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  • Reading a child process's /proc/pid/mem file from the parent

    - by Amittai Aviram
    In the program below, I am trying to cause the following to happen: Process A assigns a value to a stack variable a. Process A (parent) creates process B (child) with PID child_pid. Process B calls function func1, passing a pointer to a. Process B changes the value of variable a through the pointer. Process B opens its /proc/self/mem file, seeks to the page containing a, and prints the new value of a. Process A (at the same time) opens /proc/child_pid/mem, seeks to the right page, and prints the new value of a. The problem is that, in step 6, the parent only sees the old value of a in /proc/child_pid/mem, while the child can indeed see the new value in its /proc/self/mem. Why is this the case? Is there any way that I can get the parent to to see the child's changes to its address space through the /proc filesystem? #include <fcntl.h> #include <stdbool.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/wait.h> #include <unistd.h> #define PAGE_SIZE 0x1000 #define LOG_PAGE_SIZE 0xc #define PAGE_ROUND_DOWN(v) ((v) & (~(PAGE_SIZE - 1))) #define PAGE_ROUND_UP(v) (((v) + PAGE_SIZE - 1) & (~(PAGE_SIZE - 1))) #define OFFSET_IN_PAGE(v) ((v) & (PAGE_SIZE - 1)) # if defined ARCH && ARCH == 32 #define BP "ebp" #define SP "esp" #else #define BP "rbp" #define SP "rsp" #endif typedef struct arg_t { int a; } arg_t; void func1(void * data) { arg_t * arg_ptr = (arg_t *)data; printf("func1: old value: %d\n", arg_ptr->a); arg_ptr->a = 53; printf("func1: address: %p\n", &arg_ptr->a); printf("func1: new value: %d\n", arg_ptr->a); } void expore_proc_mem(void (*fn)(void *), void * data) { off_t frame_pointer, stack_start; char buffer[PAGE_SIZE]; const char * path = "/proc/self/mem"; int child_pid, status; int parent_to_child[2]; int child_to_parent[2]; arg_t * arg_ptr; off_t child_offset; asm volatile ("mov %%"BP", %0" : "=m" (frame_pointer)); stack_start = PAGE_ROUND_DOWN(frame_pointer); printf("Stack_start: %lx\n", (unsigned long)stack_start); arg_ptr = (arg_t *)data; child_offset = OFFSET_IN_PAGE((off_t)&arg_ptr->a); printf("Address of arg_ptr->a: %p\n", &arg_ptr->a); pipe(parent_to_child); pipe(child_to_parent); bool msg; int child_mem_fd; char child_path[0x20]; child_pid = fork(); if (child_pid == -1) { perror("fork"); exit(EXIT_FAILURE); } if (!child_pid) { close(child_to_parent[0]); close(parent_to_child[1]); printf("CHILD (pid %d, parent pid %d).\n", getpid(), getppid()); fn(data); msg = true; write(child_to_parent[1], &msg, 1); child_mem_fd = open("/proc/self/mem", O_RDONLY); if (child_mem_fd == -1) { perror("open (child)"); exit(EXIT_FAILURE); } printf("CHILD: child_mem_fd: %d\n", child_mem_fd); if (lseek(child_mem_fd, stack_start, SEEK_SET) == (off_t)-1) { perror("lseek"); exit(EXIT_FAILURE); } if (read(child_mem_fd, buffer, sizeof(buffer)) != sizeof(buffer)) { perror("read"); exit(EXIT_FAILURE); } printf("CHILD: new value %d\n", *(int *)(buffer + child_offset)); read(parent_to_child[0], &msg, 1); exit(EXIT_SUCCESS); } else { printf("PARENT (pid %d, child pid %d)\n", getpid(), child_pid); printf("PARENT: child_offset: %lx\n", child_offset); read(child_to_parent[0], &msg, 1); printf("PARENT: message from child: %d\n", msg); snprintf(child_path, 0x20, "/proc/%d/mem", child_pid); printf("PARENT: child_path: %s\n", child_path); child_mem_fd = open(path, O_RDONLY); if (child_mem_fd == -1) { perror("open (child)"); exit(EXIT_FAILURE); } printf("PARENT: child_mem_fd: %d\n", child_mem_fd); if (lseek(child_mem_fd, stack_start, SEEK_SET) == (off_t)-1) { perror("lseek"); exit(EXIT_FAILURE); } if (read(child_mem_fd, buffer, sizeof(buffer)) != sizeof(buffer)) { perror("read"); exit(EXIT_FAILURE); } printf("PARENT: new value %d\n", *(int *)(buffer + child_offset)); close(child_mem_fd); printf("ENDING CHILD PROCESS.\n"); write(parent_to_child[1], &msg, 1); if (waitpid(child_pid, &status, 0) == -1) { perror("waitpid"); exit(EXIT_FAILURE); } } } int main(void) { arg_t arg; arg.a = 42; printf("In main: address of arg.a: %p\n", &arg.a); explore_proc_mem(&func1, &arg.a); return EXIT_SUCCESS; } This program produces the output below. Notice that the value of a (boldfaced) differs between parent's and child's reading of the /proc/child_pid/mem file. In main: address of arg.a: 0x7ffffe1964f0 Stack_start: 7ffffe196000 Address of arg_ptr-a: 0x7ffffe1964f0 PARENT (pid 20376, child pid 20377) PARENT: child_offset: 4f0 CHILD (pid 20377, parent pid 20376). func1: old value: 42 func1: address: 0x7ffffe1964f0 func1: new value: 53 PARENT: message from child: 1 CHILD: child_mem_fd: 4 PARENT: child_path: /proc/20377/mem CHILD: new value 53 PARENT: child_mem_fd: 7 PARENT: new value 42 ENDING CHILD PROCESS.

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  • Facebook connect on Google App Engine with Django Patch

    - by yoav.aviram
    We are building a website on Google App Engine, using django patch. We would like to use Facebook connect for two purposes: Authenticate users. Access user's social data. Searching for a solution in the usual places (google, FB, SO) brigs up a lot of noise, many partial solutions and no clear answer. So the question is this: does anyone has a clear working solution? maybe even a recipe? Thanks.

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  • WPF EDitable ComboBox

    - by Amittai Shapira
    Hi! I want to create editable combo box with the following properties: 1. Bind the text property to my data model. 2. The data model may override the changes in the GUI, even in Selection changed. E.g. I can choose from 1, 2, 3 I choose 2, but some component down under changes it to 3. 3. Update the data model on the following events: 1. Selection Changed 2. Lose focus 3. Enter pressed (should behave the same as lost focus). I've been able to create such control, but it's pretty ugly (using many hacks) and I hoped there's a simpler way… Thanks in advance

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