procfs: preparations for remove_proc_entry() race fixes

* leave ->proc_fops alone; make ->pde_users negative instead
* trim pde_opener
* move relevant code in fs/proc/inode.c

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

4 files changed, 135 insertions, 214 deletions

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index a6a1cb5d589d..bec58323629c 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -39,7 +39,7 @@ static int proc_match(unsigned int len, const char *name, struct proc_dir_entry
 /* buffer size is one page but our output routines use some slack for overruns */
 #define PROC_BLOCK_SIZE (PAGE_SIZE - 1024)
 
-static ssize_t
+ssize_t
 __proc_file_read(struct file *file, char __user *buf, size_t nbytes,
                loff_t *ppos)
 {
@@ -171,48 +171,6 @@ __proc_file_read(struct file *file, char __user *buf, size_t nbytes,
         return retval;
 }
 
-static ssize_t
-proc_file_read(struct file *file, char __user *buf, size_t nbytes,
-               loff_t *ppos)
-{
-        struct proc_dir_entry *pde = PDE(file_inode(file));
-        ssize_t rv = -EIO;
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
-        }
-        pde->pde_users++;
-        spin_unlock(&pde->pde_unload_lock);
-
-        rv = __proc_file_read(file, buf, nbytes, ppos);
-
-        pde_users_dec(pde);
-        return rv;
-}
-
-static loff_t
-proc_file_lseek(struct file *file, loff_t offset, int orig)
-{
-        loff_t retval = -EINVAL;
-        switch (orig) {
-        case 1:
-                offset += file->f_pos;
-        /* fallthrough */
-        case 0:
-                if (offset < 0 || offset > MAX_NON_LFS)
-                        break;
-                file->f_pos = retval = offset;
-        }
-        return retval;
-}
-
-static const struct file_operations proc_file_operations = {
-        .llseek         = proc_file_lseek,
-        .read           = proc_file_read,
-};
-
 static int proc_notify_change(struct dentry *dentry, struct iattr *iattr)
 {
         struct inode *inode = dentry->d_inode;
@@ -722,41 +680,6 @@ void pde_put(struct proc_dir_entry *pde)
                 free_proc_entry(pde);
 }
 
-static void entry_rundown(struct proc_dir_entry *de)
-{
-        spin_lock(&de->pde_unload_lock);
-        /*
-         * Stop accepting new callers into module. If you're
-         * dynamically allocating ->proc_fops, save a pointer somewhere.
-         */
-        de->proc_fops = NULL;
-        /* Wait until all existing callers into module are done. */
-        if (de->pde_users > 0) {
-                DECLARE_COMPLETION_ONSTACK(c);
-
-                if (!de->pde_unload_completion)
-                        de->pde_unload_completion = &c;
-
-                spin_unlock(&de->pde_unload_lock);
-
-                wait_for_completion(de->pde_unload_completion);
-
-                spin_lock(&de->pde_unload_lock);
-        }
-
-        while (!list_empty(&de->pde_openers)) {
-                struct pde_opener *pdeo;
-
-                pdeo = list_first_entry(&de->pde_openers, struct pde_opener, lh);
-                list_del(&pdeo->lh);
-                spin_unlock(&de->pde_unload_lock);
-                pdeo->release(pdeo->inode, pdeo->file);
-                kfree(pdeo);
-                spin_lock(&de->pde_unload_lock);
-        }
-        spin_unlock(&de->pde_unload_lock);
-}
-
 /*
  * Remove a /proc entry and free it if it's not currently in use.
  */
@@ -788,7 +711,7 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
                 return;
         }
 
-        entry_rundown(de);
+        proc_entry_rundown(de);
 
         if (S_ISDIR(de->mode))
                 parent->nlink--;
@@ -837,7 +760,7 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
                 }
                 spin_unlock(&proc_subdir_lock);
 
-                entry_rundown(de);
+                proc_entry_rundown(de);
                 next = de->parent;
                 if (S_ISDIR(de->mode))
                         next->nlink--;
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index a4aaaeee3342..0cd9d80f28e8 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -129,96 +129,138 @@ static const struct super_operations proc_sops = {
         .show_options   = proc_show_options,
 };
 
