As of now, seeking in sysfs files is handled by generic_file_llseek().
There are situations where one may want to customize seeking logic:
- Many sysfs entries are fixed files while generic_file_llseek() accepts
past-the-end positions. Not only being useless by itself, this
also means a bug in userspace code will trigger not at lseek(), but at
some later point making debugging harder.
- generic_file_llseek() relies on f_mapping->host to get the file size
which might not be correct for all sysfs entries.
See commit 636b21b50152 ("PCI: Revoke mappings like devmem") as an example.
Implement llseek method to override this behavior at sysfs attribute
level. The method is optional, and if it is absent,
generic_file_llseek() is called to preserve backwards compatibility.
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Valentine Sinitsyn <valesini@xxxxxxxxxxxxxx>
----
Perhaps this got burried to deep in the original thread [a],
resubmitting afresh.
a.
https://lore.kernel.org/all/20230814190850.948032-1-valesini@xxxxxxxxxxxxxx/
Changelog:
v8:
- This is real v7; previous v7 is a buggy earlier patch sent by
mistake
v7:
- Use proper locking in kernfs_fop_llseek()
v6:
- Mark pci_llseek_resource() as __maybe_unused
- Fix a typo in pci_create_legacy_files()
v5:
- Fix builds without PCI mmap support (e.g. Alpha)
v4:
- Fix builds which #define HAVE_PCI_LEGACY (e.g. PowerPC)
v3:
- Grammar fixes
- Add base-patch: and prerequisite-patch-id: to make kernel test
robot happy
v2:
- Add infrastructure to customize llseek per sysfs entry type
- Override llseek for PCI sysfs entries with fixed_file_llseek()--
fs/kernfs/file.c | 29 ++++++++++++++++++++++++++++-
fs/sysfs/file.c | 13 +++++++++++++
include/linux/kernfs.h | 1 +
include/linux/sysfs.h | 2 ++
4 files changed, 44 insertions(+), 1 deletion(-)
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 180906c36f51..855e3f9d8dcc 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -903,6 +903,33 @@ static __poll_t kernfs_fop_poll(struct file *filp, poll_table *wait)
return ret;
}
+static loff_t kernfs_fop_llseek(struct file *file, loff_t offset, int whence)
+{
+ struct kernfs_open_file *of = kernfs_of(file);
+ const struct kernfs_ops *ops;
+ loff_t ret;
+
+ /*
+ * @of->mutex nests outside active ref and is primarily to ensure that
+ * the ops aren't called concurrently for the same open file.
+ */
+ mutex_lock(&of->mutex);
+ if (!kernfs_get_active(of->kn)) {
+ mutex_unlock(&of->mutex);
+ return -ENODEV;
+ }
+
+ ops = kernfs_ops(of->kn);
+ if (ops->llseek)
+ ret = ops->llseek(of, offset, whence);
+ else
+ ret = generic_file_llseek(file, offset, whence);
+
+ kernfs_put_active(of->kn);
+ mutex_unlock(&of->mutex);
+ return ret;
+}
+
static void kernfs_notify_workfn(struct work_struct *work)
{
struct kernfs_node *kn;
@@ -1005,7 +1032,7 @@ EXPORT_SYMBOL_GPL(kernfs_notify);
const struct file_operations kernfs_file_fops = {
.read_iter = kernfs_fop_read_iter,
.write_iter = kernfs_fop_write_iter,
- .llseek = generic_file_llseek,
+ .llseek = kernfs_fop_llseek,
.mmap = kernfs_fop_mmap,
.open = kernfs_fop_open,
.release = kernfs_fop_release,
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index a12ac0356c69..6b7652fb8050 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -167,6 +167,18 @@ static int sysfs_kf_bin_mmap(struct kernfs_open_file *of,
return battr->mmap(of->file, kobj, battr, vma);
}
+static loff_t sysfs_kf_bin_llseek(struct kernfs_open_file *of, loff_t offset,
+ int whence)
+{
+ struct bin_attribute *battr = of->kn->priv;
+ struct kobject *kobj = of->kn->parent->priv;
+
+ if (battr->llseek)
+ return battr->llseek(of->file, kobj, battr, offset, whence);
+ else
+ return generic_file_llseek(of->file, offset, whence);
+}
+
static int sysfs_kf_bin_open(struct kernfs_open_file *of)
{
struct bin_attribute *battr = of->kn->priv;
@@ -249,6 +261,7 @@ static const struct kernfs_ops sysfs_bin_kfops_mmap = {
.write = sysfs_kf_bin_write,
.mmap = sysfs_kf_bin_mmap,
.open = sysfs_kf_bin_open,
+ .llseek = sysfs_kf_bin_llseek,
};
int sysfs_add_file_mode_ns(struct kernfs_node *parent,
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 2a36f3218b51..99aaa050ccb7 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -316,6 +316,7 @@ struct kernfs_ops {
struct poll_table_struct *pt);
int (*mmap)(struct kernfs_open_file *of, struct vm_area_struct *vma);
+ loff_t (*llseek)(struct kernfs_open_file *of, loff_t offset, int whence);
};
/*
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index fd3fe5c8c17f..b717a70219f6 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -181,6 +181,8 @@ struct bin_attribute {
char *, loff_t, size_t);
ssize_t (*write)(struct file *, struct kobject *, struct bin_attribute *,
char *, loff_t, size_t);
+ loff_t (*llseek)(struct file *, struct kobject *, struct bin_attribute *,
+ loff_t, int);
int (*mmap)(struct file *, struct kobject *, struct bin_attribute *attr,
struct vm_area_struct *vma);
};
base-commit: a48fa7efaf1161c1c898931fe4c7f0070964233a
--
2.34.1
