写：无效的参数

Question

I am writing a native executable binary in C, which will write the android device node: /sys/class/leds/lcd-brightness/brightness . push it to /system/bin and run as root.

However, system call write will return error message: "write: Invalid argument"

The code is as below:

// set background brightness
int bfd = open("/sys/class/leds/lcd-backlight/brightness", O_WRONLY);

if(bfd < 0 ) {
    perror("open");
    exit(EXIT_FAILURE);
}

printf("bfd: %d\n", bfd);
printf("*p: %d\n", (int)*p);
if(write(bfd, p, 1) < 0) {
    free(p);
    perror("write");
    exit(EXIT_FAILURE);
}
close(bfd);
close(fd);

p is a pointer to the data, I print *p before write, and it's all right.

/sys/class/leds/lcd-brightness/brightness has the following file permission:

-rw-rw-rw 1 system system 4096 2018-04-18 18:46 brightness

And I can override this file successfully with following command in adb shell as root

echo 100 > /sys/class/leds/lcd-backlight/brightness

What's wrong with my code ? I have struggled with this issue for hours. Any suggestion is appreciated.

Thank you

Answer 1

Your problem seems to be permission that's why You need to set flag for writing file

Below are the diffrent flags you have to set.

Try this Code

int bfd = open("/sys/class/leds/lcd-backlight/brightness", O_WRONLY | O_APPEND | S_IWUSR |S_IXUSR |S_IXOTH);

1. S_IRUSR Set read rights for the owner to true.
2. S_IWUSR Set write rights for the owner to true.
3. S_IXUSR Set execution rights for the owner to true.
4. S_IRGRP Set read rights for the group to true.
5. S_IWGRP Set write rights for the group to true.
6. S_IXGRP Set execution rights for the group to true.
7. S_IROTH Set read rights for other users to true.
8. S_IWOTH Set write rights for other users to true.
9. S_IXOTH Set execution rights for other users to true.

Hope This helps

Answer 2

Sysfs attributes expect data in a string format, ie, an array of char null-terminated. From your code, it seems that you are passing to write a pointer to an integer:

printf("*p: %d\\n", (int)*p);

Try something like:

int ret;
int bfd;
char p_str[24];

int bfd = open("/sys/class/leds/lcd-backlight/brightness", O_WRONLY);
if (bfd < 0) {
    /* handle the error */
}

snprintf(p_str, sizeof p_str, "%i", p);

ret = write(bfd, p_str, sizeof(p_str));
if (ret < 0) {
    /* handle the error */
}