Improve User Linking and ELF Loading (#299)

* Add note to assembly to quiet GNU tools complaining about executable
stack.

* Use explicit linker script for userspace programs, to avoid unexpected behaviors from platform linking scripts.

* Remove linker -Map argument used inconsistently.

* Missing zero, how embarrassing!

* Update linker command line and scripts to generate executables that cleanly start at 0x8000000.

* Rework ELF loader to work correctly with multiple loadable segments
that each may be partially initialized.

* Align bss section independently, so that it is not combined with prior sections.

* Separate elf definition in elf.h and process loader in loader.[ch]

Author: dthain

Makefile.config

@@ -4,8 +4,8 @@ KERNEL_CCFLAGS=-Wall -c -ffreestanding -fno-pie -g -std=gnu99
 # which is likely to work on native linux-x86.
 #
 CC=gcc -m32
-LD=ld -melf_i386
-#LD=ld -arch i386
+LD=ld
+KERNEL_LD=ld -melf_i386
 AR=ar
 OBJCOPY=objcopy
 ISOGEN=genisoimage
@@ -15,6 +15,7 @@ ISOGEN=genisoimage
 # add cross/bin to your path, and uncomment these lines:
 #CC=i686-elf-gcc
 #LD=i686-elf-ld
+#KERNEL_LD=i686-elf-ld
 #AR=i686-elf-ar
 #OBJCOPY=i686-elf-objcopy
 

kernel/Makefile

@@ -1,6 +1,6 @@
 include ../Makefile.config
 
-KERNEL_OBJECTS=kernelcore.o main.o console.o page.o keyboard.o mouse.o event_queue.o clock.o interrupt.o kmalloc.o pic.o ata.o cdromfs.o string.o bitmap.o graphics.o font.o syscall_handler.o process.o mutex.o list.o pagetable.o rtc.o kshell.o fs.o hash_set.o diskfs.o serial.o elf.o device.o kobject.o pipe.o bcache.o printf.o is_valid.o window.o
+KERNEL_OBJECTS=kernelcore.o main.o console.o page.o keyboard.o mouse.o event_queue.o clock.o interrupt.o kmalloc.o pic.o ata.o cdromfs.o string.o bitmap.o graphics.o font.o syscall_handler.o process.o mutex.o list.o pagetable.o rtc.o kshell.o fs.o hash_set.o diskfs.o serial.o loader.o device.o kobject.o pipe.o bcache.o printf.o is_valid.o window.o
 
 basekernel.img: bootblock kernel
 	cat bootblock kernel /dev/zero | head -c 1474560 > basekernel.img
@@ -12,10 +12,10 @@ bootblock: bootblock.elf
 	${OBJCOPY} -O binary $< $@
 
 kernel.elf: ${KERNEL_OBJECTS}
-	${LD} ${KERNEL_LDFLAGS} -Ttext 0x10000 ${KERNEL_OBJECTS} -o $@
+	${KERNEL_LD} ${KERNEL_LDFLAGS} -Ttext 0x10000 ${KERNEL_OBJECTS} -o $@
 
 bootblock.elf: bootblock.o
-	${LD} ${KERNEL_LDFLAGS} -Ttext 0 $< -o $@
+	${KERNEL_LD} ${KERNEL_LDFLAGS} -Ttext 0 $< -o $@
 
 %.o: %.c
 	${CC} ${KERNEL_CCFLAGS} -I ../include $< -o $@

kernel/elf.c

@@ -1,5 +1,5 @@
 /*
-Copyright (C) 2015-2019 The University of Notre Dame
+Copyright (C) 2015-2025 The University of Notre Dame
 This software is distributed under the GNU General Public License.
 See the file LICENSE for details.
 */
@@ -8,16 +8,95 @@ See the file LICENSE for details.
 #define ELF_H
 
 #include "kernel/types.h"
-#include "process.h"
-#include "fs.h"
 
