Streaming files and files of not-yet-known size carried that fact as
i_size == FILE_SIZE_STREAM, i.e. (loff_t)-1. Overloading i_size this way
is fragile: a value read from media is indistinguishable from the
sentinel, so nothing can tell a genuine special file from a corrupted
inode that happens to hold -1.
Two unrelated situations were hidden behind that one value:
- a character device is a genuine sizeless stream, and
- a tftp transfer without a negotiated size has a real, definite size
that is simply not known until the file has been read to its end.
The latter is not a stream: it has an end of file and it is seekable --
tftp_lseek() moves forward by reading and discarding and backward by
reopening the transfer. Conflating the two would invite wrong
conclusions such as forbidding seeks on a tftp file.
Give the inode an i_stream flag for the former and an i_size_unknown flag
for the latter, and leave i_size at a real value in both cases. The two
agree on the only thing that matters to the I/O layer: i_size is not a
valid bound, so reads, writes and seeks must not be clamped to it and the
driver reports the end of the file itself. That shared question is folded
into the i_size_is_bound() helper. FILE_SIZE_STREAM stays only as the
stat sentinel that stat_inode() synthesises from these flags for its
callers.
No functional change intended; this only moves the "no i_size bound" bit
out of i_size so it can be trusted independently of the on-media value.
Assisted-by: Claude:claude-fable-5
---
fs/devfs.c | 7 ++++++-
fs/fs.c | 22 ++++++++++++++++++----
fs/tftp.c | 2 +-
include/linux/fs.h | 2 ++
4 files changed, 27 insertions(+), 6 deletions(-)
diff --git a/fs/devfs.c b/fs/devfs.c
index 75df330bcb..cb2980b114 100644
--- a/fs/devfs.c
+++ b/fs/devfs.c
@@ -110,7 +110,12 @@ static int devfs_open(struct inode *inode, struct file *f)
return -ENOENT;
}
- f->f_size = cdev_size(cdev);
+ if (cdev_size(cdev) == FILE_SIZE_STREAM) {
+ inode->i_stream = true;
+ f->f_size = 0;
+ } else {
+ f->f_size = cdev->size;
+ }
f->private_data = cdev;
return cdev_open(cdev, f->f_flags);
diff --git a/fs/fs.c b/fs/fs.c
index ce41f23f88..85881f6cf1 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -373,6 +373,18 @@ static int fsdev_truncate(struct file *f, loff_t length)
f->f_inode->i_fop->truncate(f, length) : -EROFS;
}
+/*
+ * Reads, writes and seeks are clamped to i_size, but only when i_size is a
+ * meaningful bound. A character device is a sizeless stream, and a file whose
+ * size is only discovered while reading (e.g. a tftp transfer without a
+ * negotiated size) starts out with i_size zero; for both, i_size must not
clamp
+ * I/O and the driver reports the end of the file itself.
+ */
+static bool i_size_is_bound(const struct inode *inode)
+{
+ return !inode->i_stream && !inode->i_size_unknown;
+}
+
int ftruncate(int fd, loff_t length)
{
struct file *f = fd_to_file(fd, false);
@@ -381,7 +393,7 @@ int ftruncate(int fd, loff_t length)
if (IS_ERR(f))
return -errno;
- if (f->f_size == FILE_SIZE_STREAM)
+ if (!i_size_is_bound(f->f_inode))
return 0;
ret = fsdev_truncate(f, length);
@@ -427,7 +439,7 @@ static ssize_t __read(struct file *f, void *buf, size_t
count)
if (fsdrv != ramfs_driver)
assert_command_context();
- if (f->f_size != FILE_SIZE_STREAM && f->f_pos + count > f->f_size)
+ if (i_size_is_bound(f->f_inode) && f->f_pos + count > f->f_size)
count = f->f_size - f->f_pos;
if (!count)
@@ -487,7 +499,7 @@ static ssize_t __write(struct file *f, const void *buf,
size_t count)
if (fsdrv != ramfs_driver)
assert_command_context();
- if (f->f_size != FILE_SIZE_STREAM && f->f_pos + count > f->f_size) {
+ if (i_size_is_bound(f->f_inode) && f->f_pos + count > f->f_size) {
ret = fsdev_truncate(f, f->f_pos + count);
if (ret) {
if (ret == -EPERM)
@@ -592,7 +604,7 @@ loff_t lseek(int fd, loff_t offset, int whence)
pos += offset;
- if (f->f_size != FILE_SIZE_STREAM && (pos < 0 || pos > f->f_size))
+ if (i_size_is_bound(f->f_inode) && (pos < 0 || pos > f->f_size))
goto out;
if (f->f_inode->i_fop->lseek) {
@@ -1105,6 +1117,8 @@ static void stat_inode(struct inode *inode, struct stat
*s)
cdev = cdev_by_name(inode->cdevname);
s->st_size = cdev ? cdev_size(cdev) : 0;
+ } else if (!i_size_is_bound(inode)) {
+ s->st_size = FILE_SIZE_STREAM;
} else {
s->st_size = inode->i_size;
}
diff --git a/fs/tftp.c b/fs/tftp.c
index e8a6b62870..dc68f2c77c 100644
--- a/fs/tftp.c
+++ b/fs/tftp.c
@@ -1088,7 +1088,7 @@ static struct dentry *tftp_lookup(struct inode *dir,
struct dentry *dentry,
if (filesize)
inode->i_size = filesize;
else
- inode->i_size = FILE_SIZE_STREAM;
+ inode->i_size_unknown = true;
d_add(dentry, inode);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d3813ad9c5..fc50d207a6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -129,6 +129,8 @@ struct inode {
gid_t i_gid;
u64 i_version;
loff_t i_size;
+ bool i_stream; /* sizeless stream, e.g. a
character device */
+ bool i_size_unknown; /* real size, not known until
read, e.g. tftp */
struct timespec i_atime;
struct timespec i_mtime;
struct timespec i_ctime;
--
2.47.3
