[Bug stdio/17830] New: dprintf fails when file descriptor is connected to /dev/kmsg

[Bug stdio/17830] New: dprintf fails when file descriptor is connected to /dev/kmsg

[mac at mcrowe dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

            Bug ID: 17830
           Summary: dprintf fails when file descriptor is connected to
                    /dev/kmsg
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: mac at mcrowe dot com

If the file descriptor passed to dprintf is connected to /dev/kmsg on a modern
Linux kernel then dprintf fails (apparently because /dev/kmsg does not support
seeking.)

Steps to reproduce:

1. Run a Linux kernel with new-style /dev/kmsg (I was running Debian Jessie's
v3.16 kernel but others as old as v3.8 show the same symptom.)

2. Compile the following code:

 #include <stdio.h>
 int main()
 {
     int r = dprintf(1, "test\n");
     fprintf(stderr, "Result: %d %m\n", r);
     return 0;
 }

3. Run ./a.out and the result is as expected

 test
 Result: 5 Success

4. Run "./a.out >/dev/kmsg" and dprintf fails:

 Result: -1 Bad file descriptor

 and no output appears in the kernel log.

Expected result:

 dprintf should work just as well on /dev/kmsg as anything else.


Reproduced in Debian Jessie's glibc v2.19 and glibc master at
56cf2763819d2f721c98f2b8bcc04a3c673837d3.

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[mac at mcrowe dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

Mike Crowe <mac at mcrowe dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|2.21                        |2.19

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
I think this is a bug in the /dev/kmsg driver.  It should return ESPIPE instead
of EBADF.

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
The problem appears to be that devkmsg_open returns without allocating a
devkmsg_user struct if opened with O_WRONLY, which causes devkmsg_llseek to
return EBADF, even though the file is properly opened.  Please report that to
the kernel devs.

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[mac at mcrowe dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #3 from Mike Crowe <mac at mcrowe dot com> ---
(In reply to Andreas Schwab from comment #2)
> The problem appears to be that devkmsg_open returns without allocating a
> devkmsg_user struct if opened with O_WRONLY, which causes devkmsg_llseek to
> return EBADF, even though the file is properly opened.  Please report that
> to the kernel devs.

I think that's intentional. A write-only /dev/kmsg is intended to be
lighter-weight than a readable one.

What seems to be wrong is unconditionally returning EBADF in that situation.
ESPIPE would presumably be better.

Even if the device were opened for reading too and we got past that point I
think dprintf would still get upset since SEEK_CUR isn't handled so
devkmsg_llseek would yield EINVAL.

I think both cases need fixing. I'll try posting a kernel patch.

Thanks for your help.

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[mac at mcrowe dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #4 from Mike Crowe <mac at mcrowe dot com> ---
(In reply to Mike Crowe from comment #3)
> Even if the device were opened for reading too and we got past that point I
> think dprintf would still get upset since SEEK_CUR isn't handled so
> devkmsg_llseek would yield EINVAL.

This case can be reproduced with "./a.out 1<>/dev/kmsg" which yields:

 Result: -1 Invalid argument

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[mac at mcrowe dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #5 from Mike Crowe <mac at mcrowe dot com> ---
(In reply to Andreas Schwab from comment #2)
> Please report that to the kernel devs.

http://lkml.iu.edu/hypermail/linux/kernel/1501.1/05813.html

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[bugdal at aerifal dot cx]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugdal at aerifal dot cx

--- Comment #6 from Rich Felker <bugdal at aerifal dot cx> ---
Isn't it something of a bug (performance, if nothing else) that dprintf is
making useless seek attempts on the file descriptor?

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #7 from Andreas Schwab <schwab@linux-m68k.org> ---
Are they useless?

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[Bug stdio/17830] dprintf fails when file descriptor is connected to /dev/kmsg

[bugdal at aerifal dot cx]

https://sourceware.org/bugzilla/show_bug.cgi?id=17830

--- Comment #8 from Rich Felker <bugdal at aerifal dot cx> ---
Yes. Presumably the seek is part of setting up the general-case FILE for
glibc's libio stdio ops to know about, but it's completely irrelevant to a
single write-only operation on a temporary FILE structure. dprintf should be
operating more like the virtual FILEs used for snprintf, fmemopen, fopencookie,
etc. rather than using the same ops as fopen so it doesn't pull in all this
overhead to every write.

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