Re: select() call corrupts stack

[Brian Gerst]

Artiom Morozov wrote:
> 
> Hello,
> 
>         Here's a sample program. Try running it and open about 2k of
> connections to port 5222 (you'll need ulimit -n 10000 or like that). It
> will segfault. Simple asm like this
>    __asm__(
>         "pushl %eax \n\t"       "movl  0(%ebp), %eax \n\t"
>         "cmp   $65535, %eax \n\t"
>         "ja isok \n\t"
>         "xor  %eax, %eax \n\t"
>         "movl  %eax, 0(%eax) \n\t"
>         "isok: \n\t"
>         "popl  %eax \n\t"
>    );
> after each subroutine call will show you that after select() [ebp] have
> weird value. While this is unlikely to be a security flaw, i think this
> is a bug.
> 
> ps: it's okay for 1k of connections or so
> pps: kernel 2.4.17 on i686, gcc 3.0.3, glibc 2.2.3.
> 
>   ------------------------------------------------------------------------
> 
>    main.cppName: main.cpp
>            Type: C++ Source file (application/x-unknown-content-type-cppfile)
> 
>    MakefileName: Makefile
>            Type: text/x-makefile

This is not a kernel problem.  You are overflowing the size of the
fd_set structure in userspace, which has room for FD_SETSIZE file
descriptors.  The kernel smashes the user stack because you told it you
had more descriptors that you gave it room for.

--

				Brian Gerst