chapters/hardware-software-interface: Add content for linking

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/README.md

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+# Fixing the Entry Point 1
+
+Access the directory `drills/tasks/entry-fix-1/support/`.
+We want to track issues with defining the `main()` function.
+
+Go to the subdirectory `a-c/`.
+Run the `make` command, interpret the encountered error, and resolve it by editing the `hello.c` file.
+
+Go to the subdirectory `b-asm/`.
+Run the `make` command, interpret the encountered error, and resolve it by editing the `hello.asm` file.
+
+**Bonus**: In the subdirectories `c-extra-nolibc/` and `d-extra-libc/`, find solutions that do not modify the source code of `hello.c`.
+These solutions instead modify the build system to use a different function, other than `main()`, as the program's entry point.
+
+If you're having difficulties solving this exercise, go through [this](../../../reading/README.md) reading material.

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/a-c/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/a-c/Makefile

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+../../support/a-c/Makefile

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/a-c/hello.c

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+// SPDX-License-Identifier: BSD-3-Clause
+
+#include <stdio.h>
+
+static void hi(void)
+{
+	puts("Hi!");
+}
+
+static void bye(void)
+{
+	puts("Bye!");
+}
+
+int main(void)
+{
+	hi();
+	bye();
+}

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/b-asm/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/b-asm/Makefile

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+../../support/b-asm/Makefile

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/b-asm/hello.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+extern puts
+extern main
+
+section .rodata
+
+    hi_str db "Hi!", 0
+    bye_str db "Bye!", 0
+
+section .text
+
+hi:
+    push ebp
+    mov ebp, esp
+
+    push hi_str
+    call puts
+
+    leave
+    ret
+
+bye:
+    push ebp
+    mov ebp, esp
+
+    push bye_str
+    call puts
+
+    leave
+    ret
+
+main:
+    push ebp
+    mov ebp, esp
+
+    call hi
+    call bye
+
+    leave
+    ret

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/c-extra-nolibc/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/c-extra-nolibc/Makefile

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+CFLAGS = -fno-PIC -m32 -g
+LDFLAGS = -nostdlib -nostdinc -no-pie -m32
+AS = nasm
+ASFLAGS = -f elf32
+
+.DEFAULT_GOAL: all
+
+.PHONY: all clean
+
+all: hello
+
+hello: hello.o start.o puts.o
+	$(CC) $(LDFLAGS) -o $@ $^
+
+hello.o: hello.c
+
+start.o: start.asm
+	$(AS) $(ASFLAGS) -o $@ $<
+
+puts.o: puts.asm
+	$(AS) $(ASFLAGS) -o $@ $<
+
+clean:
+	-rm -f hello hello.o start.o puts.o
+	-rm -f *~

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/c-extra-nolibc/hello.c

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+// SPDX-License-Identifier: BSD-3-Clause
+
+#include <stdio.h>
+
+static void hi(void)
+{
+	puts("Hi!\n");
+}
+
+static void bye(void)
+{
+	puts("Bye!\n");
+}
+
+int my_main(void)
+{
+	hi();
+	bye();
+}

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/c-extra-nolibc/puts.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+section .text
+
+global puts
+
+; /usr/include/x86_64-linux-gnu/asm/unistd_32.h
+__NR_write equ 4
+
+; Argument is message to be printed to standard output.
+puts:
+    push ebp
+    mov ebp, esp
+
+    ; Call __NR_write(1, message, message_len) (system call).
+    ;
+    ; Use x86 Linux system call convention.
+    ; https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux#Making_a_system_call
+    ;
+    ; eax stores the system call id.
+    ; ebx stores the first system call argument: 1 (standard output).
+    ; ecx stores the second system call argument: message.
+    ; edc stores the third system call argument: message length.
+
+    ; Store the write system call id in eax.
+    mov eax, __NR_write
+
+    ; Store standard output file descriptor (1) in ebx.
+    mov ebx, 1
+
+    ; Store function argument (message) in ecx.
+    mov ecx, [ebp + 8]
+
+    ; Compute message length in edx.
+    ; Find NUL byte address of message using edi. Start from message address (ecx).
+    ; Then edx <- edi - ecx.
+    mov edi, ecx
+    dec edi
+again:
+    inc edi
+    cmp byte [edi], 0
+    jne again
+
+    mov edx, edi
+    sub edx, ecx
+
+    ; Do system call.
+    int 0x80
+
+    leave
+    ret

