1 files changed, 205 insertions, 0 deletions
diff --git a/lib/darwin/objc.c b/lib/darwin/objc.c
new file mode 100644
index 0000000..398e3e7
--- /dev/null
+++ b/lib/darwin/objc.c
@@ -0,0 +1,205 @@
+#if defined(__APPLE__)
+#include "substitute.h"
+#include "substitute-internal.h"
+#include "objc.h"
+#include <stddef.h>
+#include <pthread.h>
+#include <mach/mach.h>
+#include <sys/mman.h>
+#include <sys/queue.h>
+#include <errno.h>
+
+/* These trampolines will call e->func(e->arg1, e->arg2), and jump to there,
+ * preserving all arguments.  imp_implementationWithBlock would be easier and
+ * maybe a bit faster, but it's impossible to avoid throwing away registers
+ * without having to ask whether the selector is stret or not. */
+
+struct tramp_info_page_header {
+    uint32_t magic;
+    uint32_t version;
+    struct tramp_info_page_entry *first_free;
+    size_t nfree;
+    LIST_ENTRY(tramp_info_page_header) free_pages;
+};
+
+enum {
+    TRAMP_MAGIC = 0xf00df17e,
+    TRAMP_VERSION = 0,
+};
+
+struct tramp_info_page_entry {
+    union {
+        struct tramp_info_page_entry *next_free;
+        void *func;
+    };
+    void *arg1;
+    void *arg2;
+};
+
+_Static_assert(TRAMP_INFO_PAGE_ENTRY_SIZE == sizeof(struct tramp_info_page_entry),
+               "TRAMP_INFO_PAGE_ENTRY_SIZE");
+_Static_assert(sizeof(struct tramp_info_page_header) +
+               TRAMPOLINES_PER_PAGE * sizeof(struct tramp_info_page_entry) <= _PAGE_SIZE,
+               "header+entries too big");
+
+static pthread_mutex_t tramp_mutex = PTHREAD_MUTEX_INITIALIZER;
+LIST_HEAD(tramp_info_page_list, tramp_info_page_header)
+    tramp_free_page_list = LIST_HEAD_INITIALIZER(tramp_info_page_list);
+
+extern char remap_start[];
+
+static int get_trampoline(void *func, void *arg1, void *arg2, void *tramp_ptr) {
+    int ret, rerrno = 0;
+    pthread_mutex_lock(&tramp_mutex);
+
+    struct tramp_info_page_header *header = LIST_FIRST(&tramp_free_page_list);
+    if (!header) {
+        if (PAGE_SIZE > _PAGE_SIZE)
+            panic("%s: strange PAGE_SIZE %x\n", __func__, PAGE_SIZE);
+        void *new_pages = mmap(NULL, _PAGE_SIZE * 2, PROT_READ | PROT_WRITE,
+                               MAP_SHARED | MAP_ANON, -1, 0);
+        if (new_pages == MAP_FAILED) {
+            ret = SUBSTITUTE_ERR_OOM;
+            rerrno = errno;
+            goto out;
+        }
+        vm_address_t tramp_page = (vm_address_t) new_pages;
+        vm_prot_t cur_prot, max_prot;
+        kern_return_t kr = vm_remap(
+            mach_task_self(),
+            &tramp_page,
+            _PAGE_SIZE,
+            _PAGE_SIZE - 1,
+            VM_FLAGS_OVERWRITE | VM_FLAGS_FIXED,
+            mach_task_self(),
+            (vm_address_t) remap_start,
+            FALSE, /* copy */
+            &cur_prot,
+            &max_prot,
+            VM_INHERIT_NONE);
+        if (kr != KERN_SUCCESS || tramp_page != (vm_address_t) new_pages) {
+            ret = SUBSTITUTE_ERR_VM;
+            goto out;
+        }
+        header = new_pages + _PAGE_SIZE * 2 - sizeof(*header);
+        header->magic = TRAMP_MAGIC;
+        header->version = TRAMP_VERSION;
+        header->first_free = NULL;
+        header->nfree = TRAMPOLINES_PER_PAGE;
+        LIST_INSERT_HEAD(&tramp_free_page_list, header, free_pages);
+    }
+
+    void *page = (void *) (((uintptr_t) header) & ~(_PAGE_SIZE - 1));
+    struct tramp_info_page_entry *entries = page;
+    struct tramp_info_page_entry *entry = header->first_free;
+    if (entry == NULL) {
+        entry = &entries[TRAMPOLINES_PER_PAGE - header->nfree];
