[Bug stdio/21534] New: vfprintf: user-defined specifiers with multiple arguments broken with positional arguments

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21534

            Bug ID: 21534
           Summary: vfprintf: user-defined specifiers with multiple
                    arguments broken with positional arguments
           Product: glibc
           Version: 2.25
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

Created attachment 10077
  --> https://sourceware.org/bugzilla/attachment.cgi?id=10077&action=edit
tst-vfprintf-user-type.c

If %P has an arginfo function which requests more than one argument, and it is
used as a positional argument specified (%1$P), the additional arguments are
not taken into account when computing the total number of arguments in
vfprintf/printf_positional (and possibly the offsets as well).

This means that the last subtest in the attached test program crashes.

This could perhaps be a documentation issue.  It is not entirely clear how
positional indexes would be determined if a format specifier consumes multiple
arguments.

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