On 03/14/2017 12:11 PM, Matthew Wilcox wrote:
Hello Matthew :-)
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.
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.
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)
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.
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.
I'm including Michael Kerrisk here if he has any comments since he
touched the manual page several times ;-)
--
Cheers,
Carlos.