[PATCH 07/39] introduce struct fderr, convert overlayfs uses to that

viro
2024-07-30T05:14:53Z

From: Al Viro <viro@zeniv.linux.org.uk>

Similar to struct fd; unlike struct fd, it can represent
error values.

Accessors:

* fd_empty(f): true if f represents an error
* fd_file(f): just as for struct fd it yields a pointer to
struct file if fd_empty(f) is false. If
fd_empty(f) is true, fd_file(f) is guaranteed
_not_ to be an address of any object (IS_ERR()
will be true in that case)
* fd_error(f): if f represents an error, returns that error,
otherwise the return value is junk.

Constructors:

* ERR_FD(-E...): an instance encoding given error [ERR_FDERR, perhaps?]
* BORROWED_FDERR(file): if file points to a struct file instance,
return a struct fderr representing that file
reference with no flags set.
if file is an ERR_PTR(-E...), return a struct
fderr representing that error.
file MUST NOT be NULL.
* CLONED_FDERR(file): similar, but in case when file points to
a struct file instance, set FDPUT_FPUT in flags.

Same destructor as for struct fd; I'm not entirely convinced that
playing with _Generic is a good idea here, but for now let's go
that way...

See fs/overlayfs/file.c for example of use.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/overlayfs/file.c | 125 +++++++++++++++++++++----------------------
include/linux/file.h | 38 +++++++++++--
2 files changed, 95 insertions(+), 68 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 2b7a5a3a7a2f..4b9e145bc7b8 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -89,56 +89,47 @@ static int ovl_change_flags(struct file *file, unsigned int flags)
return 0;
}

-static int ovl_real_fdget_meta(const struct file *file, struct fd *real,
- bool allow_meta)
+static struct fderr ovl_real_fdget_meta(const struct file *file, bool allow_meta)
{
struct dentry *dentry = file_dentry(file);
struct file *realfile = file->private_data;
struct path realpath;
int err;

- real->word = (unsigned long)realfile;
-
if (allow_meta) {
ovl_path_real(dentry, &realpath);
} else {
/* lazy lookup and verify of lowerdata */
err = ovl_verify_lowerdata(dentry);
if (err)
- return err;
+ return ERR_FD(err);

ovl_path_realdata(dentry, &realpath);
}
if (!realpath.dentry)
- return -EIO;
+ return ERR_FD(-EIO);

/* Has it been copied up since we'd opened it? */
if (unlikely(file_inode(realfile) != d_inode(realpath.dentry))) {
- struct file *f = ovl_open_realfile(file, &realpath);
- if (IS_ERR(f))
- return PTR_ERR(f);
- real->word = (unsigned long)ovl_open_realfile(file, &realpath) | FDPUT_FPUT;
- return 0;
+ return CLONED_FDERR(ovl_open_realfile(file, &realpath));
}

/* Did the flags change since open? */
- if (unlikely((file->f_flags ^ realfile->f_flags) & ~OVL_OPEN_FLAGS))
- return ovl_change_flags(realfile, file->f_flags);
+ if (unlikely((file->f_flags ^ realfile->f_flags) & ~OVL_OPEN_FLAGS)) {
+ err = ovl_change_flags(realfile, file->f_flags);
+ if (err)
+ return ERR_FD(err);
+ }

- return 0;
+ return BORROWED_FDERR(realfile);
}

-static int ovl_real_fdget(const struct file *file, struct fd *real)
+static struct fderr ovl_real_fdget(const struct file *file)
{
- if (d_is_dir(file_dentry(file))) {
- struct file *f = ovl_dir_real_file(file, false);
- if (IS_ERR(f))
- return PTR_ERR(f);
- real->word = (unsigned long)f;
- return 0;
- }
+ if (d_is_dir(file_dentry(file)))
+ return BORROWED_FDERR(ovl_dir_real_file(file, false));

- return ovl_real_fdget_meta(file, real, false);
+ return ovl_real_fdget_meta(file, false);
}

static int ovl_open(struct inode *inode, struct file *file)
@@ -183,7 +174,7 @@ static int ovl_release(struct inode *inode, struct file *file)
static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
{
struct inode *inode = file_inode(file);
- struct fd real;
+ struct fderr real;
const struct cred *old_cred;
loff_t ret;

@@ -199,9 +190,9 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
return vfs_setpos(file, 0, 0);
}

