Problem - Kevin Tafuro

ate on the name of the file, especially if multiple queries are required for the same file
or if you intend to open it based on the information obtained from queries. Operating
on filenames introduces the possibility of race conditions because filenames can
change between calls.
On Unix, use the fstat( ) function instead of the stat( ) function. Both functions
return the same information, but fstat( ) uses an open file descriptor while stat( )
uses a filename. Doing so removes the possibility of a race condition, because the file
to which the file descriptor points can never change unless you reopen the file
descriptor. When operating on just the filename, there is no guarantee that the
underlying file pointed to by the filename remains the same after the call to stat( ).
On Windows, use the function GetFileInformationByHandle( ) instead of functions
like FindFirstFile( ) or FindFirstFileEx( ). As with fstat( ) versus stat( ) on Unix
(which are also available on Windows if you’re using the C runtime API), the primary
difference between these functions is that one uses a file handle while the others
use filenames. If the only information you need is the size of the file, you can use
GetFileSize( ) instead of GetFileInformationByHandle( ).
Discussion
Accessing file information using filenames can lead to race conditions, particularly if
multiple queries are necessary or if you intend to open the file depending on information
previously obtained. In particular, if symbolic links are involved, an attacker
could potentially change the file to which the link points between queries or between
the time information is queried and the time the file is actually opened. This type of
race condition, known as a Time of Check, Time of Use (TOCTOU) race condition,
was also discussed in Recipe 2.3.
In most cases, when you need information about a file, such as its size, you also have
some intention of opening the file and using it in some way. For example, if you’re
checking to see whether a file exists before trying to create it, you might think to use
stat( ) or FindFirstFile( ) first, and if the function fails with an error indicating the
file does not exist, create the file with creat( ) or CreateFile( ). A better solution is
to use open( ) with the O_CREAT and O_EXCL flags, or to use CreateFile( ) with CREATE_
NEW specified as the creation disposition.
See Also
Recipe 2.3
54 | Chapter 2: Access Control
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