Re: [Caml-list] Re: Unix.getlogin () fails when stdin is redirected

[Till Varoquaux <till@pps.jussieu.fr>]

As far as I can tell getpwuid(geteuid()) is what the (deprecated)
cuserid function does. The linux man page explains the difference
between the two functions like this:

  _ These functions let your program identify positively the user who
is running (cuserid()) or the user who logged in this session
(getlogin()). (These can differ when set-user-ID programs are
involved.)

I expect there to be more points were the behaviours of those two
functions diverge (e.g.: sudo, deamons etc...).

In general I am very skeptic any time I see a one liner that is sold
as a working alternative to a glibc function. The glibc has issues and
pitfalls but they tend to be very well documented. "Better the devil
you know"...

Till

On Tue, Mar 20, 2012 at 3:41 PM, Yaron Minsky <yminsky@gmail.com> wrote:
> Is there a concrete difference in behavior you're concerned about?
>
> y
>
>
> On Tue, Mar 20, 2012 at 2:48 PM, Till Varoquaux <till@pps.jussieu.fr> wrote:
>>
>> getpwuid(getuid()) is not a synonym for get_login (refer to the
>> discussion in the POSIX standard[^1]). You should not shadow posix
>> functions by functions with different semantics in the Unix modules;
>> providing your own abstraction over the OS is a commendable goal but
>> you should do so without silently bypassing core functions.
>>
>> Till
>> [1]:http://pubs.opengroup.org/onlinepubs/007904975/functions/getlogin.html
>>
>>
>> On Tue, Mar 20, 2012 at 2:28 PM, David House <dhouse@janestreet.com>
>> wrote:
>> > Note that Jane Street's core library [1] does not use getlogin(3) in its
>> > replacement Unix module, for exactly this reason:
>> >
>> > (* The standard getlogin function goes through utmp which is unreliable,
>> >  see the BUGS section of getlogin(3) *)
>> > let getlogin_orig = Unix.getlogin
>> > let getlogin () = (Unix.getpwuid (getuid ())).Unix.pw_name
>> >
>> > [1]: https://bitbucket.org/yminsky/ocaml-core/wiki/Home
>> >
>> > I just tested your specific example, and it worked fine.
>> >
>> >
>> > On Tue 20 Mar 2012 06:07:59 PM GMT, Ricardo Catalinas Jiménez wrote:
>> >>
>> >> On Tue, Mar 20, 2012 at 06:51:13PM +0100, Ricardo Catalinas Jiménez
>> >> wrote:
>> >>>
>> >>> I found out the next issue in this simple code:
>> >>>
>> >>> let () =
>> >>>     print_endline "Hello";
>> >>>     print_endline (Unix.getlogin ())
>> >>>
>> >>> Running in the normal case, with `./a.out' gives:
>> >>>
>> >>> Hello
>> >>> ricardo
>> >>>
>> >>> But running like `./a.out</dev/null' makes Unix.getlogin fail:
>> >>>
>> >>> Hello
>> >>> Fatal error: exception Unix.Unix_error(20, "getlogin", "")
>> >>>
>> >>> A simple strace reveals the problem:
>> >>>
>> >>>    open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
>> >>>    fstat(3, {st_mode=S_IFREG|0644, st_size=509, ...}) = 0
>> >>>    mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
>> >>> -1,
>> >>> 0) = 0x7fb125554000
>> >>>    read(3, "root:x:0:0:root:/root:/bin/bash\n"..., 4096) = 509
>> >>>    read(3, "", 4096)                       = 0
>> >>>    close(3)                                = 0
>> >>>    munmap(0x7fb125554000, 4096)            = 0
>> >>> ->  ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or
>> >>> TCGETS, 0x7fff12682c98) = -1 ENOTTY (Inappropriate ioctl for device)
>> >>>    write(2, "Fatal error: exception Unix.Unix"..., 59) = 59
>> >>>    exit_group(2)                           = ?
>> >>
>> >>
>> >>
>> >> Someone knew the answer, man 3 getlogin reads:
>> >>
>> >>        Note that glibc does not follow the POSIX specification and uses
>> >>        stdin instead of /dev/tty.  A bug.  (Other recent systems, like
>> >>        SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8 all return the login
>> >>        name also when stdin is redirected.)
>> >>
>> >>
>> >> Regards
>> >
>> >
>> >
>> >
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>>
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>