On Mon 07-04-14 20:16:27, Heinrich Schuchardt wrote:
> Hello Jan,
>
> >> FAN_ACCESS_PERM
> >> The name is misleading. The permission check is done before executing a file
> >> or listing a directory.
> > AFAIK it doesn't happen before listing a directory. But if you are
> >convinced it does, I can check more carefully ;)
>
> iterate_dir calls
> security_file_permission(file, MAY_READ);
> which in turn calls
> fsnotify_perm(file, mask);
> Which has these lines:
> 51 else if (mask & MAY_READ)
> 52 fsnotify_mask = FS_ACCESS_PERM;
Ah, OK, for FAN_ACCESS_PERM. But as you note below FAN_ACCESS isn't
generated for directories. That actually looks pretty inconsistent and I
guess we should fix that.
> This is the result I get for
> #include <stdlib.h>
> #include <sys/types.h>
> #include <dirent.h>
> #include <libgen.h>
> int main() {
> DIR *dir;
> struct dirent *dp;
> if ((dir = opendir ("/home/test")) == NULL) {
> exit (1);
> }
> while ((dp = readdir (dir)) != NULL) {
> }
> closedir(dir);
> return 0;
> }
>
> FAN_OPEN: File /home/test
> FAN_ACCESS_PERM: File /home/test
> FAN_ACCESS_PERM: File /home/test
>
> I guess it is somewhat inconsistent that
> readdir results in FAN_ACCESS_PERM while
> read results in FAN_ACCESS_PERM and FAN_ACCESS.
>
> ===
>
> This is the result I get for executing
> /home/test/test
> which contains the following lines
> !#/bin/sh
> echo Hello world
>
> FAN_OPEN: File /home/test/test
> FAN_ACCESS_PERM: File /home/test/test
> FAN_ACCESS: File /home/test/test
> FAN_OPEN: File /home/test/test
> FAN_ACCESS_PERM: File /home/test/test
> FAN_ACCESS: File /home/test/test
> FAN_ACCESS_PERM: File /home/test/test
>
> The last FAN_ACCESS_PERM probably does not relate to a read() as it
> is not followed by FAN_ACCESS.
>
> ===
>
> When running
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> int main(int argc, char *argv[]) {
> int fd;
> char buf[4096];
> ssize_t len;
> fd = open("/home/test/test", O_RDONLY);
> len = read(fd, buf, 4096);
> buf[len] = '\0';
> printf("Bytes read %d\n%s\n", len, buf);
> close(fd);
> return 0;
> }
>
> I get
>
> FAN_OPEN: File /home/test/test
> FAN_ACCESS_PERM: File /home/test/test
> FAN_ACCESS: File /home/test/test
>
> When running
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
>
> int main(int argc, char *argv[]) {
> int fd;
> char buf[4096];
> ssize_t len;
> fd = open("/home/test/test", O_WRONLY);
> len = write(fd, "visited" , 4);
> close(fd);
> return 0;
> }
>
> I get
>
> FAN_OPEN: File /home/test/test
> FAN_MODIFY: File /home/test/test
>
> You only receive a permission event before reading and not before writing.
>
> The fanotify API was meant for use in a hierarchical storage
> management system. Should it not be able to inhibit writing, e.g. if
> the remote file is on a read-only medium?
>
> I guess it would make sense to add a FAN_MODIFY_PERM event.
Well, there were different uses for fanotify() (antivirus scanners for
file servers for windows workstations was the one I remember). Storage
management might be one of those. If there's a real demand for
FAN_MODIFY_PERM, I guess we can add that. But I wouldn't like to add that
just because it sounds useful...
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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