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authorAlain Knaff <alain@knaff.lu>2008-11-10 17:08:08 -0800
committerAl Viro <viro@zeniv.linux.org.uk>2009-01-05 11:53:07 -0500
commit5b6f1eb97d462a45be3b30759758b5fdbb562c8c (patch)
treee8f664c34dde71b95955f57caad3093581be87ef /fs
parent7d3b56ba37a95f1f370f50258ed3954c304c524b (diff)
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>
Diffstat (limited to 'fs')
-rw-r--r--fs/read_write.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/fs/read_write.c b/fs/read_write.c
index 969a6d9c020..5cc6924eb15 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;
}