Theodore Tso wrote:
You could, but then you're succeptible to a memory allocation attack.
If you have an arbitrarily large directory (say, one with multiple
millions of entries), and the attacker program calls seekdir() after
every single readdir() call, you would then force the kernel to
allocate and then pin arbitrarily large amounts of memory, which as
you point out, as currently specified by the POSIX specification, you
are not allowed to release until closedir().
This could be done in userspace, by forcing glibc to readdir() the
entire directory into memory, at which point seekdir()/telldir() will
work just fine. But for a really big directory, this could consume a
huge amount of space.
If we had the 64-byte telldir cookie that I had proposed, then in
userspace we could simply associate that 64-byte telldir cookie with a
small 32-bit integer, either in memory, or in some berkdb or tdb
interface, at least until the use of telldir/seekdir had actually
disappeared. (Which probably wouldn't take that long; I really doubt
there are that many users of it out there, so it's probably OK if they
suffer a performance penality if they use this really wretched and
horrible interface.)
If you want to have a large cookies, you could have glibc allocate a
memory block to store it, and have glibc responsible for keeping track
of it. As far as I know, off_t can hold a pointer on all our
implementations (only 32-bit machines have 32-bit off_t as an option;
Alpha *might* be an exception but I don't think so.)
I'll also note, by the way, that there are those who have been much
more cavalier with breaking the wireless interface or the udev/sys
interface after one year. Not that I would agree with that, but over
some deprecation period measured in years, I think it is possible to
nuke what was a horribly misguided interface that should have never
existed. Whoever invented it really should receive the brown paper
award for one of the worst design decisions of all time.
Readdir/telldir are much, much, more fundamental than that. We're
talking interfaces which have been standardized for longer than Linux
itself has existed.
-hpa
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
