Hello.
Below is a loadable kernel module which attempts to read (for example) /proc/interrupts from
kernel using usermode driver interface. What is strange is that the total bytes obtained by
doing "wc -c /proc/interrupts" from userspace's shell and trying to insmod this kernel module
differs; for unknown reason, kernel_read() returns "#!/bin/cat /proc/interrupts\n" (28 bytes)
at the end of input.
---------- Start of sample module ----------
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/usermode_driver.h>
static int __init umd_test_init(void)
{
static const char program[] = "#!/bin/cat /proc/interrupts\n";
struct umd_info umd = { .driver_name = "umd_test" };
static char buffer[512];
loff_t pos = 0;
int total = 0;
int ret;
if (umd_load_blob(&umd, program, sizeof(program) - 1))
return -EINVAL;
ret = fork_usermode_driver(&umd);
if (ret == 0) {
memset(buffer, 0, sizeof(buffer));
while ((ret = kernel_read(umd.pipe_from_umh, buffer, sizeof(buffer) - 1, &pos)) > 0) {
buffer[ret] = '\0';
printk("buffer='%s'\n", buffer);
total += ret;
}
printk("ret=%d total=%u\n", ret, total);
}
umd_unload_blob(&umd);
return -EINVAL;
}
module_init(umd_test_init);
MODULE_LICENSE("GPL");
---------- End of sample module ----------
If I don't use "#!" (i.e. replacing
"#!/bin/cat /proc/kallsyms\n"
with binary data generated by compiling
---------- Start of sample usermode code ----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
const int fd = open("/proc/interrupts", O_RDONLY);
char buffer[4096];
int len;
while ((len = read(fd, buffer, sizeof(buffer))) > 0 &&
write(1, buffer, len) == len);
return !!len;
}
---------- End of sample usermode code ----------
and converted by ./scripts/bin2c ), the total bytes obtained by doing
"wc -c /proc/interrupts" from userspace's shell and trying to insmod
this kernel module matches (i.e. there is no garbage).
Why there is garbage data if I use "#!" ?
