vfs: lseek(fd, 0, SEEK_CUR) race condition

This patch fixes a race condition in lseek. While it is expected that
unpredictable behaviour may result while repositioning the offset of a
file descriptor concurrently with reading/writing to the same file
descriptor, this should not happen when merely *reading* the file
descriptor's offset.

Unfortunately, the only portable way in Unix to read a file
descriptor's offset is lseek(fd, 0, SEEK_CUR); however executing this
concurrently with read/write may mess up the position.

[with fixes from akpm]

Signed-off-by: Alain Knaff <alain@knaff.lu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 13 insertions, 0 deletions

diff --git a/fs/read_write.c b/fs/read_write.c
index 969a6d9c020b..5cc6924eb158 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -50,6 +50,14 @@ generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
                 offset += inode->i_size;
                 break;
         case SEEK_CUR:
+                /*
+                 * Here we special-case the lseek(fd, 0, SEEK_CUR)
+                 * position-querying operation.  Avoid rewriting the "same"
+                 * f_pos value back to the file because a concurrent read(),
+                 * write() or lseek() might have altered it
+                 */
+                if (offset == 0)
+                        return file->f_pos;
                 offset += file->f_pos;
                 break;
         }
@@ -105,6 +113,10 @@ loff_t default_llseek(struct file *file, loff_t offset, int origin)
                         offset += i_size_read(file->f_path.dentry->d_inode);
                         break;
                 case SEEK_CUR:
+                        if (offset == 0) {
+                                retval = file->f_pos;
+                                goto out;
+                        }
                         offset += file->f_pos;
         }
         retval = -EINVAL;
@@ -115,6 +127,7 @@ loff_t default_llseek(struct file *file, loff_t offset, int origin)
                 }
                 retval = offset;
         }
+out:
         unlock_kernel();
         return retval;
 }