04: able to map dependency & resolve reolcs and execture main program (initial commit)

5 files changed, 578 insertions, 0 deletions

diff --git a/04_dynld_nostd/Makefile b/04_dynld_nostd/Makefile
new file mode 100644
index 0000000..8cd9d35
--- /dev/null
+++ b/04_dynld_nostd/Makefile
@@ -0,0 +1,60 @@
+# Copyright (c) 2020 Johannes Stoelp
+
+COMMON_CFLAGS := -g -O0 -Wall -Wextra \
+                 -I../lib/include     \
+                 -nostartfiles -nodefaultlibs
+
+run: main
+	./$<
+
+main: dynld.so main.c ../lib/libcommon.a
+	@# For now ew only add support for ELF hash tables (DT_HASH).
+	@# Therefore we specify the `hash-style` below.
+	gcc -o libgreet.so        \
+	    $(COMMON_CFLAGS)      \
+	    -fPIC -shared         \
+	    -Wl,--hash-style=sysv \
+	    libgreet.c
+
+	@if readelf -W -S libgreet.so | grep plt >& /dev/null; then \
+		echo "ERROR: libgreet.so contains PLT while we don't support relocation calls in libgreet.so!"; \
+		echo "       All function calls in libgreet.so must be statically resolved!"; \
+		exit 1; \
+	fi
+
+	@# For now ew only add support for ELF hash tables (DT_HASH).
+	@# Therefore we specify the `hash-style` below.
+	gcc -o $@                                   \
+	    $(COMMON_CFLAGS)                        \
+	    -L$(CURDIR) -lgreet                     \
+	    -Wl,--dynamic-linker=$(CURDIR)/dynld.so \
+	    -Wl,--hash-style=sysv                   \
+	    -no-pie \
+	    $(filter %.c %.a, $^)
+
+	readelf -W --dynamic $@
+	readelf -W --program-headers $@
+	objdump --disassemble -j .plt -M intel $@
+	objdump --disassemble=_start -M intel $@
+
+dynld.so: dynld.S dynld.c ../lib/libcommon.a
+	gcc -o $@                          \
+	    $(COMMON_CFLAGS)               \
+	    -fPIC -static-pie              \
+	    -fvisibility=hidden            \
+	    -Wl,--entry=dl_start           \
+	    -Wl,--no-allow-shlib-undefined \
+	    $^
+
+	@if readelf -W -S $@ | grep plt >& /dev/null; then \
+		echo "ERROR: $@ contains PLT while we don't support relocation calls in $@!"; \
+		echo "       All function calls in $@ must be statically resolved!"; \
+		exit 1; \
+	fi
+
+../lib/libcommon.a:
+	make -C ../lib
+
+clean:
+	rm -f main libgreet.so
+	rm -f dynld.so
diff --git a/04_dynld_nostd/dynld.S b/04_dynld_nostd/dynld.S
new file mode 100644
index 0000000..8a634f5
--- /dev/null
+++ b/04_dynld_nostd/dynld.S
@@ -0,0 +1,27 @@
+// Copyright (c) 2020 Johannes Stoelp
+
+#include <asm/unistd.h>
+
+.intel_syntax noprefix
+
+.section .text, "ax", @progbits
+.global dl_start
+dl_start:
+    // $rsp is guaranteed to be 16-byte aligned.
+
+    // Clear $rbp as specified by the SysV AMD64 ABI.
+    xor rbp, rbp
+
+    // Load pointer to process context prepared by execve(2) syscall as
+    // specified in the SysV AMD64 ABI.
+    // Save pointer in $rdi which is the arg0 (int/ptr) register.
+    lea rdi, [rsp]
+
+    // Stack frames must be 16-byte aligned before control is transfered to the
+    // callees entry point.
+    call dl_entry
+
+    // Call exit(1) syscall to indicate error, dl_entry should not return.
