Re: pkg/59096: qt6 apps with qt6-qtbase don't work on NetBSD/earmv7hf 10.1

["Izumi Tsutsui via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR pkg/59096; it has been noted by GNATS.

From: Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: tsutsui%ceres.dti.ne.jp@localhost
Subject: Re: pkg/59096: qt6 apps with qt6-qtbase don't work on NetBSD/earmv7hf
	 10.1
Date: Mon, 8 Sep 2025 04:28:56 +0900

 > >Synopsis:       qt6 apps with qt6-qtbase don't work on NetBSD/earmv7hf 10.1
 
 This is caused by incorrect alignment rounding in ELF note parser.
 
 src/corelib/plugin/qelfparser_p.cpp:scanProgramHeadersForNotes()
 has the following calculations to search and parse note sections:
 
  https://github.com/qt/qtbase/blob/6.8.3/src/corelib/plugin/qelfparser_p.cpp#L619-L630
 
 ```
             // overflow check: calculate where the next note will be, if it exists
             T::Off next_offset = offset;
             next_offset += sizeof(T::Nhdr);          // can't overflow (we checked above)
             next_offset += NoteAlignment - 3;        // offset is aligned, this can't overflow
             if (qAddOverflow<T::Off>(next_offset, n_namesz, &next_offset))
                 break;
             next_offset &= -NoteAlignment;
 
             next_offset += NoteAlignment - 3;        // offset is aligned, this can't overflow
             if (qAddOverflow<T::Off>(next_offset, n_descsz, &next_offset))
                 break;
             next_offset &= -NoteAlignment;
 ```
 
 I'm not sure where 'NoteAlignment - 3' comes from (3 should be 1?),
 but NetBSD/earmv7hf binaries has .note.netbsd.march section where
 n_descsz==9 and in that case the above rounding calculation add
 only 8 bytes to `next_offset` rather than 12 bytes, so it fails
 to find the next .note.qt.metadata section.
 
 It would be simpler to use explicit roundup calculations
 (pulled from local patches/patch-src_corelib_plugin_qelfparser__p.cpp):
 
 
 ```
 $NetBSD$
 
 - fix wrong alignment calculations that could fail on NetBSD/earmv7hf
   where n_descsz==9 in .note.netbsd.march section just before
   .not.qt.metadata
   
 --- src/corelib/plugin/qelfparser_p.cpp.orig	2024-10-01 10:46:30.000000000 +0000
 +++ src/corelib/plugin/qelfparser_p.cpp
 @@ -619,15 +619,15 @@ static QLibraryScanResult scanProgramHea
              // overflow check: calculate where the next note will be, if it exists
              T::Off next_offset = offset;
              next_offset += sizeof(T::Nhdr);          // can't overflow (we checked above)
 -            next_offset += NoteAlignment - 3;        // offset is aligned, this can't overflow
 -            if (qAddOverflow<T::Off>(next_offset, n_namesz, &next_offset))
 +            // roundup n_namesz and n_descsz to NoteAlignment
 +            const T::Off AlignOffset = T::Off(NoteAlignment - 1);
 +            const T::Off AlignMask = ~AlignOffset;
 +            const T::Off round_namesz = (T::Off(n_namesz) + AlignOffset) & AlignMask;
 +            const T::Off round_descsz = (T::Off(n_descsz) + AlignOffset) & AlignMask;
 +            if (qAddOverflow<T::Off>(next_offset, round_namesz, &next_offset))
                  break;
 -            next_offset &= -NoteAlignment;
 -
 -            next_offset += NoteAlignment - 3;        // offset is aligned, this can't overflow
 -            if (qAddOverflow<T::Off>(next_offset, n_descsz, &next_offset))
 +            if (qAddOverflow<T::Off>(next_offset, round_descsz, &next_offset))
                  break;
 -            next_offset &= -NoteAlignment;
              if (next_offset > end_offset)
                  break;
  
 ```
 
 ---
 Izumi Tsutsui