- ret = ovl_real_fdget(file, &real);
- if (ret)
- return ret;
+ real = ovl_real_fdget(file);
+ if (fd_empty(real))
+ return fd_error(real);

/*
* Overlay file f_pos is the master copy that is preserved
@@ -262,7 +253,7 @@ static void ovl_file_accessed(struct file *file)
static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
{
struct file *file = iocb->ki_filp;
- struct fd real;
+ struct fderr real;
ssize_t ret;
struct backing_file_ctx ctx = {
.cred = ovl_creds(file_inode(file)->i_sb),
@@ -273,9 +264,9 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
if (!iov_iter_count(iter))
return 0;

- ret = ovl_real_fdget(file, &real);
- if (ret)
- return ret;
+ real = ovl_real_fdget(file);
+ if (fd_empty(real))
+ return fd_error(real);

ret = backing_file_read_iter(fd_file(real), iter, iocb, iocb->ki_flags,
&ctx);
@@ -288,7 +279,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
{
struct file *file = iocb->ki_filp;
struct inode *inode = file_inode(file);
- struct fd real;
+ struct fderr real;
ssize_t ret;
int ifl = iocb->ki_flags;
struct backing_file_ctx ctx = {
@@ -304,9 +295,11 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
/* Update mode */
ovl_copyattr(inode);

- ret = ovl_real_fdget(file, &real);
- if (ret)
+ real = ovl_real_fdget(file);
+ if (fd_empty(real)) {
+ ret = fd_error(real);
goto out_unlock;
+ }

if (!ovl_should_sync(OVL_FS(inode->i_sb)))
ifl &= ~(IOCB_DSYNC | IOCB_SYNC);
@@ -329,7 +322,7 @@ static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
struct pipe_inode_info *pipe, size_t len,
unsigned int flags)
{
- struct fd real;
+ struct fderr real;
ssize_t ret;
struct backing_file_ctx ctx = {
.cred = ovl_creds(file_inode(in)->i_sb),
@@ -337,9 +330,9 @@ static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
.accessed = ovl_file_accessed,
};

- ret = ovl_real_fdget(in, &real);
- if (ret)
- return ret;
+ real = ovl_real_fdget(in);
+ if (fd_empty(real))
+ return fd_error(real);

ret = backing_file_splice_read(fd_file(real), ppos, pipe, len, flags, &ctx);
fdput(real);
@@ -358,7 +351,7 @@ static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
static ssize_t ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
loff_t *ppos, size_t len, unsigned int flags)
{
- struct fd real;
+ struct fderr real;
struct inode *inode = file_inode(out);
ssize_t ret;
struct backing_file_ctx ctx = {
@@ -371,9 +364,11 @@ static ssize_t ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
/* Update mode */
ovl_copyattr(inode);

- ret = ovl_real_fdget(out, &real);
- if (ret)
+ real = ovl_real_fdget(out);
+ if (fd_empty(real)) {
+ ret = fd_error(real);
goto out_unlock;
+ }

ret = backing_file_splice_write(pipe, fd_file(real), ppos, len, flags, &ctx);
fdput(real);
@@ -386,7 +381,7 @@ static ssize_t ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,

static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
{
- struct fd real;
+ struct fderr real;
const struct cred *old_cred;
int ret;

@@ -394,9 +389,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
if (ret <= 0)
return ret;

- ret = ovl_real_fdget_meta(file, &real, !datasync);
- if (ret)
- return ret;
+ real = ovl_real_fdget_meta(file, !datasync);
+ if (fd_empty(real))
+ return fd_error(real);

/* Don't sync lower file for fear of receiving EROFS error */
if (file_inode(fd_file(real)) == ovl_inode_upper(file_inode(file))) {
@@ -425,7 +420,7 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
{
struct inode *inode = file_inode(file);
- struct fd real;
+ struct fderr real;
const struct cred *old_cred;
int ret;

@@ -435,10 +430,11 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
ret = file_remove_privs(file);
if (ret)
goto out_unlock;
-
- ret = ovl_real_fdget(file, &real);
- if (ret)
+ real = ovl_real_fdget(file);
+ if (fd_empty(real)) {
+ ret = fd_error(real);
goto out_unlock;
+ }

old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = vfs_fallocate(fd_file(real), mode, offset, len);
@@ -457,13 +453,13 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len

static int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
{
- struct fd real;
+ struct fderr real;
const struct cred *old_cred;
int ret;