+enum {BIAS = -1U<<31};
+
+static inline int use_pde(struct proc_dir_entry *pde)
+{
+        int res = 1;
+        spin_lock(&pde->pde_unload_lock);
+        if (unlikely(pde->pde_users < 0))
+                res = 0;
+        else
+                pde->pde_users++;
+        spin_unlock(&pde->pde_unload_lock);
+        return res;
+}
+
 static void __pde_users_dec(struct proc_dir_entry *pde)
 {
-        pde->pde_users--;
-        if (pde->pde_unload_completion && pde->pde_users == 0)
+        if (--pde->pde_users == BIAS)
                 complete(pde->pde_unload_completion);
 }
 
-void pde_users_dec(struct proc_dir_entry *pde)
+static void unuse_pde(struct proc_dir_entry *pde)
 {
         spin_lock(&pde->pde_unload_lock);
         __pde_users_dec(pde);
         spin_unlock(&pde->pde_unload_lock);
 }
 
-static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
+void proc_entry_rundown(struct proc_dir_entry *de)
 {
-        struct proc_dir_entry *pde = PDE(file_inode(file));
-        loff_t rv = -EINVAL;
-        loff_t (*llseek)(struct file *, loff_t, int);
+        spin_lock(&de->pde_unload_lock);
+        de->pde_users += BIAS;
+        /* Wait until all existing callers into module are done. */
+        if (de->pde_users != BIAS) {
+                DECLARE_COMPLETION_ONSTACK(c);
+                de->pde_unload_completion = &c;
+                spin_unlock(&de->pde_unload_lock);
 
-        spin_lock(&pde->pde_unload_lock);
-        /*
-         * remove_proc_entry() is going to delete PDE (as part of module
-         * cleanup sequence). No new callers into module allowed.
-         */
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+                wait_for_completion(de->pde_unload_completion);
+
+                spin_lock(&de->pde_unload_lock);
         }
-        /*
-         * Bump refcount so that remove_proc_entry will wail for ->llseek to
-         * complete.
-         */
-        pde->pde_users++;
-        /*
-         * Save function pointer under lock, to protect against ->proc_fops
-         * NULL'ifying right after ->pde_unload_lock is dropped.
-         */
-        llseek = pde->proc_fops->llseek;
-        spin_unlock(&pde->pde_unload_lock);
 
-        if (!llseek)
-                llseek = default_llseek;
-        rv = llseek(file, offset, whence);
+        while (!list_empty(&de->pde_openers)) {
+                struct pde_opener *pdeo;
+                struct file *file;
 
-        pde_users_dec(pde);
-        return rv;
+                pdeo = list_first_entry(&de->pde_openers, struct pde_opener, lh);
+                list_del(&pdeo->lh);
+                spin_unlock(&de->pde_unload_lock);
+                file = pdeo->file;
+                de->proc_fops->release(file_inode(file), file);
+                kfree(pdeo);
+                spin_lock(&de->pde_unload_lock);
+        }
+        spin_unlock(&de->pde_unload_lock);
 }
 
-static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
+/* ->read_proc() users - legacy crap */
+static ssize_t
+proc_file_read(struct file *file, char __user *buf, size_t nbytes,
+               loff_t *ppos)
 {
         struct proc_dir_entry *pde = PDE(file_inode(file));
         ssize_t rv = -EIO;
-        ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
+        if (use_pde(pde)) {
+                rv = __proc_file_read(file, buf, nbytes, ppos);
+                unuse_pde(pde);
+        }
+        return rv;
+}
 
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+static loff_t
+proc_file_lseek(struct file *file, loff_t offset, int orig)
+{
+        loff_t retval = -EINVAL;
+        switch (orig) {
+        case 1:
+                offset += file->f_pos;
+        /* fallthrough */
+        case 0:
+                if (offset < 0 || offset > MAX_NON_LFS)
+                        break;
+                file->f_pos = retval = offset;
         }
-        pde->pde_users++;
-        read = pde->proc_fops->read;
-        spin_unlock(&pde->pde_unload_lock);
+        return retval;
+}
 
-        if (read)
-                rv = read(file, buf, count, ppos);
+const struct file_operations proc_file_operations = {
+        .llseek         = proc_file_lseek,
+        .read           = proc_file_read,
+};
 
-        pde_users_dec(pde);
+static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
+{
+        struct proc_dir_entry *pde = PDE(file_inode(file));
+        loff_t rv = -EINVAL;
+        if (use_pde(pde)) {
+                loff_t (*llseek)(struct file *, loff_t, int);
+                llseek = pde->proc_fops->llseek;
+                if (!llseek)
+                        llseek = default_llseek;
+                rv = llseek(file, offset, whence);
+                unuse_pde(pde);
+        }
         return rv;
 }
 
