elf: make offset pages writeable

This fixes crashes with eager GOT population, where the interpreter
makes the table read-only when done.

src/plt-elf.c

@@ -32,6 +32,8 @@
 #include "trampoline.h"
 #include "vitals.h"
 
+#include <sys/mman.h>
+
 #if MMK_BITS == 32
 typedef Elf32_Word ElfWord;
 typedef Elf32_Sword ElfSWord;
@@ -272,11 +274,21 @@ plt_offset *plt_get_offsets(plt_ctx ctx, plt_lib lib, const char *name, size_t *
     return NULL;
 }
 
+#define align2_down(v, d) ((v) & ~((d) - 1))
+
 void plt_set_offsets(plt_offset *offset, size_t nb_off, plt_fn *newval)
 {
     for (size_t i = 0; i < nb_off; ++i) {
         if (!offset[i].oldval)
             offset[i].oldval = *offset[i].offset;
+        void *page_start = (void *)align2_down((uintptr_t)offset[i].offset, 4096);
+
+        /* making a page in a rx segment rwx is usually very bad practice,
+           but this is a test context, we don't have to care as much.
+           Implementing this right assumes that we have a way to know
+           the protection of an existing page, which is not necessarily
+           available on all unices. */
+        mmk_mprotect(page_start, 4096, PROT_READ|PROT_WRITE|PROT_EXEC);
         *offset[i].offset = newval;
     }
 }

src/vitals.c

@@ -98,6 +98,12 @@ int mmk_isspace(int c)
     return c == ' ' || c == '\t' || c == '\r' || c == '\n';
 }
 
+int (*mmk_mprotect_)(void *, size_t, int);
+int mmk_mprotect(void *addr, size_t len, int prot)
+{
+    return mmk_mprotect_(addr, len, prot);
+}
+
 void *(*mmk_malloc_)(size_t);
 void *mmk_malloc(size_t size)
 {
@@ -190,4 +196,7 @@ void mmk_init_vital_functions(plt_ctx ctx)
     INIT_VITAL_FUNC(malloc);
     INIT_VITAL_FUNC(realloc);
     INIT_VITAL_FUNC(free);
+#ifdef MMK_EXE_FMT_ELF
+    INIT_VITAL_FUNC(mprotect);
+#endif
 }

src/vitals.h

@@ -40,6 +40,7 @@ char *mmk_strcpy(char *dst, const char *src);
 char *mmk_strncpy(char *dst, const char *src, size_t n);
 size_t mmk_strlen(const char *s);
 int mmk_isspace(int c);
+int mmk_mprotect(void *addr, size_t len, int prot);
 
 mmk_noreturn void mmk_panic(const char *, ...);