             n = (Node *) get_pointer(&n[1]);
             break;
+         case OPCODE_NOP:
+            /* no-op */
+            break;
          case OPCODE_END_OF_LIST:
             done = GL_TRUE;
             break;
@@ -9947,6 +10003,9 @@ print_list(struct gl_context *ctx, GLuint list, const char *fname)
             fprintf(f, "DISPLAY-LIST-CONTINUE\n");
             n = (Node *) get_pointer(&n[1]);
             break;
+         case OPCODE_NOP:
+            fprintf(f, "NOP\n");
+            break;
          case OPCODE_END_OF_LIST:
             fprintf(f, "END-LIST %u\n", list);
             done = GL_TRUE;
@@ -10097,6 +10156,8 @@ _mesa_init_display_list(struct gl_context *ctx)
    ctx->List.ListBase = 0;
 
    save_vtxfmt_init(&ctx->ListState.ListVtxfmt);
+
+   InstSize[OPCODE_NOP] = 1;
 }