-static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
+static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 {
+        ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
         struct proc_dir_entry *pde = PDE(file_inode(file));
         ssize_t rv = -EIO;
-        ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+        if (use_pde(pde)) {
+                read = pde->proc_fops->read;
+                if (read)
+                        rv = read(file, buf, count, ppos);
+                unuse_pde(pde);
         }
-        pde->pde_users++;
-        write = pde->proc_fops->write;
-        spin_unlock(&pde->pde_unload_lock);
-
-        if (write)
-                rv = write(file, buf, count, ppos);
+        return rv;
+}
 
-        pde_users_dec(pde);
+static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
+{
+        ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
+        struct proc_dir_entry *pde = PDE(file_inode(file));
+        ssize_t rv = -EIO;
+        if (use_pde(pde)) {
+                write = pde->proc_fops->write;
+                if (write)
+                        rv = write(file, buf, count, ppos);
+                unuse_pde(pde);
+        }
         return rv;
 }
 
@@ -227,20 +269,12 @@ static unsigned int proc_reg_poll(struct file *file, struct poll_table_struct *p
         struct proc_dir_entry *pde = PDE(file_inode(file));
         unsigned int rv = DEFAULT_POLLMASK;
         unsigned int (*poll)(struct file *, struct poll_table_struct *);
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+        if (use_pde(pde)) {
+                poll = pde->proc_fops->poll;
+                if (poll)
+                        rv = poll(file, pts);
+                unuse_pde(pde);
         }
-        pde->pde_users++;
-        poll = pde->proc_fops->poll;
-        spin_unlock(&pde->pde_unload_lock);
-
-        if (poll)
-                rv = poll(file, pts);
-
-        pde_users_dec(pde);
         return rv;
 }
 
@@ -249,20 +283,12 @@ static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigne
         struct proc_dir_entry *pde = PDE(file_inode(file));
         long rv = -ENOTTY;
         long (*ioctl)(struct file *, unsigned int, unsigned long);
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+        if (use_pde(pde)) {
+                ioctl = pde->proc_fops->unlocked_ioctl;
+                if (ioctl)
+                        rv = ioctl(file, cmd, arg);
+                unuse_pde(pde);
         }
-        pde->pde_users++;
-        ioctl = pde->proc_fops->unlocked_ioctl;
-        spin_unlock(&pde->pde_unload_lock);
-
-        if (ioctl)
-                rv = ioctl(file, cmd, arg);
-
-        pde_users_dec(pde);
         return rv;
 }
 
@@ -272,20 +298,12 @@ static long proc_reg_compat_ioctl(struct file *file, unsigned int cmd, unsigned
         struct proc_dir_entry *pde = PDE(file_inode(file));
         long rv = -ENOTTY;
         long (*compat_ioctl)(struct file *, unsigned int, unsigned long);
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+        if (use_pde(pde)) {
+                compat_ioctl = pde->proc_fops->compat_ioctl;
+                if (compat_ioctl)
+                        rv = compat_ioctl(file, cmd, arg);
+                unuse_pde(pde);
         }
-        pde->pde_users++;
-        compat_ioctl = pde->proc_fops->compat_ioctl;
-        spin_unlock(&pde->pde_unload_lock);
-
-        if (compat_ioctl)
-                rv = compat_ioctl(file, cmd, arg);
-
-        pde_users_dec(pde);
         return rv;
 }
 #endif
@@ -295,20 +313,12 @@ static int proc_reg_mmap(struct file *file, struct vm_area_struct *vma)
         struct proc_dir_entry *pde = PDE(file_inode(file));
         int rv = -EIO;
         int (*mmap)(struct file *, struct vm_area_struct *);
-
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
-                return rv;
+        if (use_pde(pde)) {
+                mmap = pde->proc_fops->mmap;
+                if (mmap)
+                        rv = mmap(file, vma);
+                unuse_pde(pde);
         }
-        pde->pde_users++;
-        mmap = pde->proc_fops->mmap;
-        spin_unlock(&pde->pde_unload_lock);
-
-        if (mmap)
-                rv = mmap(file, vma);
-
-        pde_users_dec(pde);
         return rv;
 }
 
@@ -334,16 +344,12 @@ static int proc_reg_open(struct inode *inode, struct file *file)
         if (!pdeo)
                 return -ENOMEM;
 