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/c-extra-nolibc/start.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+extern my_main
+
+section .text
+
+; /usr/include/x86_64-linux-gnu/asm/unistd_32.h
+__NR_exit equ 1
+
+global _start
+
+_start:
+    call my_main
+
+    ; Call __NR_exit(main_return_value) (system call).
+    ;
+    ; Use x86 Linux system call convention.
+    ; https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux#Making_a_system_call
+    ;
+    ; ebx stores the first system call argument.
+    ; eax stores the system call id.
+
+    ; eax is main return value. Store it in ebx.
+    mov ebx, eax
+
+    ; Store the exit system call id in rax.
+    mov eax, __NR_exit
+
+    ; Do system call.
+    int 0x80

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/d-extra-libc/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/d-extra-libc/Makefile

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+CFLAGS = -fno-PIC -m32 -g
+LDFLAGS = -no-pie -m32
+AS = nasm
+ASFLAGS = -f elf32
+
+.DEFAULT_GOAL: all
+
+.PHONY: all clean
+
+all: hello
+
+hello: hello.o start.o
+	ld -static -m elf_i386 -L/usr/lib/gcc/x86_64-linux-gnu/7/32/ -o $@ $^ /usr/lib32/crti.o --start-group -lc -lgcc -lgcc_eh --end-group /usr/lib32/crtn.o
+
+hello.o: hello.c
+
+start.o: start.asm
+	$(AS) $(ASFLAGS) -o $@ $<
+
+clean:
+	-rm -f hello hello.o start.o
+	-rm -f *~

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/d-extra-libc/hello.c

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+// SPDX-License-Identifier: BSD-3-Clause
+
+#include <stdio.h>
+
+static void hi(void)
+{
+	puts("Hi!");
+}
+
+static void bye(void)
+{
+	puts("Bye!");
+}
+
+int my_main(void)
+{
+	hi();
+	bye();
+}

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/solution/d-extra-libc/start.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+extern __libc_csu_init
+extern __libc_csu_fini
+extern __libc_start_main
+extern my_main
+
+section .text
+
+global _start
+global _dl_relocate_static_pie
+
+; Adapted from GNU Libc i386 start.S
+;   https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/i386/start.S;hb=HEAD
+_start:
+    ; Align stack.
+    xor ebp, ebp
+    pop esi
+    mov ecx, esp
+    and esp, 0xfffffff0
+
+    push eax
+    push esp
+    push edx
+
+    push __libc_csu_fini
+    push __libc_csu_init
+    push ecx
+    push esi
+    push my_main
+    call __libc_start_main
+
+    hlt
+
+_dl_relocate_static_pie:
+    repz ret

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/support/a-c/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/support/a-c/Makefile

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+CFLAGS = -fno-PIC -m32 -g
+LDFLAGS = -no-pie -m32
+
+.DEFAULT_GOAL: all
+
+.PHONY: all clean
+
+all: hello
+
+hello: hello.o
+	$(CC) $(LDFLAGS) -o $@ $^
+
+hello.o: hello.c
+
+clean:
+	-rm -f hello hello.o
+	-rm -f *~

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/support/a-c/hello.c

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+// SPDX-License-Identifier: BSD-3-Clause
+
+#include <stdio.h>
+
+static void hi(void)
+{
+	puts("Hi!");
+}
+
+static void bye(void)
+{
+	puts("Bye!");
+}
+
+int my_main(void)
+{
+	hi();
+	bye();
+}

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/support/b-asm/.gitignore

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+/hello

chapters/hardware-software-interface/linking/drills/tasks/entry-fix-1/support/b-asm/Makefile

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+CFLAGS = -fno-PIC -m32 -g
+LDFLAGS = -no-pie -m32
+AS = nasm
+ASFLAGS = -f elf32
+
+.DEFAULT_GOAL: all
+
+.PHONY: all clean
+
+all: hello
+
+hello: hello.o
+	$(CC) $(LDFLAGS) -o $@ $^
+
+hello.o: hello.asm
+	$(AS) $(ASFLAGS) -o $@ $<
+
+clean:
+	-rm -f hello hello.o
+	-rm -f *~