-/*
-elf_load opens the given filename, and if it contains a valid
-ELF executable, allocates space in the current process' pagetable,
-loads the text, data, and bss into memory, and updates the 
-entry point value in the current process structure.
-*/
+struct elf_header {
+	char ident[16];
+	uint16_t type;
+	uint16_t machine;
+	uint32_t version;
+	uint32_t entry;
+	uint32_t program_offset;
+	uint32_t section_offset;
+	uint32_t flags;
+	uint16_t header_size;
+	uint16_t phentsize;
+	uint16_t phnum;
+	uint16_t shentsize;
+	uint16_t shnum;
+	uint16_t shstrndx;
+};
+
+#define ELF_HEADER_TYPE_NONE         0
+#define ELF_HEADER_TYPE_OBJECT       1
+#define ELF_HEADER_TYPE_EXECUTABLE   2
+#define ELF_HEADER_TYPE_DYNAMIC      3
+#define ELF_HEADER_TYPE_CORE         4
+
+#define ELF_HEADER_MACHINE_I386   3
+#define ELF_HEADER_MACHINE_ARM    40
+#define ELF_HEADER_MACHINE_X86_64 62
+
+#define ELF_HEADER_VERSION     1
+
+struct elf_program {
+	uint32_t type;
+	uint32_t offset;
+	uint32_t vaddr;
+	uint32_t paddr;
+	uint32_t file_size;
+	uint32_t memory_size;
+	uint32_t flags;
+	uint32_t align;
+};
+
+#define ELF_PROGRAM_TYPE_NULL 0
+#define ELF_PROGRAM_TYPE_LOADABLE 1
+#define ELF_PROGRAM_TYPE_DYNAMIC 2
+#define ELF_PROGRAM_TYPE_INTERPRETER 3
+#define ELF_PROGRAM_TYPE_NOTE 4
+#define ELF_PROGRAM_TYPE_SHARED_LIBRARY 5
+#define ELF_PROGRAM_TYPE_PROGRAM_HEADER 6
+#define ELF_PROGRAM_TYPE_THREAD_LOCAL 7
+#define ELF_PROGRAM_TYPE_GNU_EH_FRAME 0x6474e550
+#define ELF_PROGRAM_TYPE_GNU_STACK 0x6474e551
+#define ELF_PROGRAM_TYPE_GNU_RELRO 0x6474e552
+
+#define ELF_PROGRAM_FLAGS_EXEC  1
+#define ELF_PROGRAM_FLAGS_WRITE 2
+#define ELF_PROGRAM_FLAGS_READ  4
+
+struct elf_section {
+	uint32_t name;
+	uint32_t type;
+	uint32_t flags;
+	uint32_t address;
+	uint32_t offset;
+	uint32_t size;
+	uint32_t link;
+	uint32_t info;
+	uint32_t alignment;
+	uint32_t entry_size;
+};
+
+#define ELF_SECTION_TYPE_NULL         0
+#define ELF_SECTION_TYPE_PROGRAM      1
+#define ELF_SECTION_TYPE_SYMBOL_TABLE 2
+#define ELF_SECTION_TYPE_STRING_TABLE 3
+#define ELF_SECTION_TYPE_RELA         4
+#define ELF_SECTION_TYPE_HASH         5
+#define ELF_SECTION_TYPE_DYNAMIC      6
+#define ELF_SECTION_TYPE_NOTE         7
+#define ELF_SECTION_TYPE_BSS          8
 
-int elf_load(struct process *p, struct fs_dirent *d, addr_t * entry);
+#define ELF_SECTION_FLAGS_WRITE    1
+#define ELF_SECTION_FLAGS_MEMORY   2
+#define ELF_SECTION_FLAGS_EXEC     8
+#define ELF_SECTION_FLAGS_MERGE    16
+#define ELF_SECTION_FLAGS_STRINGS  32
+#define ELF_SECTION_FLAGS_INFO_LINK 64
+#define ELF_SECTION_FLAGS_LINK_ORDER 128
+#define ELF_SECTION_FLAGS_NONSTANDARD 256
+#define ELF_SECTION_FLAGS_GROUP 512
+#define ELF_SECTION_FLAGS_TLS 1024
 
 #endif