-        spin_lock(&pde->pde_unload_lock);
-        if (!pde->proc_fops) {
-                spin_unlock(&pde->pde_unload_lock);
+        if (!use_pde(pde)) {
                 kfree(pdeo);
                 return -ENOENT;
         }
-        pde->pde_users++;
         open = pde->proc_fops->open;
         release = pde->proc_fops->release;
-        spin_unlock(&pde->pde_unload_lock);
 
         if (open)
                 rv = open(inode, file);
@@ -351,10 +357,8 @@ static int proc_reg_open(struct inode *inode, struct file *file)
         spin_lock(&pde->pde_unload_lock);
         if (rv == 0 && release) {
                 /* To know what to release. */
-                pdeo->inode = inode;
                 pdeo->file = file;
                 /* Strictly for "too late" ->release in proc_reg_release(). */
-                pdeo->release = release;
                 list_add(&pdeo->lh, &pde->pde_openers);
         } else
                 kfree(pdeo);
@@ -364,12 +368,12 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 }
 
 static struct pde_opener *find_pde_opener(struct proc_dir_entry *pde,
-                                        struct inode *inode, struct file *file)
+                                        struct file *file)
 {
         struct pde_opener *pdeo;
 
         list_for_each_entry(pdeo, &pde->pde_openers, lh) {
-                if (pdeo->inode == inode && pdeo->file == file)
+                if (pdeo->file == file)
                         return pdeo;
         }
         return NULL;
@@ -383,8 +387,8 @@ static int proc_reg_release(struct inode *inode, struct file *file)
         struct pde_opener *pdeo;
 
         spin_lock(&pde->pde_unload_lock);
-        pdeo = find_pde_opener(pde, inode, file);
-        if (!pde->proc_fops) {
+        pdeo = find_pde_opener(pde, file);
+        if (pde->pde_users < 0) {
                 /*
                  * Can't simply exit, __fput() will think that everything is OK,
                  * and move on to freeing struct file. remove_proc_entry() will
@@ -396,7 +400,7 @@ static int proc_reg_release(struct inode *inode, struct file *file)
                 if (pdeo) {
                         list_del(&pdeo->lh);
                         spin_unlock(&pde->pde_unload_lock);
-                        rv = pdeo->release(inode, file);
+                        rv = pde->proc_fops->release(inode, file);
                         kfree(pdeo);
                 } else
                         spin_unlock(&pde->pde_unload_lock);
@@ -413,7 +417,7 @@ static int proc_reg_release(struct inode *inode, struct file *file)
         if (release)
                 rv = release(inode, file);
 
-        pde_users_dec(pde);
+        unuse_pde(pde);
         return rv;
 }
 
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 9c93a53f371d..c43d536f93b9 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -151,12 +151,13 @@ int proc_readdir_de(struct proc_dir_entry *de, struct file *filp, void *dirent,
                 filldir_t filldir);
 
 struct pde_opener {
-        struct inode *inode;
         struct file *file;
-        int (*release)(struct inode *, struct file *);
         struct list_head lh;
 };
-void pde_users_dec(struct proc_dir_entry *pde);
+
+ssize_t __proc_file_read(struct file *, char __user *, size_t, loff_t *);
+extern const struct file_operations proc_file_operations;
+void proc_entry_rundown(struct proc_dir_entry *);
 
 extern spinlock_t proc_subdir_lock;
 
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index bcc0e10ef1df..947ae7eb63ef 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -60,20 +60,13 @@ struct proc_dir_entry {
         kgid_t gid;
         loff_t size;
         const struct inode_operations *proc_iops;
-        /*
-         * NULL ->proc_fops means "PDE is going away RSN" or
-         * "PDE is just created". In either case, e.g. ->read_proc won't be
-         * called because it's too late or too early, respectively.
-         *
-         * If you're allocating ->proc_fops dynamically, save a pointer
-         * somewhere.
-         */
         const struct file_operations *proc_fops;
         struct proc_dir_entry *next, *parent, *subdir;
         void *data;
         read_proc_t *read_proc;
         atomic_t count;         /* use count */
-        int pde_users;  /* number of callers into module in progress */
+        int pde_users;  /* number of callers into module in progress; */
+                        /* negative -> it's going away RSN */
         struct completion *pde_unload_completion;
         struct list_head pde_openers;   /* who did ->open, but not ->release */
         spinlock_t pde_unload_lock; /* proc_fops checks and pde_users bumps */