Hi Carlos,

On 03/15/2017 06:34 AM, Carlos O'Donell wrote:

On 03/14/2017 12:11 PM, Matthew Wilcox wrote:

Hello Matthew :-)
 

quoted

Quoting the manpage:

       int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout);

       nfds  is the highest-numbered file descriptor in any of the three sets,
       plus 1.

OK.

quoted

       EBADF  An  invalid file descriptor was given in one of the sets.  (Per‐
              haps a file descriptor that was already closed, or one on  which
              an error has occurred.)

That's not quite how Linux behaves.  We only check the fd_set up to the
maximum number of fds allocated to this task:

        rcu_read_lock();
        fdt = files_fdtable(current->files);
        max_fds = fdt->max_fds;
        rcu_read_unlock();
        if (n > max_fds)
                n = max_fds;

(then we copy in up to 'n' bits worth of bitmaps).

Right, and that doesn't match the POSIX requirement either.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pselect.html
~~~
The nfds argument specifies the range of descriptors to be tested. 
The first nfds descriptors shall be checked in each set; that is, 
the descriptors from zero through nfds-1 in the descriptor sets 
shall be examined.
~~~

That is to say you should "test" `nfds - 1` descriptors, potentially
returning EBADF for invalid descriptors, not some random number that
depends on how many you have open right now.

Thanks for checking POSIX!

quoted

It is pretty straightforward to demonstrate that Linux doesn't check:

int main(void)
{
	int ret;
	struct timeval tv = { };
	fd_set fds;
	FD_ZERO(&fds);
	FD_SETFD(FD_SETSIZE - 1, &fds);
	ret = select(FD_SETSIZE, &fds, NULL, NULL, &tv);
	assert(ret == -1 && errno == EBADF);
	return 0;
}

And one that compiles and works...

cat >> select-verify.c <<EOF
#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <assert.h>
#include <errno.h>

int 
main(void)
{
  int ret;
  struct timeval tv = {0, 0};
  fd_set fds;
  FD_ZERO(&fds);
  FD_SET(FD_SETSIZE - 1, &fds);
  ret = select(FD_SETSIZE, &fds, NULL, NULL, &tv);
  assert(ret == -1 && errno == EBADF);
  return 0;
}
EOF
gcc -O0 -g3 -Wall -pedantic -o ./select-verify ./select-verify.c
./select-verify 
select-verify: ./select-verify.c:19: main: Assertion `ret == -1 && errno == EBADF' failed.
Aborted (core dumped)

And thanks for the CWE.
 

quoted

Linux has behaved this way since 2.6.12, and I can't be bothered to get
out the historical git trees to find out what happened before 2005.

So ... if I change this behaviour by checking all the file descriptors, I
do stand a chance of breaking an application.  On the other hand, that
application could already have been broken by the shell deciding to open
a really high file descriptor (I'm looking at you, bash), which the program
then inherits.

The user application would have to:

(a) Use a sloppy limit for nfds e.g. `FD_SETSIZE - 1`.

_and_

(b) Have an fd selected that they were not interested in selecting.

Exactly what the test application above does.

quoted

Worth fixing this bug?  Worth documenting this bug, at least?

I would fix it on the grounds of making the interface as robust as
it was designed to be.

Right now it randomly depends on your task max_fds if it's going to
work or not. That's pretty terrible.

It's certainly at least sloppy. Of course, fixing this might
conceivably break some (broken) code. But, the violation of POSIX
suggests we should at least try a fix and see if anything does
get broken (presumably, it's rather unlikely).

I'm including Michael Kerrisk here if he has any comments since he
touched the manual page several times ;-)

Thanks ;-).

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/