+    mov rdi, 1
+    mov rax, __NR_exit
+    syscall
diff --git a/04_dynld_nostd/dynld.c b/04_dynld_nostd/dynld.c
new file mode 100644
index 0000000..ad600f5
--- /dev/null
+++ b/04_dynld_nostd/dynld.c
@@ -0,0 +1,464 @@
+// Copyright (c) 2021 Johannes Stoelp
+
+#include <auxv.h>
+#include <common.h>
+#include <elf.h>
+#include <io.h>
+#include <syscalls.h>
+
+#include <stdbool.h>
+#include <stdint.h>
+
+
+/// ----------------
+/// Global Constants
+/// ----------------
+
+enum {
+    // Hard-coded page size.
+    // We assert against the `AT_PAGESZ` auxiliary vector entry.
+    PAGE_SIZE = 4096,
+    // Hard-coded upper limit of `DT_NEEDED` entries per dso
+    // (for simplicity to not require allocations).
+    MAX_NEEDED = 1,
+};
+
+
+/// --------
+/// Execinfo
+/// --------
+
+typedef struct {
+    uint64_t argc;              // Number of commandline arguments.
+    const char** argv;          // List of pointer to command line arguments.
+    uint64_t envc;              // Number of environment variables.
+    const char** envv;          // List of pointers to environment variables.
+    uint64_t auxv[AT_MAX_CNT];  // Auxiliary vector entries.
+} ExecInfo;
+
+// Interpret and extract data passed on the stack by the Linux Kernel
+// when loading the initial process image.
+// The data is organized according to the SystemV x86_64 ABI.
+ExecInfo get_exec_info(const uint64_t* prctx) {
+    ExecInfo info = {0};
+
+    info.argc = *prctx;
+    info.argv = (const char**)(prctx + 1);
+    info.envv = (const char**)(info.argv + info.argc + 1);
+
+    // Count the number of environment variables in the `ENVP` segment.
+    for (const char** env = info.envv; *env; ++env) {
+        info.envc += 1;
+    }
+
+    // Decode auxiliary vector `AUXV`.
+    for (const Auxv64Entry* auxvp = (const Auxv64Entry*)(info.envv + info.envc + 1); auxvp->tag != AT_NULL; ++auxvp) {
+        if (auxvp->tag < AT_MAX_CNT) {
+            info.auxv[auxvp->tag] = auxvp->val;
+        }
+    }
+
+    return info;
+}
+
+
+/// ---
+/// Dso
+/// ---
+
+typedef struct {
+    uint8_t* base;                 // Base address.
+    void (*entry)();               // Entry function.
+    uint64_t dynamic[DT_MAX_CNT];  // `.dynamic` section entries.
+    uint64_t needed[MAX_NEEDED];   // Shared object dependencies (`DT_NEEDED` entries).
+    uint8_t needed_len;            // Number of `DT_NEEDED` entries (SO dependencies).
+} Dso;
+
+void decode_dynamic(Dso* dso, uint64_t dynoff) {
+    // Decode `.dynamic` section of the `dso`.
+    for (const Elf64Dyn* dyn = (const Elf64Dyn*)(dso->base + dynoff); dyn->tag != DT_NULL; ++dyn) {
+        if (dyn->tag == DT_NEEDED) {
+            ERROR_ON(dso->needed_len == MAX_NEEDED, "Too many dso dependencies!");
+            dso->needed[dso->needed_len++] = dyn->val;
+        } else if (dyn->tag < DT_MAX_CNT) {
+            dso->dynamic[dyn->tag] = dyn->val;
+        }
+    }
+
+    // Check for string table entries.
+    ERROR_ON(dso->dynamic[DT_STRTAB] == 0, "DT_STRTAB missing in dynamic section!");
+    ERROR_ON(dso->dynamic[DT_STRSZ] == 0, "DT_STRSZ missing in dynamic section!");
+
+    // Check for symbol table entries.
+    ERROR_ON(dso->dynamic[DT_SYMTAB] == 0, "DT_SYMTAB missing in dynamic section!");
+    ERROR_ON(dso->dynamic[DT_SYMENT] == 0, "DT_SYMENT missing in dynamic section!");
+    ERROR_ON(dso->dynamic[DT_SYMENT] != sizeof(Elf64Sym), "ELf64Sym size miss-match!");
+
+    // Check for relocation entries related to PLT.
+    // ERROR_ON(dso->dynamic[DT_JMPREL] == 0, "DT_JMPREL missing in dynamic section!");
+    // ERROR_ON(dso->dynamic[DT_PLTRELSZ] == 0, "DT_PLTRELSZ missing in dynamic section!");
+    // ERROR_ON(dso->dynamic[DT_PLTREL] == 0, "DT_PLTREL missing in dynamic section!");
+    // ERROR_ON(dso->dynamic[DT_PLTREL] != DT_RELA, "x86_64 only uses RELA entries!");
+
+    // Check for SystemV hash table. We only support SystemV hash tables
+    // `DT_HASH`, not gnu hash tables `DT_GNU_HASH`.
+    ERROR_ON(dso->dynamic[DT_HASH] == 0, "DT_HASH missing in dynamic section!");
+}
+
+Dso get_prog_info(const ExecInfo* info) {
+    Dso prog = {0};
+
+    // Determine the base address of the user program.
+    //   We only support the case where the Kernel already mapped the
+    // user program into the virtual address space and therefore the
+    // auxiliary vector contains an `AT_PHDR` entry pointing to the
+    // Program Headers of the user program.
+    // In that case, the base address of the user program can be
+    // computed by taking the absolute address of the `AT_PHDR` entry
+    // and subtracting the relative address `p_vaddr` of the `PT_PHDR`
+    // entry from the user programs Program Header iself.
+    //
+    //              VMA
+    //              |         |
+    // PROG BASE -> |         |  ^
+    //              |         |  |
+    //              |         |  | <---------------------+
+    //              |         |  |                       |
+    //   AT_PHDR -> +---------+  v                       |
+    //              |         |                          |
+    //              |         |                          |
+    //              | PT_PHDR | -----> Elf64Phdr { .., vaddr, .. }
+    //              |         |
+    //              |         |
+    //              +---------+
+    //              |         |
+    //
+    // PROG BASE = AT_PHDR - PT_PHDR.vaddr
+    ERROR_ON(info->auxv[AT_PHDR] == 0 || info->auxv[AT_EXECFD] != 0, "AT_PHDR entry missing in the AUXV!");
+
+    // Offset to the `.dynamic` section from the user programs `base addr`.
+    uint64_t dynoff = 0;
+
+    // Program header of the user program.
+    const Elf64Phdr* phdr = (const Elf64Phdr*)info->auxv[AT_PHDR];
+
+    ERROR_ON(info->auxv[AT_PHENT] != sizeof(Elf64Phdr), "Elf64Phdr size miss-match!");
+
+    // Decode PHDRs of the user program.
+    for (unsigned phdrnum = info->auxv[AT_PHNUM]; --phdrnum; ++phdr) {
+        if (phdr->type == PT_PHDR) {
+            ERROR_ON(info->auxv[AT_PHDR] < phdr->vaddr, "Expectation auxv[AT_PHDR] >= phdr->vaddr failed!");
+            prog.base = (uint8_t*)(info->auxv[AT_PHDR] - phdr->vaddr);
+        } else if (phdr->type == PT_DYNAMIC) {
+            dynoff = phdr->vaddr;
+        }
+    }
+    ERROR_ON(dynoff == 0, "PT_DYNAMIC entry missing in the user programs PHDR!");
+
+    // Decode `.dynamic` section.
+    decode_dynamic(&prog, dynoff);
+
+    // Get the entrypoint of the user program form the auxiliary vector.
+    ERROR_ON(info->auxv[AT_ENTRY] == 0, "AT_ENTRY entry missing in the AUXV!");
+    prog.entry = (void (*)())info->auxv[AT_ENTRY];
+
+    return prog;
+}
+
+uint64_t get_num_dynsyms(const Dso* dso) {
+    ERROR_ON(dso->dynamic[DT_HASH] == 0, "DT_HASH missing in dynamic section!");
+
+    // Get SystemV hash table.
+    const uint32_t* hashtab = (const uint32_t*)(dso->base + dso->dynamic[DT_HASH]);
+
+    // SystemV hash table layout:
+    //   nbucket
+    //   nchain
+    //   bucket[nbuckets]
+    //   chain[nchains]
+    //
+    // From the SystemV ABI - Dynamic Linking - Hash Table:
+    //   Both `bucket` and `chain` hold symbol table indexes. Chain
+    //   table entries parallel the symbol table. The number of symbol
+    //   table entries should equal `nchain`.
+    return hashtab[1];
+}
+
+const char* get_str(const Dso* dso, const uint64_t idx) {
+    ERROR_ON(dso->dynamic[DT_STRSZ] < idx, "String table indexed out-of-bounds!");
+    return (const char*)(dso->base + dso->dynamic[DT_STRTAB] + idx);
+}
+
+const Elf64Sym* get_sym(const Dso* dso, const uint64_t idx) {
+    ERROR_ON(get_num_dynsyms(dso) < idx, "Symbol table index out-of-bounds!");
+    return (const Elf64Sym*)(dso->base + dso->dynamic[DT_SYMTAB]) + idx;
+}
+
+const Elf64Rela* get_pltreloc(const Dso* dso, const uint64_t idx) {
+    ERROR_ON(dso->dynamic[DT_PLTRELSZ] < sizeof(Elf64Rela) * idx, "PLT relocation table indexed out-of-bounds!");
+    return (const Elf64Rela*)(dso->base + dso->dynamic[DT_JMPREL]) + idx;
+}
+
+
+/// -------------
+/// Symbol lookup
+/// -------------
+
+int strcmp(const char* s1, const char* s2) {
+    while (*s1 == *s2 && *s1) {
+        ++s1;
+        ++s2;
+    }
+    return *(unsigned char*)s1 - *(unsigned char*)s2;
+}
+
+void* lookup_sym(const Dso* dso, const char* sym_name) {
+    for (unsigned i = 0; i < get_num_dynsyms(dso); ++i) {
+        const Elf64Sym* sym = get_sym(dso, i);
+
+        if (ELF64_ST_TYPE(sym->info) == STT_FUNC && ELF64_ST_BIND(sym->info) == STB_GLOBAL && sym->shndx != SHN_UNDEF) {
+            if (strcmp(sym_name, get_str(dso, sym->name)) == 0) {
+                return dso->base + sym->value;
+            }
+        }
+    }
+
+    return 0;
+}
+
+
+/// -----------------------------
+/// Map Shared Library Dependency
+/// -----------------------------
+
+Dso map_dependency(const char* dependency) {
+    // For simplicity we only search for SO dependencies in the current working dir.
+    ERROR_ON(access(dependency, R_OK) != 0, "Dependency '%s' does not exist!\n", dependency);
+
+    const int fd = open(dependency, O_RDONLY);
+    ERROR_ON(fd < 0, "Failed to open '%s'", dependency);
+
+    Elf64Ehdr ehdr;
+    // Read ELF header.
+    ERROR_ON(read(fd, &ehdr, sizeof(ehdr)) != (ssize_t)sizeof(ehdr), "Failed to read Elf64Ehdr!");
+
+    // Check ELF magic.
+    ERROR_ON(ehdr.ident[EI_MAG0] != '\x7f' || ehdr.ident[EI_MAG1] != 'E' || ehdr.ident[EI_MAG2] != 'L' || ehdr.ident[EI_MAG3] != 'F',
+             "Dependency '%s' wrong ELF magic value!\n", dependency);
+    // Check ELF header size.
+    ERROR_ON(ehdr.ehsize != sizeof(ehdr), "Elf64Ehdr size miss-match!");
+    // Check for 64bit ELF.
+    ERROR_ON(ehdr.ident[EI_CLASS] != ELFCLASS64, "Dependency '%s' is not 64bit ELF!\n", dependency);
+    // Check for OS ABI.
+    ERROR_ON(ehdr.ident[EI_OSABI] != ELFOSABI_SYSV, "Dependency '%s' is not built for SysV OS ABI!\n", dependency);
+    // Check ELF type.
+    ERROR_ON(ehdr.type != ET_DYN, "Dependency '%s' is not a dynamic library!");
+    // Check for Phdr.
+    ERROR_ON(ehdr.phnum == 0, "Dependency '%s' has no Phdr!\n", dependency);
+
+
+    Elf64Phdr phdr[ehdr.phnum];
+    // Check PHDR header size.
+    ERROR_ON(ehdr.phentsize != sizeof(phdr[0]), "Elf64Phdr size miss-match!");
+
+    // Read Program headers at offset `phoff`.
+    ERROR_ON(pread(fd, &phdr, sizeof(phdr), ehdr.phoff) != (ssize_t)sizeof(phdr), "Failed to read Elf64Phdr[%d]!\n", ehdr.phnum);
+
+    // Compute start and end address used by the library based on the all the `PT_LOAD` program headers.
+    uint64_t dynoff = 0;
+    uint64_t addr_start = (uint64_t)-1;
+    uint64_t addr_end = 0;
+    for (unsigned i = 0; i < ehdr.phnum; ++i) {
+        const Elf64Phdr* p = &phdr[i];
+        if (p->type == PT_DYNAMIC) {
+            // Offset to `.dynamic` section.
+            dynoff = p->vaddr;
+        } else if (p->type == PT_LOAD) {
+            // Find start & end address.
+            if (p->vaddr < addr_start) {
+                addr_start = p->vaddr;
+            } else if (p->vaddr + p->memsz > addr_end) {
+                addr_end = p->vaddr + p->memsz;
+            }
+        }
+    }
+
+    // Align start address to the next lower page boundary.
+    addr_start = addr_start & ~(PAGE_SIZE - 1);
+    // Align end address to the next higher page boundary.
+    addr_end = (addr_end + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1);
+
+    // Reserve region big enough to map all `PT_LOAD` sections of `dependency`.
+    uint8_t* map = mmap(0 /* addr */, addr_end - addr_start /* len */, PROT_EXEC | PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
+                        -1 /* fd */, 0 /* file offset */);
+    ERROR_ON(map == MAP_FAILED, "Failed to mmap address space for dependency '%s'\n", dependency);
+
+    // Compute base address for library.
+    uint8_t* base = map - addr_start;
+
+    // Map in all `PT_LOAD` segments from the `dependency`.
+    for (unsigned i = 0; i < ehdr.phnum; ++i) {
+        const Elf64Phdr* p = &phdr[i];
+        ;
+        if (p->type != PT_LOAD) {
+            continue;
+        }
+
+        // Page align start & end address.
+        uint64_t addr_start = p->vaddr & ~(PAGE_SIZE - 1);
+        uint64_t addr_end = (p->vaddr + p->memsz + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1);
+
+        // Page align file offset.
+        uint64_t off = p->offset & ~(PAGE_SIZE - 1);
+
+        // Compute segment permissions.
+        uint32_t prot = (p->flags & PF_X ? PROT_EXEC : 0) | (p->flags & PF_R ? PROT_READ : 0) | (p->flags & PF_W ? PROT_WRITE : 0);
+
+        // Mmap segment.
+        ERROR_ON(mmap(base + addr_start, addr_end - addr_start, prot, MAP_PRIVATE | MAP_FIXED, fd, off) != base + addr_start,
+                 "Failed to map `PT_LOAD` section %d for dependency '%s'.", i, dependency);
+
+        // From the SystemV ABI - Program Headers:
+        //   If the segment’s memorysize (memsz) is larger than the file size (filesz), the "extra" bytes are defined to hold the value
+        //   `0` and to follow the segment’s initialized are
+        //
+        // This is typically used by the `.bss` section.
+        if (p->memsz > p->filesz) {
+            memset(base + addr_start + p->filesz, 0 /* byte */, p->memsz - p->filesz /*len*/);
+        }
+    }
+
+    // Close file descriptor.
+    close(fd);
+
+    Dso dso = {0};
+    dso.base = base;
+    decode_dynamic(&dso, dynoff);
+    return dso;
+}
+
+
+/// ------------------------------
+/// Dynamic Linking (lazy resolve)
+/// ------------------------------
+
+struct LinkMap {
+    const Dso* dso;              // Pointer to Dso list object.
+    const struct LinkMap* next;  // Pointer to next LinkMap entry.
+};
+typedef struct LinkMap LinkMap;
+
+void resolve_relocs(const Dso* dso, const LinkMap* map) {
+    for (unsigned long relocidx = 0; relocidx < (dso->dynamic[DT_PLTRELSZ] / sizeof(Elf64Rela)); ++relocidx) {
+        const Elf64Rela* reloc = get_pltreloc(dso, relocidx);
+        ERROR_ON(ELF64_R_TYPE(reloc->info) != R_X86_64_JUMP_SLOT, "Expected relocation entry of type X86_64_JUMP_SLOT!");
+
+        const int symidx = ELF64_R_SYM(reloc->info);
+        const char* symname = get_str(dso, get_sym(dso, symidx)->name);
+
+        void* symaddr = 0;
+        for (const LinkMap* lmap = map; lmap && symaddr == 0; lmap = lmap->next) {
+            symaddr = lookup_sym(lmap->dso, symname);
+        }
+        ERROR_ON(symaddr == 0, "Failed lookup symbol %s while resolving relocations!", symname);
+
+        pfmt("Resolved reloc %s to %p\n", symname, symaddr);
+
+        // Patch storage unit of relocation with absolute address of the symbol.
+        *(uint64_t*)(dso->base + reloc->offset) = (uint64_t)symaddr;
+    }
+}
+
+
+/// ------------------------------
+/// Dynamic Linking (lazy resolve)
+/// ------------------------------
+
+// `noreturn`  Function never returns.
+// `naked`     Don't generate prologue/epilogue sequences.
+__attribute__((noreturn)) __attribute__((naked)) static void dynresolve_entry() {
+    asm("dynresolve_entry:\n\t"
+        // Pop arguments on stack from PLT0 into rdi/rsi argument registers.
+        "pop %rdi\n\t"  // GOT[1] entry (pushed by PLT0 pad).
+        "pop %rsi\n\t"  // Relocation index (pushed by PLT0 pad).
+        "jmp dynresolve");
+}
+
+// `used`    Foce to emit code for function.
+// `unused`  Don't warn about unused function.
+__attribute__((used)) __attribute__((unused)) static void dynresolve(uint64_t got1, uint64_t reloc_idx) {
+    ERROR_ON(true,
+             "ERROR: dynresolve request not supported!"
+             "\n\tGOT[1]    = 0x%x"
+             "\n\treloc_idx = %d\n",
+             got1, reloc_idx);
+}
+
+
+/// -------------------------
+/// Dynamic Linker Entrypoint
+/// -------------------------
+
+void dl_entry(const uint64_t* prctx) {
+    // Parse SystemV ABI block.
+    const ExecInfo exec_info = get_exec_info(prctx);
+
+    // Ensure hard-coded page size value is correct.
