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POOS X: write syscall is not executed when loading binary to memory
 

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  1. POOS X: write syscall is not executed when loading binary to memory
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    <p>I have a simple hello world program written in assembly. It prints "Hello, world!" and exits. When compiling with <code>nasm</code> and executing the code directly, everything works fine.</p> <p>I also wrote some C code to load a flat binary and execute its code using <code>mmap</code>. When loading the flat binary, the text is not printed out. I'm pretty sure the call is happening, because adding an extra call to <code>exit</code> causes the program to exit prematurely. Here's the code for the assembly program:</p> <pre><code> global start section .text start: ; write is not run mov rax, 0x2000004 mov rdi, 1 mov rsi, msg mov rdx, msg.len syscall ; but exit is... mov rax, 0x2000001 mov rdi, 0 syscall section .data msg: db "Hello, world!", 10 .len: equ $ - msg </code></pre> <p>Compile it with <code>nasm -f macho64 hello.s &amp;&amp; ld -macosx_version_min 10.7.0 hello.o -o hello</code> and generate a flat binary with <code>objcopy -O binary hello.o hello.bin</code>.</p> <p>The code to load and execute the binary looks like this:</p> <pre><code>#include &lt;sys/mman.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; /* allocate executable memory size-bytes long */ void* memexec_alloc(size_t size) { void* ptr = mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE | MAP_ANON, -1, 0); if (ptr == (void *)-1) { printf("mmap error\n"); return NULL; } return ptr; } /* Load contents of file f into mem */ void memload(const char *f, unsigned char *mem) { FILE *file; size_t size; unsigned char *data; file = fopen(f, "r"); fseek(file, 0, SEEK_END); size = ftell(file); rewind(file); data = (unsigned char*) malloc(sizeof(char) * size); fread(data, sizeof(char), size, file); fclose(file); memcpy(mem, data, sizeof(char) * size); free(data); } static const size_t SIZE = 1024; typedef void (*voidfunc)(void); int main(int argc, char **argv) { if (argc != 2) { puts("no file specified!"); return 1; } void *mem = memexec_alloc(SIZE); memload(argv[1], mem); voidfunc func = mem; func(); return 1; } </code></pre> <p>Inspecting the code in a debugger shows that the first <code>syscall</code> <em>is indeed</em> executed, but nothing is printed out, the program then exits with a value of 0 instead of 1. How do I get it to print?</p> <p>(The OS is OS X Mavericks, 64 bit.)</p>
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