Add support code needed for gdb-darling

platform-include/mach-o/dyld_images.h

@@ -25,7 +25,11 @@
 
 #include <stdbool.h>
 #include <unistd.h>
+#ifndef MLDR_BUILD
 #include <mach/mach.h>
+#else
+#include <stdint.h>
+#endif
 
 #ifdef __cplusplus
 extern "C" {
@@ -73,10 +77,12 @@ struct dyld_image_info {
 													/* then file has been modified since dyld loaded it */
 };
 
+#ifndef MLDR_BUILD
 struct dyld_uuid_info {
 	const struct mach_header*	imageLoadAddress;	/* base address image is mapped into */
 	uuid_t						imageUUID;			/* UUID of image */
 };
+#endif
 
 typedef void (*dyld_image_notifier)(enum dyld_image_mode mode, uint32_t infoCount, const struct dyld_image_info info[]);
 

src/dyld/CMakeLists.txt

@@ -9,8 +9,8 @@ endif(COMMAND cmake_policy)
 
 enable_language(C ASM)
 
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11")
-add_definitions(-DINSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}" -D_GNU_SOURCE)
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11 -ggdb")
+add_definitions(-DINSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}" -D_GNU_SOURCE -DMLDR_BUILD)
 
 add_executable(darling darling.c)
 
@@ -19,6 +19,7 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR})
 set(mldr_sources
 	mldr.c
 	threads.c
+	gdb.c
 )
 add_executable(mldr ${mldr_sources})
 target_link_libraries(mldr -lpthread -ldl)

src/dyld/gdb.c

@@ -0,0 +1,58 @@
+#include "gdb.h"
+#include <string.h>
+#include <stdio.h>
+
+__attribute__ ((noinline))
+static void gdb_notifier(enum dyld_image_mode mode, uint32_t infoCount, const struct dyld_image_info info[]);
+static void dyld_notification_wrapper(enum dyld_image_mode mode, uint32_t infoCount, const struct dyld_image_info info[]);
+
+static struct dyld_all_image_infos* orig_dyld_all_image_infos;
+
+// This is the symbol GDB looks for
+struct dyld_all_image_infos _dyld_all_image_infos = {
+	.version = 15,
+	.infoArrayCount = 0,
+	.infoArray = NULL,
+	.notification = &gdb_notifier,
+};
+
+void gdb_notifier(enum dyld_image_mode mode, uint32_t infoCount, const struct dyld_image_info info[])
+{
+	// GDB sets a breakpoint in this function
+}
+
+struct jump
+{
+	uint16_t mov;
+	void* addr;
+	uint16_t jump;
+} __attribute__ ((packed));
+
+void setup_gdb_notifications(uint64_t slide, uint64_t addr)
+{
+	orig_dyld_all_image_infos = (struct dyld_all_image_infos*)(addr + slide);
+
+	// dyld will later rebase the address in notification,
+	// but at this point we must add slide manually.
+	struct jump* jump = (struct jump*)(((uint64_t)orig_dyld_all_image_infos->notification) + slide);
+
+	// Rewrite instructions in the notification function to redirect the call to us.
+#ifdef __x86_64__
+	jump->mov = 0xb948; // movabs imm,%rcx
+	jump->addr = (void*) &dyld_notification_wrapper; // immediate for preceding movabs
+	jump->jump = 0xe1ff; // jmpq *%ecx
+#else
+#	error TODO: Unsupported platform
+#endif
+}
+
+void dyld_notification_wrapper(enum dyld_image_mode mode, uint32_t infoCount, const struct dyld_image_info info[])
+{
+	// Copy over all data from dyld's copy of the structure to the one GDB can find (ours)
+	memcpy(&_dyld_all_image_infos, orig_dyld_all_image_infos, sizeof(_dyld_all_image_infos));
+	_dyld_all_image_infos.notification = &gdb_notifier;
+
+	printf("Got notification from dyld\n");
+	gdb_notifier(mode, infoCount, info);
+}
+

src/dyld/gdb.h

@@ -0,0 +1,9 @@
+#ifndef _GDB_H_
+#define _GDB_H_
+#include <mach-o/dyld_images.h>
+#include <stdint.h>
+
+void setup_gdb_notifications(uint64_t slide, uint64_t addr);
+
+#endif
+

src/dyld/mldr.c

@@ -14,6 +14,7 @@
 #include <dlfcn.h>
 #include "elfcalls.h"
 #include "threads.h"
+#include "gdb.h"
 
 #ifndef PAGE_SIZE
 #	define PAGE_SIZE	4096
@@ -233,6 +234,23 @@ void load64(int fd, uint64_t* entryPoint_out, uint64_t* mh_out)
 					if (seg->fileoff == 0)
 						mappedHeader = (struct mach_header_64*) (seg->vmaddr + slide);
 				}
+
+				if (strcmp(SEG_DATA, seg->segname) == 0)
+				{
+					// Look for section named __all_image_info for GDB integration
+					struct section_64* sect = (struct section_64*) (seg+1);
+					struct section_64* end = (struct section_64*) (&cmds[p + lc->cmdsize]);
+
+					while (sect < end)
+					{
+						if (strncmp(sect->sectname, "__all_image_info", 16) == 0)
+						{
+							setup_gdb_notifications(slide, sect->addr);
+							break;
+						}
+						sect++;
+					}
+				}
 				break;
 			}
 			case LC_UNIXTHREAD: