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From: "fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug stdio/30039] New: __vsprintf_internal does not handle
 unespecified buffer length in fortify mode
Date: Tue, 24 Jan 2023 09:44:56 +0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: stdio
X-Bugzilla-Version: 2.37
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X-Bugzilla-Who: fweimer at redhat dot com
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D30039

            Bug ID: 30039
           Summary: __vsprintf_internal does not handle unespecified
                    buffer length in fortify mode
           Product: glibc
           Version: 2.37
            Status: NEW
          Severity: normal
          Priority: P1
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

>From libio/iovsprintf.c:

  /* When called from fortified sprintf/vsprintf, erase the destination
     buffer and try to detect overflows.  When called from regular
     sprintf/vsprintf, do not erase the destination buffer, because
     known user code relies on this behavior (even though its undefined
     by ISO C), nor try to detect overflows.  */
  if ((mode_flags & PRINTF_CHK) !=3D 0)
    {
      string[0] =3D '\0';
      __printf_buffer_init (&buf, string, maxlen,
                            __printf_buffer_mode_sprintf_chk);
    }
  else
    {
      __printf_buffer_init (&buf, string, 0, __printf_buffer_mode_sprintf);
      buf.write_end =3D (char *) ~(uintptr_t) 0; /* End of address space.  =
*/
    }

However, in some cases, this code is called for an inline expansion of spri=
ntf
where the buffer size is not known and maxlen =3D=3D -1. This makes buf->wr=
ite_end
=3D=3D buf->write_ptr - 1 (so end before active write position). This cause=
s an
assertion failure in __printf_buffer_as_file_commit:

Fatal glibc error: printf_buffer_as_file.c:31 (__printf_buffer_as_file_comm=
it):
assertion failed: file->stream._IO_write_ptr <=3D file->next->write_end

I believe the assertion is not unreasonable, so I think we need to fix the
buffer setup.

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