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

["mac at mcrowe dot com" <sourceware-bugzilla@sourceware.org>]

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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