+        entry->next_free = NULL;
+    }
+
+    header->first_free = entry->next_free;
+    if (--header->nfree == 0)
+        LIST_REMOVE(header, free_pages);
+
+    entry->func = func;
+    entry->arg1 = arg1;
+    entry->arg2 = arg2;
+    void *tramp = (page - PAGE_SIZE) + (entry - entries) * TRAMPOLINE_SIZE;
+#ifdef __arm__
+    tramp += 1;
+#endif
+    *(void **) tramp_ptr = tramp;
+    ret = SUBSTITUTE_OK;
+out:
+    pthread_mutex_unlock(&tramp_mutex);
+    errno = rerrno;
+    return ret;
+}
+
+static void free_trampoline(void *tramp) {
+    pthread_mutex_lock(&tramp_mutex);
+    void *page = (void *) (((uintptr_t) tramp) & ~(_PAGE_SIZE - 1));
+    size_t i = (tramp - page) / TRAMPOLINE_SIZE;
+    struct tramp_info_page_entry *entries = page + _PAGE_SIZE;
+    struct tramp_info_page_entry *entry = &entries[i];
+    struct tramp_info_page_header *header = page + 2 * _PAGE_SIZE - sizeof(*header);
+
+    if (header->magic != TRAMP_MAGIC)
+        panic("%s: bad pointer\n", __func__);
+    if (header->version != TRAMP_VERSION) {
+        /* shouldn't happen, but just in case multiple versions of this library
+         * are mixed up */
+        return;
+    }
+
+    entry->next_free = header->first_free;
+    header->first_free = entry;
+    header->nfree++;
+    if (header->nfree == 1)
+        LIST_INSERT_HEAD(&tramp_free_page_list, header, free_pages);
+    else if (header->nfree == TRAMPOLINES_PER_PAGE &&
+        /* have others? */
+        (LIST_FIRST(&tramp_free_page_list) != header ||
+         LIST_NEXT(header, free_pages))) {
+        /* free the trampoline and info pages */
+        LIST_REMOVE(header, free_pages);
+        munmap(page, 2 * _PAGE_SIZE);
+    }
+
+    pthread_mutex_unlock(&tramp_mutex);
+}
+
+static IMP dereference(IMP *old_ptr, UNUSED void *_) {
+    return *old_ptr;
+}
+
+EXPORT
+int substitute_hook_objc_message(Class class, SEL selector, void *replacement,
+                                 void *old_ptr, bool *created_imp_ptr) {
+    int ret;
+    Method meth = class_getInstanceMethod(class, selector);
+    if (meth == NULL)
+        return SUBSTITUTE_ERR_NO_SUCH_SELECTOR;
+    const char *types = method_getTypeEncoding(meth);
+
+    if (created_imp_ptr)
+        *created_imp_ptr = false;
+
+    /* temporary trampoline just tries again */
+    IMP temp = NULL;
+    if (old_ptr) {
+        if ((ret = get_trampoline(dereference, old_ptr, NULL, &temp)))
+            return ret;
+        *(IMP *) old_ptr = temp;
+    }
+
+    IMP old = class_replaceMethod(class, selector, replacement, types);
+    if (old) {
+        if (old_ptr)
+            *(IMP *) old_ptr = old;
+    } else {
+        if (old_ptr) {
+            Class super = class_getSuperclass(class);
+            if (!super) {
+                /* this ought to only be possible if the method was removed in
+                 * the meantime, since we found the method above and it
+                 * couldn't have been found in a superclass, but the objc2
+                 * runtime doesn't allow removing methods. */
+                panic("%s: no superclass but the method didn't exist\n", __func__);
+            }
+            ret = get_trampoline(class_getMethodImplementation, super, selector, old_ptr);
+            if (created_imp_ptr)
+                *created_imp_ptr = true;
+        }
+    }
+
+    if (temp)
+        free_trampoline(temp);
+    return SUBSTITUTE_OK;
+}
+
+EXPORT
+void substitute_free_created_imp(IMP imp) {
+    free_trampoline(imp);
+}
+#endif