need help understanding the read part of read_write.c - c

Question

Hi I was hoping someone might help me understand the read section of the read_write.c kernel file when I look at it I don't really understand a thing.
I can't really tell which part is actually reading the file considering there are several instances where read functions are called. I ask because I have to know where to modify it and how for an assignment I have where I have to modify the output of the read without actually modifying the file.
By the way I am using the latest version of the Linux kernel from kernel.org version 4.9 any and all help is appreciated thank you. Below is where I believe the read is happening.
typedef ssize_t (*iter_fn_t)(struct kiocb *, struct iov_iter *);
const struct file_operations generic_ro_fops = {
.llseek = generic_file_llseek,
.read_iter = generic_file_read_iter,
.mmap = generic_file_readonly_mmap,
.splice_read = generic_file_splice_read,
};
static ssize_t do_iter_readv_writev(struct file *filp, struct iov_iter *iter,
loff_t *ppos, iter_fn_t fn, int flags)
{
struct kiocb kiocb;
ssize_t ret;
if (flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC))
return -EOPNOTSUPP;
init_sync_kiocb(&kiocb, filp);
if (flags & RWF_HIPRI)
kiocb.ki_flags |= IOCB_HIPRI;
if (flags & RWF_DSYNC)
kiocb.ki_flags |= IOCB_DSYNC;
if (flags & RWF_SYNC)
kiocb.ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
kiocb.ki_pos = *ppos;
ret = fn(&kiocb, iter);
BUG_ON(ret == -EIOCBQUEUED);
*ppos = kiocb.ki_pos;
return ret;
}
ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
unsigned long nr_segs, unsigned long fast_segs,
struct iovec *fast_pointer,
struct iovec **ret_pointer)
{
unsigned long seg;
ssize_t ret;
struct iovec *iov = fast_pointer;
/*
* SuS says "The readv() function *may* fail if the iovcnt argument
* was less than or equal to 0, or greater than {IOV_MAX}. Linux has
* traditionally returned zero for zero segments, so...
*/
if (nr_segs == 0) {
ret = 0;
goto out;
}
/*
* First get the "struct iovec" from user memory and
* verify all the pointers
*/
if (nr_segs > UIO_MAXIOV) {
ret = -EINVAL;
goto out;
}
if (nr_segs > fast_segs) {
iov = kmalloc(nr_segs*sizeof(struct iovec), GFP_KERNEL);
printk(KERN_DEBUG "Hello from read_write.c\n");
printk(KERN_DEBUG "Inside the copy check uvector method\n");
if (iov == NULL) {
ret = -ENOMEM;
goto out;
}
}
if (copy_from_user(iov, uvector, nr_segs*sizeof(*uvector))) {
ret = -EFAULT;
goto out;
}
/*
* According to the Single Unix Specification we should return EINVAL
* if an element length is < 0 when cast to ssize_t or if the
* total length would overflow the ssize_t return value of the
* system call.
*
* Linux caps all read/write calls to MAX_RW_COUNT, and avoids the
* overflow case.
*/
ret = 0;
for (seg = 0; seg < nr_segs; seg++) {
void __user *buf = iov[seg].iov_base;
ssize_t len = (ssize_t)iov[seg].iov_len;
/* see if we we're about to use an invalid len or if
* it's about to overflow ssize_t */
if (len < 0) {
ret = -EINVAL;
goto out;
}
if (type >= 0
&& unlikely(!access_ok(vrfy_dir(type), buf, len))) {
ret = -EFAULT;
goto out;
}
if (len > MAX_RW_COUNT - ret) {
len = MAX_RW_COUNT - ret;
iov[seg].iov_len = len;
}
ret += len;
}
out:
*ret_pointer = iov;
return ret;
}
/* Do it by hand, with file-ops */
static ssize_t do_loop_readv_writev(struct file *filp, struct iov_iter *iter,
loff_t *ppos, io_fn_t fn, int flags)
{
ssize_t ret = 0;
if (flags & ~RWF_HIPRI)
return -EOPNOTSUPP;
while (iov_iter_count(iter)) {
struct iovec iovec = iov_iter_iovec(iter);
ssize_t nr;
nr = fn(filp, iovec.iov_base, iovec.iov_len, ppos);
if (nr < 0) {
if (!ret)
ret = nr;
break;
}
ret += nr;
if (nr != iovec.iov_len)
break;
iov_iter_advance(iter, nr);
}
return ret;
}
static ssize_t do_readv_writev(int type, struct file *file,
const struct iovec __user * uvector,
unsigned long nr_segs, loff_t *pos,
int flags)
{
size_t tot_len;
struct iovec iovstack[UIO_FASTIOV];
struct iovec *iov = iovstack;
struct iov_iter iter;
ssize_t ret;
io_fn_t fn;
iter_fn_t iter_fn;
ret = import_iovec(type, uvector, nr_segs,
ARRAY_SIZE(iovstack), &iov, &iter);
if (ret < 0)
return ret;
tot_len = iov_iter_count(&iter);
if (!tot_len)
goto out;
ret = rw_verify_area(type, file, pos, tot_len);
if (ret < 0)
goto out;
if (type == READ) {
fn = file->f_op->read;
iter_fn = file->f_op->read_iter;
} else {
fn = (io_fn_t)file->f_op->write;
iter_fn = file->f_op->write_iter;
file_start_write(file);
}
if (iter_fn)
