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Subject[PATCH 3/7] VFS: Make generic lseek lockless safe
Date
From: Andi Kleen <ak@linux.intel.com>

- use f_lock to protect SEEK_CUR
- use i_size_read to safely read file sizes on 32bit

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
fs/read_write.c | 52 +++++++++++++++++++++++++++++++++++-----------------
include/linux/fs.h | 3 ++-
2 files changed, 37 insertions(+), 18 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 24f0001..8e8aab3 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -35,6 +35,21 @@ static inline int unsigned_offsets(struct file *file)
return file->f_mode & FMODE_UNSIGNED_OFFSET;
}

+static loff_t lseek_execute(struct file *file, struct inode *inode, loff_t offset,
+ loff_t maxsize)
+{
+ if (offset < 0 && !unsigned_offsets(file))
+ return -EINVAL;
+ if (offset > maxsize)
+ return -EINVAL;
+
+ if (offset != file->f_pos) {
+ file->f_pos = offset;
+ file->f_version = 0;
+ }
+ return offset;
+}
+
/**
* generic_file_llseek - generic llseek implementation for regular files
* @file: file structure to seek on
@@ -44,6 +59,12 @@ static inline int unsigned_offsets(struct file *file)
* This is a generic implemenation of ->llseek useable for all normal local
* filesystems. It just updates the file offset to the value specified by
* @offset and @origin under i_mutex.
+ *
+ * Synchronization:
+ * SEEK_SET is unsynchronized (but atomic on 64bit platforms)
+ * SEEK_CUR is synchronized against other SEEK_CURs, but not read/writes.
+ * read/writes behave like SEEK_SET against seeks.
+ * SEEK_END
*/
loff_t
generic_file_llseek(struct file *file, loff_t offset, int origin)
@@ -63,14 +84,22 @@ generic_file_llseek(struct file *file, loff_t offset, int origin)
*/
if (offset == 0)
return file->f_pos;
- offset += file->f_pos;
- break;
+ /*
+ * f_lock protects against read/modify/write race with other
+ * SEEK_CURs. Note that parallel writes and reads behave
+ * like SEEK_SET.
+ */
+ spin_lock(&file->f_lock);
+ offset = lseek_execute(file, inode, file->f_pos + offset,
+ inode->i_sb->s_maxbytes);
+ spin_unlock(&file->f_lock);
+ return offset;
case SEEK_DATA:
/*
* In the generic case the entire file is data, so as long as
* offset isn't at the end of the file then the offset is data.
*/
- if (offset >= inode->i_size)
+ if (offset >= i_size_read(inode))
return -ENXIO;
break;
case SEEK_HOLE:
@@ -78,24 +107,13 @@ generic_file_llseek(struct file *file, loff_t offset, int origin)
* There is a virtual hole at the end of the file, so as long as
* offset isn't i_size or larger, return i_size.
*/
- if (offset >= inode->i_size)
+ if (offset >= i_size_read(inode))
return -ENXIO;
- offset = inode->i_size;
+ offset = i_size_read(inode);
break;
}

- if (offset < 0 && !unsigned_offsets(file))
- return -EINVAL;
- if (offset > inode->i_sb->s_maxbytes)
- return -EINVAL;
-
- /* Special lock needed here? */
- if (offset != file->f_pos) {
- file->f_pos = offset;
- file->f_version = 0;
- }
-
- return offset;
+ return lseek_execute(file, inode, offset, inode->i_sb->s_maxbytes);
}
EXPORT_SYMBOL(generic_file_llseek);

diff --git a/include/linux/fs.h b/include/linux/fs.h
index d9efdf7..fdcf2eb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -965,7 +965,8 @@ struct file {
#define f_dentry f_path.dentry
#define f_vfsmnt f_path.mnt
const struct file_operations *f_op;
- spinlock_t f_lock; /* f_ep_links, f_flags, no IRQ */
+ spinlock_t f_lock; /* f_ep_links, f_flags, no IRQ,
+ SEEK_SET */
#ifdef CONFIG_SMP
int f_sb_list_cpu;
#endif
--
1.7.4.4


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