proc: store cookie in private data

Store the cookie to detect concurrent seeks on directories in
file->private_data.

Link: https://lore.kernel.org/r/20240830-vfs-file-f_version-v1-14-6d3e4816aa7b@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 72a1acd03675cc77da7320a14426f40e6fc9513f..988f6bbfc4bdf33480493b98b05558deb90f06d4 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3870,12 +3870,12 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
        if (!dir_emit_dots(file, ctx))
                return 0;
 
-       /* f_version caches the tgid value that the last readdir call couldn't
-        * return. lseek aka telldir automagically resets f_version to 0.
+       /* We cache the tgid value that the last readdir call couldn't
+        * return and lseek resets it to 0.
         */
        ns = proc_pid_ns(inode->i_sb);
-       tid = (int)file->f_version;
-       file->f_version = 0;
+       tid = (int)(intptr_t)file->private_data;
+       file->private_data = NULL;
        for (task = first_tid(proc_pid(inode), tid, ctx->pos - 2, ns);
             task;
             task = next_tid(task), ctx->pos++) {
@@ -3890,7 +3890,7 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
                                proc_task_instantiate, task, NULL)) {
                        /* returning this tgid failed, save it as the first
                         * pid for the next readir call */
-                       file->f_version = (u64)tid;
+                       file->private_data = (void *)(intptr_t)tid;
                        put_task_struct(task);
                        break;
                }
@@ -3915,6 +3915,24 @@ static int proc_task_getattr(struct mnt_idmap *idmap,
        return 0;
 }
 
+/*
+ * proc_task_readdir() set @file->private_data to a positive integer
+ * value, so casting that to u64 is safe. generic_llseek_cookie() will
+ * set @cookie to 0, so casting to an int is safe. The WARN_ON_ONCE() is
+ * here to catch any unexpected change in behavior either in
+ * proc_task_readdir() or generic_llseek_cookie().
+ */
+static loff_t proc_dir_llseek(struct file *file, loff_t offset, int whence)
+{
+       u64 cookie = (u64)(intptr_t)file->private_data;
+       loff_t off;
+
+       off = generic_llseek_cookie(file, offset, whence, &cookie);
+       WARN_ON_ONCE(cookie > INT_MAX);
+       file->private_data = (void *)(intptr_t)cookie; /* serialized by f_pos_lock */
+       return off;
+}
+
 static const struct inode_operations proc_task_inode_operations = {
        .lookup         = proc_task_lookup,
        .getattr        = proc_task_getattr,
@@ -3925,7 +3943,7 @@ static const struct inode_operations proc_task_inode_operations = {
 static const struct file_operations proc_task_operations = {
        .read           = generic_read_dir,
        .iterate_shared = proc_task_readdir,
-       .llseek         = generic_file_llseek,
+       .llseek         = proc_dir_llseek,
 };
 
 void __init set_proc_pid_nlink(void)