user: a super primitive ELF loader

holy shit.
this was simpler than i expected it to be

6 files changed, 293 insertions, 2 deletions

diff --git a/src/user/app/shell.c b/src/user/app/shell.c
index 952b9b7..9085aed 100644
--- a/src/user/app/shell.c
+++ b/src/user/app/shell.c
@@ -1,8 +1,9 @@
+#include <shared/syscalls.h>
+#include <stdbool.h>
 #include <user/app/shell.h>
+#include <user/lib/elfload.h>
 #include <user/lib/stdlib.h>
 #include <user/tests/main.h>
-#include <shared/syscalls.h>
-#include <stdbool.h>
 
 static bool isspace(char c) {
 	return c == ' ' || c == '\t' || c == '\n';
@@ -157,6 +158,15 @@ void shell_loop(void) {
 
 			if (!strcmp(cmd, "echo")) {
 				printf("%s\n", args);
+			} else if (!strcmp(cmd, "exec")) {
+				libc_file *file = file_open(args, 0);
+				if (!file) {
+					printf("couldn't open file\n");
+				} else {
+					elf_execf(file);
+					file_close(file);
+					printf("elf_execf failed\n");
+				}
 			} else if (!strcmp(cmd, "cat")) {
 				cmd_cat_ls(args, false);
 			} else if (!strcmp(cmd, "ls")) {
diff --git a/src/user/lib/elf.h b/src/user/lib/elf.h
new file mode 100644
index 0000000..8dc6242
--- /dev/null
+++ b/src/user/lib/elf.h
@@ -0,0 +1,188 @@
+/* shamelessly stolen from https://github.com/adachristine/sophia/tree/main/api/elf */
+#pragma once
+#include <stdint.h>
+
+typedef uint8_t Elf_Byte;
+
+#define EI_MAG0 0
+#define EI_MAG1 1
+#define EI_MAG2 2
+#define EI_MAG3 3
+#define EI_CLASS 4
+#define EI_DATA 5
+#define EI_VERSION 6
+#define EI_OSABI 7
+#define EI_ABIVERSION 8
+#define EI_PAD 9
+#define EI_NIDENT 16
+
+#define ELFMAG0 0x7f
+#define ELFMAG1 'E'
+#define ELFMAG2 'L'
+#define ELFMAG3 'F'
+
+#define ELFCLASSNONE 0
+#define ELFCLASS32 1
+#define ELFCLASS64 2
+
+#define ELFDATANONE 0
+#define ELFDATA2LSB 1
+#define ELFDATA2MSB 2
+
+#define EI_VERSION 6
+#define EV_NONE 0
+#define EV_CURRENT 1
+
+#define ELFOSABI_NONE 0
+#define ELFOSABI_SYSV EI_OSABI_NONE
+
+#define ET_NONE 0
+#define ET_REL 1
+#define ET_EXEC 2
+#define ET_DYN 3
+#define ET_CORE 4
+#define ET_LOOS 0xfe00
+#define ET_HIOS 0xfeff
+
+#define EM_NONE 0
+
+#define PT_NULL 0
+#define PT_LOAD 1
+#define PT_DYNAMIC 2
+
+#define PF_X 0x01
+#define PF_W 0x02
+#define PF_R 0x04
+
+#define SHT_NULL 0
+#define SHT_PROGITS 1
+#define SHT_SYMTAB 2
+#define SHT_STRTAB 3
+#define SHT_RELA 4
+#define SHT_HASH 5
+#define SHT_DYNAMIC 6
+
+#define SHF_WRITE 0x1
+#define SHF_ALLOC 0x2
+#define SHF_EXEC  0x4
+#define SHF_MERGE 0x10
+#define SHF_STRINGS 0x20
+
+#define DT_NULL 0
+#define DT_NEEDED 1
+#define DT_PLTRELSZ 2
+#define DT_PLTGOT 3
+#define DT_HASH 4
+#define DT_STRTAB 5
+#define DT_SYMTAB 6
+#define DT_RELA 7
+#define DT_RELASZ 8
+#define DT_RELAENT 9
+#define DT_STRSZ 10
+#define DT_SYMENT 11
+#define DT_JMPREL 0x17
+
+typedef uint64_t Elf64_Addr;
+typedef uint64_t Elf64_Off;
+typedef uint16_t Elf64_Section;
+typedef uint16_t Elf64_Versym;
+typedef uint16_t Elf64_Half;
+typedef int32_t Elf64_Sword;
+typedef uint32_t Elf64_Word;
+typedef int64_t Elf64_Sxword;
+typedef uint64_t Elf64_Xword;
+
+typedef struct Elf64_Ehdr Elf64_Ehdr;
+typedef struct Elf64_Phdr Elf64_Phdr;
+typedef struct Elf64_Shdr Elf64_Shdr;
+typedef struct Elf64_Sym Elf64_Sym;
+typedef struct Elf64_Dyn Elf64_Dyn;
+
+typedef struct Elf64_Rel Elf64_Rel;
+typedef struct Elf64_Rela Elf64_Rela;
+
+#define EM_X86_64 62
+
+struct Elf64_Ehdr
+{
+    Elf_Byte e_ident[EI_NIDENT];
+    Elf64_Half e_type;
+    Elf64_Half e_machine;
+    Elf64_Word e_version;
+    Elf64_Addr e_entry;
+    Elf64_Off e_phoff;
+    Elf64_Off e_shoff;
+    Elf64_Word e_flags;
+    Elf64_Half e_ehsize;
+    Elf64_Half e_phentsize;
+    Elf64_Half e_phnum;
+    Elf64_Half e_shentsize;
+    Elf64_Half e_shnum;
+    Elf64_Half e_shstrndx;
+};
+
