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fs: use backing_file container for internal files with "fake" f_path

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Overlayfs uses open_with_fake_path() to allocate internal kernel files,
with a "fake" path - whose f_path is not on the same fs as f_inode.

Allocate a container struct backing_file for those internal files, that
is used to hold the "fake" ovl path along with the real path.

backing_file_real_path() can be used to access the stored real path.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Message-Id: <20230615112229.2143178-5-amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Amir Goldstein
2023-06-15 14:22:28 +03:00
committed by Christian Brauner
parent 8a05a8c31d
commit 62d53c4a1d
5 changed files with 114 additions and 23 deletions
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50
fs/file_table.c
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@@ -44,18 +44,40 @@ static struct kmem_cache *filp_cachep __read_mostly;
static struct percpu_counter nr_files __cacheline_aligned_in_smp;
/* Container for backing file with optional real path */
struct backing_file {
struct file file;
struct path real_path;
};
static inline struct backing_file *backing_file(struct file *f)
{
return container_of(f, struct backing_file, file);
}
struct path *backing_file_real_path(struct file *f)
{
return &backing_file(f)->real_path;
}
EXPORT_SYMBOL_GPL(backing_file_real_path);
static void file_free_rcu(struct rcu_head *head)
{
struct file *f = container_of(head, struct file, f_rcuhead);
put_cred(f->f_cred);
kmem_cache_free(filp_cachep, f);
if (unlikely(f->f_mode & FMODE_BACKING))
kfree(backing_file(f));
else
kmem_cache_free(filp_cachep, f);
}
static inline void file_free(struct file *f)
{
security_file_free(f);
if (!(f->f_mode & FMODE_NOACCOUNT))
if (unlikely(f->f_mode & FMODE_BACKING))
path_put(backing_file_real_path(f));
if (likely(!(f->f_mode & FMODE_NOACCOUNT)))
percpu_counter_dec(&nr_files);
call_rcu(&f->f_rcuhead, file_free_rcu);
}
@@ -226,6 +248,30 @@ struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred)
return f;
}
/*
* Variant of alloc_empty_file() that allocates a backing_file container
* and doesn't check and modify nr_files.
*
* This is only for kernel internal use, and the allocate file must not be
* installed into file tables or such.
*/
struct file *alloc_empty_backing_file(int flags, const struct cred *cred)
{
struct backing_file *ff;
int error;
ff = kzalloc(sizeof(struct backing_file), GFP_KERNEL);
if (unlikely(!ff))
return ERR_PTR(-ENOMEM);
error = init_file(&ff->file, flags, cred);
if (unlikely(error))
return ERR_PTR(error);
ff->file.f_mode |= FMODE_BACKING | FMODE_NOACCOUNT;
return &ff->file;
}
/**
* alloc_file - allocate and initialize a 'struct file'
*