+    ERROR_ON(exec_info.auxv[AT_PAGESZ] != PAGE_SIZE, "Hard-coded PAGE_SIZE miss-match!");
+
+    // Initialize dso handle for user program but extracting necesarry
+    // information from `AUXV` and the `PHDR`.
+    const Dso dso_prog = get_prog_info(&exec_info);
+
+    // Map dependency.
+    //
+    // In this chapter the user program should have a single shared
+    // object dependency, which is our `libgreet.so` no-std shared
+    // library.
+    // The `libgreet.so` library itself should not have any dynamic
+    // dependencies.
+    ERROR_ON(dso_prog.needed_len != 1, "User program should have exactly one dependency!");
+
+    const Dso dso_lib = map_dependency(get_str(&dso_prog, dso_prog.needed[0]));
+    ERROR_ON(dso_lib.needed_len != 0, "The programs dependency should be stand-alone!");
+
+    // Setup LinkMap.
+    //
+    // Create a list of DSOs as link map with the following order:
+    //   main -> libgreet.so
+    // The link map determines the symbol lookup order.
+    const LinkMap map_lib = {.dso = &dso_lib, .next = 0};
+    const LinkMap map_prog = {.dso = &dso_prog, .next = &map_lib};
+
+    // Resolve relocations for the main program.
+    resolve_relocs(&dso_prog, &map_prog);
+
+
+    // Install dynamic resolve handler.
+    //
+    // The dynamic resolve handler is used when binding symbols lazily. Hence
+    // it should not be called in this example as we resolve all relocations
+    // before transfering controll to the user program.
+    // For safety we still install a handler which will terminate the program
+    // once it is called, if we wouldn't install this handler the program would
+    // most probably SEGFAULT.
+    //
+    // The handler is installed in the `GOT[2]` entry for each DSO object that
+    // has an GOT. It is jumped to from the `PLT0` pad with the following two
+    // arguments passed via the stack:
+    //    pop %rdi  // GOT[1] entry.
+    //    pop %rsi  // Relocation index.
+    {
+        uint64_t* got = (uint64_t*)(dso_prog.base + dso_prog.dynamic[DT_PLTGOT]);
+        // Jump target for PLT0 pad.
+        got[2] = (uint64_t)&dynresolve_entry;
+    }
+
+    // GOT[0];  // Hold address of dynamic structure referenced by `_DYNAMIC`.
+    // GOT[1];  // Pushed by PLT0 pad on stack before jumping to got[2] -> Word the dynamic linker can use to identify the caller.
+    // GOT[2];  // Jump target for PLT0 pad (when doing lazy resolving).
+
+    dso_prog.entry();
+
+    _exit(0);
+}
diff --git a/04_dynld_nostd/libgreet.c b/04_dynld_nostd/libgreet.c
new file mode 100644
index 0000000..f2a96a3
--- /dev/null
+++ b/04_dynld_nostd/libgreet.c
@@ -0,0 +1,9 @@
+// Copyright (c) 2020 Johannes Stoelp
+
+const char* get_greet() {
+    return "Hello from libgreet.so!";
+}
+
+const char* get_greet2() {
+    return "Hello 2 from libgreet.so!";
+}
diff --git a/04_dynld_nostd/main.c b/04_dynld_nostd/main.c
new file mode 100644
index 0000000..ca250ef
--- /dev/null
+++ b/04_dynld_nostd/main.c
@@ -0,0 +1,18 @@
+// Copyright (c) 2020 Johannes Stoelp
+
+#include <io.h>
+#include <syscalls.h>
+
+// API of `libgreet.so`.
+extern const char* get_greet();
+extern const char* get_greet2();
+
+void _start() {
+    pfmt("Running _start() @ %s\n", __FILE__);
+
+    // Call function from libgreet.so -> generates PLT entry.
+    pfmt("get_greet()  -> %s\n", get_greet());
+    pfmt("get_greet2() -> %s\n", get_greet2());
+
+    _exit(0);
+}