Re: [Full-disclosure] Re: readdir_r considered harmful

From: Andrew Farmer (andfarm_at_gmail.com)
Date: 11/07/05

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    Date: Sun, 6 Nov 2005 17:00:48 -0800
To: full-disclosure@lists.grok.org.uk

    On 06 Nov 05, at 01:00, Casper.***@Sun.COM wrote:
    >> Then you never really understood the implementation, seems. Of
    >> course
    >> all implementations keep the content of the directory as read with
    >> getdents or so in the DIR descriptor. But it is usually not the case
    >> that the whole content fits into the buffer allocated. One could, of
    >> course, resize the buffer to fit the content of the directory read,
    >> even if this means reserving hundreds or thousands of kBs. But this
    >> is not how most implementations work.
    >>
    >
    > I don't see how that is relevant; the typical use of readdir() is
    > as follows:
    >
    > DIR *dirp = opendir(name);
    >
    > while ((dent = readdir(dirp)) != NULL) {
    > ...
    > }
    >
    > closedir(dirp);
    >
    > Nothing other threads do with readdir() on different dirp's will
    > influence
    > what "dent" points to.
    >
    > I have *never* seen a program where multiple threads read from a
    > single
    > dirp; and I can't image the use.
    >

    In practice, you're correct. In theory, however, consider the
    following code
    path.

    > THREAD 1 THREAD 2
    > ------------------------------ ------------------------------
    > DIR *d1 = opendir(dir1);
    > DIR *d2 = opendir(dir2);
    > dent1 = readdir(dir1);
    > dent2 = readdir(dir2);
    > use(dent1);
    >

    In most implementations, dent1 != dent2. HOWEVER, there is no
    guarantee that
    they will not both point to the same statically allocated buffer, and
    some
    implementations may do so. For example, this is why ctime_r exists:
    ctime
    returns a pointer to a statically allocated buffer, and hence is not
    thread
    safe.

    You are correct, though, that the glibc implementation of readdir is
    thread-safe, so readdir_r is unnecessary in all common situations.

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