Re: SECURITY.NNOV: Multiple applications fd_set structure bitmap

[Damien Miller]

David LeBlanc wrote:
> if (__i == ((fd_set FAR *)(set))->fd_count) { \
> if (((fd_set FAR *)(set))->fd_count < FD_SETSIZE) { \
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ((fd_set FAR *)(set))->fd_array[__i] = (fd); \
> ((fd_set FAR *)(set))->fd_count++; \
> } \
> } \
> } while(0)
>
> So if you attempted to put FD_SETSIZE + 1 sockets into an fd_set, it
> would just fail.

This effectively limits select to a maximum of FD_SETSIZE descriptors on
Windows. I don't think that this limitiation exists on other platforms.

Correctly written programs dynamically allocate their FD_SETs to avoid
these problems (or they use poll or some other mechanism instead).

-d

[Lee Dilkie]

David LeBlanc wrote:

>
>
>-----Original Message-----
>From: Damien Miller [mailto:djm@mindrot.org] said:
>
>
>
>>This effectively limits select to a maximum of FD_SETSIZE descriptors
>>
>>
>on Windows. I don't think that this limitiation exists on other
>platforms.
>
>---------------------------
>
>Note the bit where it says:
>
>#ifndef FD_SETSIZE
>#define FD_SETSIZE 64
>#endif /* FD_SETSIZE */
>
>So to make FD_SETSIZE any arbitrarily large value up to whatever your
>system can handle, you just redefine FD_SETSIZE before you #include
>winsock.h.
>
>
>
Something you can't do in linux, is enlarge FD_SETSIZE.

from linux/posix_types/h:
#undef __FD_SETSIZE
#define __FD_SETSIZE 1024

Well, you *can* change it, but it requires a recompile of the kernel and
all userland programs that create an fd_set.

In this regard, windows did get it right. However, the earlier comment
on using the windows async sockets is spot on, if you want performance.
Windows fd_set's are structured more like unix poll() arrays (un-ordered
array of fd's) and are not very efficient if there are many sockets on
one set. Also, a linux fd_set limits the fd *value* to < 1024, not just
the number of fd's in the set. So it's possible to only want to put one
fd on a fd_set but be unable to do so if it's value is > FD_SETSIZE
(which can be done by increasing the maximum number of file handles a
process is permitted to open).

-lee