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

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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>
This commit is contained in:
Alain Knaff 2008-11-10 17:08:08 -08:00 committed by Al Viro
parent 7d3b56ba37
commit 5b6f1eb97d
1 changed files with 13 additions and 0 deletions
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13
fs/read_write.c
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@ -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;
}