On i386, it's better for a jit to ask for exec memory upfront, so uvm
knows to place it in the exec segment. Otherwise, if you mmap non-exec
and mprotect it, you wind up with a high mapping and the segment must
be enlarged to cover the whole process.
I sent Mike a slightly different diff; this is what he ended up
committing to git. I think we should add it to the port.
diff --git a/src/lj_mcode.c b/src/lj_mcode.c
index f8f8406..d95ebeb 100644
--- a/src/lj_mcode.c
+++ b/src/lj_mcode.c
@@ -145,7 +145,7 @@ static void mcode_free(jit_State *J, void *p, size_t sz)
/* -- MCode area protection ----------------------------------------------- */
-/* Define this ONLY if the page protection twiddling becomes a bottleneck. */
+/* Define this ONLY if page protection twiddling becomes a bottleneck. */
#ifdef LUAJIT_UNPROTECT_MCODE
/* It's generally considered to be a potential security risk to have
@@ -252,7 +252,20 @@ static void *mcode_alloc(jit_State *J, size_t sz)
#else
/* All memory addresses are reachable by relative jumps. */
-#define mcode_alloc(J, sz) mcode_alloc_at((J), 0, (sz), MCPROT_GEN)
+static void *mcode_alloc(jit_State *J, size_t sz)
+{
+#ifdef __OpenBSD__
+ /* Allow better executable memory allocation for OpenBSD W^X mode. */
+ void *p = mcode_alloc_at(J, 0, sz, MCPROT_RUN);
+ if (p && mcode_setprot(p, sz, MCPROT_GEN)) {
+ mcode_free(J, p, sz);
+ return NULL;
+ }
+ return p;
+#else
+ return mcode_alloc_at(J, 0, sz, MCPROT_GEN);
+#endif
+}
#endif