ret = do_iter_readv_writev(file, &iter, pos, iter_fn, flags);
else
ret = do_loop_readv_writev(file, &iter, pos, fn, flags);
if (type != READ)
file_end_write(file);
out:
kfree(iov);
if ((ret + (type == READ)) > 0) {
if (type == READ)
fsnotify_access(file);
else
fsnotify_modify(file);
}
return ret;
}
ssize_t vfs_readv(struct file *file, const struct iovec __user *vec,
unsigned long vlen, loff_t *pos, int flags)
{
if (!(file->f_mode & FMODE_READ))
return -EBADF;
if (!(file->f_mode & FMODE_CAN_READ))
return -EINVAL;
return do_readv_writev(READ, file, vec, vlen, pos, flags);
}
EXPORT_SYMBOL(vfs_readv);
static ssize_t do_readv(unsigned long fd, const struct iovec __user *vec,
unsigned long vlen, int flags)
{
struct fd f = fdget_pos(fd);
ssize_t ret = -EBADF;
if (f.file) {
loff_t pos = file_pos_read(f.file);
ret = vfs_readv(f.file, vec, vlen, &pos, flags);
if (ret >= 0)
file_pos_write(f.file, pos);
fdput_pos(f);
}
if (ret > 0)
add_rchar(current, ret);
inc_syscr(current);
return ret;
}
static ssize_t do_preadv(unsigned long fd, const struct iovec __user *vec,
unsigned long vlen, loff_t pos, int flags)
{
struct fd f;
ssize_t ret = -EBADF;
if (pos < 0)
return -EINVAL;
f = fdget(fd);
if (f.file) {
ret = -ESPIPE;
if (f.file->f_mode & FMODE_PREAD)
ret = vfs_readv(f.file, vec, vlen, &pos, flags);
fdput(f);
}
if (ret > 0)
add_rchar(current, ret);
inc_syscr(current);
return ret;
}
static ssize_t compat_do_readv_writev(int type, struct file *file,
const struct compat_iovec __user *uvector,
unsigned long nr_segs, loff_t *pos,
int flags)
{
compat_ssize_t tot_len;
struct iovec iovstack[UIO_FASTIOV];
struct iovec *iov = iovstack;
struct iov_iter iter;
ssize_t ret;
io_fn_t fn;
iter_fn_t iter_fn;
ret = compat_import_iovec(type, uvector, nr_segs,
UIO_FASTIOV, &iov, &iter);
if (ret < 0)
return ret;
tot_len = iov_iter_count(&iter);
if (!tot_len)
goto out;
ret = rw_verify_area(type, file, pos, tot_len);
if (ret < 0)
goto out;
if (type == READ) {
fn = file->f_op->read;
iter_fn = file->f_op->read_iter;
} else {
fn = (io_fn_t)file->f_op->write;
iter_fn = file->f_op->write_iter;
file_start_write(file);
}
if (iter_fn)
ret = do_iter_readv_writev(file, &iter, pos, iter_fn, flags);
else
ret = do_loop_readv_writev(file, &iter, pos, fn, flags);
if (type != READ)
file_end_write(file);
out:
kfree(iov);
if ((ret + (type == READ)) > 0) {
if (type == READ)
fsnotify_access(file);
else
fsnotify_modify(file);
}
return ret;
}
static size_t compat_readv(struct file *file,
const struct compat_iovec __user *vec,
unsigned long vlen, loff_t *pos, int flags)
{
ssize_t ret = -EBADF;
if (!(file->f_mode & FMODE_READ))
goto out;
ret = -EINVAL;
if (!(file->f_mode & FMODE_CAN_READ))
goto out;
ret = compat_do_readv_writev(READ, file, vec, vlen, pos, flags);
out:
if (ret > 0)
add_rchar(current, ret);
inc_syscr(current);
return ret;
}
static size_t do_compat_readv(compat_ulong_t fd,
const struct compat_iovec __user *vec,
compat_ulong_t vlen, int flags)
{
struct fd f = fdget_pos(fd);
ssize_t ret;
loff_t pos;
if (!f.file)
return -EBADF;
pos = f.file->f_pos;
ret = compat_readv(f.file, vec, vlen, &pos, flags);
if (ret >= 0)
f.file->f_pos = pos;
fdput_pos(f);
return ret;
}
COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd,
const struct compat_iovec __user *,vec,
compat_ulong_t, vlen)
{
return do_compat_readv(fd, vec, vlen, 0);
}
static long do_compat_preadv64(unsigned long fd,
const struct compat_iovec __user *vec,
unsigned long vlen, loff_t pos, int flags)
{
struct fd f;
ssize_t ret;
if (pos < 0)
return -EINVAL;
f = fdget(fd);
if (!f.file)
return -EBADF;
ret = -ESPIPE;
if (f.file->f_mode & FMODE_PREAD)
ret = compat_readv(f.file, vec, vlen, &pos, flags);
fdput(f);
return ret;
}

Answer 1

Apart from bad formatting you can easily see that do_readv and do_preadv both call vfs_readv. In those functions no hint is seen that they do some reading on their own.
You can also see that vfs_readv doesn't do any reading but only calls do_readv_writev.
The actual reading is done here:
if (type == READ) {
fn = file->f_op->read;
iter_fn = file->f_op->read_iter;
}
...
if (iter_fn)
ret = do_iter_readv_writev(file, &iter, pos, iter_fn, flags);
else
ret = do_loop_readv_writev(file, &iter, pos, fn, flags);
Well, it is not the actual reading, but it is the closest to reading that you can get from your code snippet.
What happens in these functions and more important, what was stored in iter_fn and fn is not visible from your code.
And I am no Linux expert to tell you more details.