+struct Elf64_Phdr
+{
+    Elf64_Word p_type;
+    Elf64_Word p_flags;
+    Elf64_Off p_offset;
+    Elf64_Addr p_vaddr;
+    Elf64_Addr p_paddr;
+    Elf64_Xword p_filesz;
+    Elf64_Xword p_memsz;
+    Elf64_Xword p_align;
+};
+
+struct Elf64_Shdr
+{
+    Elf64_Word sh_name;
+    Elf64_Word sh_type;
+    Elf64_Xword sh_flags;
+    Elf64_Addr sh_addr;
+    Elf64_Off sh_offset;
+    Elf64_Xword sh_size;
+    Elf64_Word sh_link;
+    Elf64_Word sh_info;
+    Elf64_Off sh_addralign;
+    Elf64_Xword sh_entsize;
+};
+
+struct Elf64_Sym
+{
+    Elf64_Word st_name;
+    Elf_Byte st_info;
+    Elf_Byte st_other;
+    Elf64_Half st_shndx;
+    Elf64_Addr st_value;
+    Elf64_Xword st_size;
+};
+
+struct Elf64_Dyn
+{
+    Elf64_Xword d_tag;
+    union
+    {
+        Elf64_Xword d_val;
+        Elf64_Addr d_ptr;
+    };
+};
+
+#define ELF64_R_SYM(info) ((info)>>32)
+#define ELF64_R_TYPE(info) ((Elf64_Word)(info))
+#define ELF64_R_INFO(sym, type) (((Elf64_Xword)(sym)<<32)+(Elf64_Xword)(type))
+
+#define R_X86_64_JUMP_SLOT 7
+#define R_X86_64_RELATIVE 8
+
+struct Elf64_Rel
+{
+    Elf64_Addr r_offset;
+    Elf64_Xword r_info;
+};
+
+struct Elf64_Rela
+{
+    Elf64_Addr r_offset;
+    Elf64_Xword r_info;
+    Elf64_Sxword r_addend;
+};
diff --git a/src/user/lib/elfload.c b/src/user/lib/elfload.c
new file mode 100644
index 0000000..67cef68
--- /dev/null
+++ b/src/user/lib/elfload.c
@@ -0,0 +1,59 @@
+#include <shared/flags.h>
+#include <shared/syscalls.h>
+#include <user/lib/elf.h>
+#include <user/lib/elfload.h>
+
+void elf_execf(libc_file *f) {
+	const size_t cap = 0x60000;
+	size_t pos = 0;
+	void *buf = malloc(cap); // TODO a way to get file size
+	if (!buf) goto fail;
+
+	while (!f->eof) {
+		long ret = file_read(f, buf, cap - pos);
+		if (ret < 0) goto fail;
+		pos += ret;
+		if (pos >= cap) goto fail;
+	}
+	elf_exec(buf);
+
+fail:
+	free(buf);
+}
+
+static bool valid_ehdr(const struct Elf64_Ehdr *h) {
+	return h->e_ident[0] == 0x7f
+		&& h->e_ident[1] == 'E'
+		&& h->e_ident[2] == 'L'
+		&& h->e_ident[3] == 'F'
+		&& h->e_type == ET_EXEC
+		&& h->e_machine == EM_X86_64
+		&& h->e_version == 1;
+}
+
+static bool load_phdr(const void *base, size_t idx) {
+	const struct Elf64_Ehdr *ehdr = base;
+	const struct Elf64_Phdr *phdr = base + ehdr->e_phoff + idx * ehdr->e_phentsize;
+
+	if (phdr->p_type != PT_LOAD) {
+		printf("unknown type %x\n", phdr->p_type);
+		return false;
+	}
+	// TODO overlap check
+	// TODO don't ignore flags
+	_syscall_memflag((void*)phdr->p_vaddr, phdr->p_memsz, MEMFLAG_PRESENT);
+	// TODO check that filesz <= memsz
+	memcpy((void*)phdr->p_vaddr, base + phdr->p_offset, phdr->p_filesz);
+	return true;
+}
+
+void elf_exec(void *base) {
+	struct Elf64_Ehdr *ehdr = base;
+	if (!valid_ehdr(ehdr)) return;
+	for (size_t phi = 0; phi < ehdr->e_phnum; phi++) {
+		if (!load_phdr(base, phi))
+			return;
+	}
+	((void(*)())ehdr->e_entry)();
+	_syscall_exit(1);
+}
diff --git a/src/user/lib/elfload.h b/src/user/lib/elfload.h
new file mode 100644
index 0000000..c6afbdd
--- /dev/null
+++ b/src/user/lib/elfload.h
@@ -0,0 +1,5 @@
+#pragma once
+#include <user/lib/stdlib.h>
+
+void elf_execf(libc_file *f);
+void elf_exec(void *elf);
diff --git a/src/usertestelf.c b/src/usertestelf.c
new file mode 100644
index 0000000..19f0096
--- /dev/null
+++ b/src/usertestelf.c
@@ -0,0 +1,6 @@
+#include <user/lib/syscall.c>
+
+int main(void) {
+	_syscall_write(1, "Hello!", 6, 0);
+	_syscall_exit(0);
+}
diff --git a/src/usertestelf.ld b/src/usertestelf.ld
new file mode 100644
index 0000000..2567f77
--- /dev/null
+++ b/src/usertestelf.ld
@@ -0,0 +1,23 @@
+ENTRY(main)
+
+SECTIONS
+{
+	. = 8M;
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		*(.text)
+	}
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+	}
+}