[PATCH -next V2] proc: support file->f_pos checking in mem_lseek

[WoZ1zh1]

In mem_lseek, file->f_pos may overflow. And it's not a problem that
mem_open set file mode with FMODE_UNSIGNED_OFFSET(memory_lseek). However,
another file use mem_lseek do lseek can have not FMODE_UNSIGNED_OFFSET
(kpageflags_proc_ops/proc_pagemap_operations...), so in order to prevent
file->f_pos updated to an abnormal number, fix it by checking overflow and
FMODE_UNSIGNED_OFFSET.

Signed-off-by: WoZ1zh1 <wozizhi@xxxxxxxxxx>
---
 fs/proc/base.c     | 30 ++++++++++++++++++++++--------
 fs/read_write.c    |  5 -----
 include/linux/fs.h |  5 ++++-
 3 files changed, 26 insertions(+), 14 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index dd31e3b6bf77..0fd986e861d9 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -903,18 +903,32 @@ static ssize_t mem_write(struct file *file, const char __user *buf,
 
 loff_t mem_lseek(struct file *file, loff_t offset, int orig)
 {
+	loff_t ret = 0;
+
+	spin_lock(&file->f_lock);
 	switch (orig) {
-	case 0:
-		file->f_pos = offset;
-		break;
-	case 1:
-		file->f_pos += offset;
+	case SEEK_CUR:
+		offset += file->f_pos;
+	case SEEK_SET:
+		/* to avoid userland mistaking f_pos=-9 as -EBADF=-9 */
+		if ((unsigned long long)offset >= -MAX_ERRNO)
+			ret = -EOVERFLOW;
 		break;
 	default:
-		return -EINVAL;
+		ret = -EINVAL;
 	}
-	force_successful_syscall_return();
-	return file->f_pos;
+	if (!ret) {
+		if (offset < 0 && !(unsigned_offsets(file))) {
+			ret = -EINVAL;
+		} else {
+			file->f_pos = offset;
+			ret = file->f_pos;
+			force_successful_syscall_return();
+		}
+	}
+
+	spin_unlock(&file->f_lock);
+	return ret;
 }
 
 static int mem_release(struct inode *inode, struct file *file)
diff --git a/fs/read_write.c b/fs/read_write.c
index 4771701c896b..2f456d5a1df5 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -34,11 +34,6 @@ const struct file_operations generic_ro_fops = {
 
 EXPORT_SYMBOL(generic_ro_fops);
 
-static inline bool unsigned_offsets(struct file *file)
-{
-	return file->f_mode & FMODE_UNSIGNED_OFFSET;
-}
-
 /**
  * vfs_setpos - update the file offset for lseek
  * @file:	file structure in question
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 98b7a7a8c42e..dde0756d2350 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2994,7 +2994,10 @@ extern ssize_t iter_file_splice_write(struct pipe_inode_info *,
 extern long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
 		loff_t *opos, size_t len, unsigned int flags);
 
-
+static inline bool unsigned_offsets(struct file *file)
+{
+	return file->f_mode & FMODE_UNSIGNED_OFFSET;
+}
 extern void
 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
 extern loff_t noop_llseek(struct file *file, loff_t offset, int whence);
-- 
2.39.2