- ret = ovl_real_fdget(file, &real);
- if (ret)
- return ret;
+ real = ovl_real_fdget(file);
+ if (fd_empty(real))
+ return fd_error(real);

old_cred = ovl_override_creds(file_inode(file)->i_sb);
ret = vfs_fadvise(fd_file(real), offset, len, advice);
@@ -485,7 +481,7 @@ static loff_t ovl_copyfile(struct file *file_in, loff_t pos_in,
loff_t len, unsigned int flags, enum ovl_copyop op)
{
struct inode *inode_out = file_inode(file_out);
- struct fd real_in, real_out;
+ struct fderr real_in, real_out;
const struct cred *old_cred;
loff_t ret;

@@ -498,13 +494,16 @@ static loff_t ovl_copyfile(struct file *file_in, loff_t pos_in,
goto out_unlock;
}

- ret = ovl_real_fdget(file_out, &real_out);
- if (ret)
+ real_out = ovl_real_fdget(file_out);
+ if (fd_empty(real_out)) {
+ ret = fd_error(real_out);
goto out_unlock;
+ }

- ret = ovl_real_fdget(file_in, &real_in);
- if (ret) {
+ real_in = ovl_real_fdget(file_in);
+ if (fd_empty(real_in)) {
fdput(real_out);
+ ret = fd_error(real_in);
goto out_unlock;
}

@@ -577,13 +576,13 @@ static loff_t ovl_remap_file_range(struct file *file_in, loff_t pos_in,

static int ovl_flush(struct file *file, fl_owner_t id)
{
- struct fd real;
+ struct fderr real;
const struct cred *old_cred;
- int err;
+ int err = 0;

- err = ovl_real_fdget(file, &real);
- if (err)
- return err;
+ real = ovl_real_fdget(file);
+ if (fd_empty(real))
+ return fd_error(real);

if (fd_file(real)->f_op->flush) {
old_cred = ovl_override_creds(file_inode(file)->i_sb);
diff --git a/include/linux/file.h b/include/linux/file.h
index 3353d70fd460..d3165d7a8112 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -10,6 +10,7 @@
#include <linux/types.h>
#include <linux/posix_types.h>
#include <linux/errno.h>
+#include <linux/err.h>
#include <linux/cleanup.h>

struct file;
@@ -37,13 +38,26 @@ extern struct file *alloc_file_clone(struct file *, int flags,
struct fd {
unsigned long word;
};
+
+/* either a reference to struct file + flags
+ * (cloned vs. borrowed, pos locked), with
+ * flags stored in lower bits of value,
+ * or an error (represented by small negative value).
+ */
+struct fderr {
+ unsigned long word;
+};
+
#define FDPUT_FPUT 1
#define FDPUT_POS_UNLOCK 2

+#define fd_empty(f) _Generic((f), \
+ struct fd: unlikely(!(f).word), \
+ struct fderr: IS_ERR_VALUE((f).word))
#define fd_file(f) ((struct file *)((f).word & ~3))
-static inline bool fd_empty(struct fd f)
+static inline long fd_error(struct fderr f)
{
- return unlikely(!f.word);
+ return (long)f.word;
}

#define EMPTY_FD (struct fd){0}
@@ -56,11 +70,25 @@ static inline struct fd CLONED_FD(struct file *f)
return (struct fd){(unsigned long)f | FDPUT_FPUT};
}

-static inline void fdput(struct fd fd)
+static inline struct fderr ERR_FD(long n)
+{
+ return (struct fderr){(unsigned long)n};
+}
+static inline struct fderr BORROWED_FDERR(struct file *f)
{
- if (fd.word & FDPUT_FPUT)
- fput(fd_file(fd));
+ return (struct fderr){(unsigned long)f};
}
+static inline struct fderr CLONED_FDERR(struct file *f)
+{
+ if (IS_ERR(f))
+ return BORROWED_FDERR(f);
+ return (struct fderr){(unsigned long)f | FDPUT_FPUT};
+}
+
+#define fdput(f) (void) (_Generic((f), \
+ struct fderr: IS_ERR_VALUE((f).word), \
+ struct fd: true) && \
+ ((f).word & FDPUT_FPUT) && (fput(fd_file(f)),0))

extern struct file *fget(unsigned int fd);
extern struct file *fget_raw(unsigned int fd);
--
2.39.2