Date: Mon, 10 Sep 2001 20:41:34 +0100 From: David Taylor <davidt@yadt.co.uk> To: Jonathan Lemon <jlemon@flugsvamp.com> Cc: kent@erix.ericsson.se, hackers@freebsd.org Subject: Re: How to force small TCP packets? Message-ID: <20010910204134.A65177@gattaca.yadt.co.uk> In-Reply-To: <200109101817.f8AIH7t11927@prism.flugsvamp.com>; from jlemon@flugsvamp.com on Mon, Sep 10, 2001 at 13:17:07 -0500 References: <local.mail.freebsd-hackers/d2y9nogetm.fsf@erix.ericsson.se>, <local.mail.freebsd-hackers/20010909182745.O2965@elvis.mu.org> <local.mail.freebsd-hackers/d2heubk4p2.fsf@erix.ericsson.se> <200109101817.f8AIH7t11927@prism.flugsvamp.com>
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On Mon, 10 Sep 2001, Jonathan Lemon wrote:
> One thing you might want to try doing is to write your own read()
> function and link against that. Your modified version could then
> replace the nbytes value with something smaller, for debugging purposes.
Infact, when I saw this thread I whipped up a simple .so to override the
syscalls to test a network program of my own, and it helpfully found a
obscure bug where I wasn't correctly using the return value from read()..
The main problem with the below code is that it unconditionally reads X
bytes at a time. You might want to override socket/close (and maybe dup
too, if the code does things like that) so you're only restricting syscalls
on sockets. Or make a is_socket() function that does some runtime test I
can't think of to figure out what to do...
(compile using something like:
gcc -fPIC -shared -O2 -g test.c -o test.so;
use with something like:
LD_PRELOAD=/path/to/test.so /your/program/bin/blah)
#define NBYTES 1
ssize_t read(int d, void *buf, size_t nbytes)
{
return syscall(SYS_read, d, buf, ((nbytes<NBYTES) ? nbytes : NBYTES));
}
ssize_t write(int d, const void *buf, size_t nbytes)
{
return syscall(SYS_write, d, buf, ((nbytes<NBYTES) ? nbytes : NBYTES));
}
--
David Taylor
davidt@yadt.co.uk
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