On Tuesday, July 31, 2012 2:25:01 pm John Baldwin wrote:
> Author: jhb
> Date: Tue Jul 31 18:25:00 2012
> New Revision: 238952
> URL: http://svn.freebsd.org/changeset/base/238952
>
> Log:
> Reorder the managament of advisory locks on open files so that the advisory
> lock is obtained before the write count is increased during open() and the
> lock is released after the write count is decreased during close().
>
> The first change closes a race where an open() that will block with O_SHLOCK
> or O_EXLOCK can increase the write count while it waits. If the process
> holding the current lock on the file then tries to call exec() on the file
> it has locked, it can fail with ETXTBUSY even though the advisory lock is
> preventing other threads from succesfully completeing a writable open().
>
> The second change closes a race where a read-only open() with O_SHLOCK or
> O_EXLOCK may return successfully while the write count is non-zero due to
> another descriptor that had the advisory lock and was blocking the open()
> still being in the process of closing. If the process that completed the
> open() then attempts to call exec() on the file it locked, it can fail with
> ETXTBUSY even though the other process that held a write lock has closed
> the file and released the lock.
>
> Reviewed by: kib
> MFC after: 1 month
Oops, should have included:
Tested by: pho
I have a regression test (of sorts) for this. If you run it on an unpatched
system it should start emitting errors within a few seconds.
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static void
usage(void)
{
fprintf(stderr, "Usage: flock_close_race <binary> [args]\n");
exit(1);
}
static void
child(const char *binary)
{
int fd;
/* Exit as soon as our parent exits. */
while (getppid() != 1) {
fd = open(binary, O_RDWR | O_EXLOCK);
if (fd < 0) {
/*
* This may get ETXTBSY since exit() will
* close its open fd's (thus releasing the
* lock), before it releases the vmspace (and
* mapping of the binary).
*/
if (errno == ETXTBSY)
continue;
err(1, "can't open %s", binary);
}
close(fd);
}
exit(0);
}
static void
exec_child(char **av)
{
int fd;
fd = open(av[0], O_RDONLY | O_SHLOCK);
execv(av[0], av);
err(127, "execv");
}
int
main(int ac, char **av)
{
struct stat sb;
pid_t pid;
if (ac < 2)
usage();
if (stat(av[1], &sb) != 0)
err(1, "stat(%s)", av[1]);
if (!S_ISREG(sb.st_mode))
errx(1, "%s not an executable", av[1]);
pid = fork();
if (pid < 0)
err(1, "fork");
if (pid == 0)
child(av[1]);
for (;;) {
pid = fork();
if (pid < 0)
err(1, "fork");
if (pid == 0)
exec_child(av + 1);
wait(NULL);
}
return (0);
}
--
John Baldwin
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