A few systems, like OpenWRT, may strip the section table off the
resulting binaries (see [1]), making it impossible for us to pinpoint
the exact location of the Qt plugin metadata. This commit moves the meta
data to a location that is identifiable even in fully stripped binaries:
an ELF note.
By naming our section ".note.qt.metadata", we instruct the linker to
place it along the other notes and to mark it in the program header
section. Another advantage is that the notes are usually in the very
beginning of the file, as they are used by the dynamic linker itself, so
we'll need to read much less of the full contents.
The unit test is modified not to attempt to strip the plugin of
debugging data. In fact, we add something to the end that would,
otherwise, be matched as (invalid) metadata.
The following was produced with GCC 11 and GNU binutils ld 2.36.1.
Section Headers:
[Nr] Name Type Addr Off Size ES Flags Lk Inf Al
[ 0] NULL 0000000000000000 00000000 00000000 0 0 0 0
[ 1] .note.gnu.property NOTE 00000000000002a8 000002a8 00000030 0 A 0 0 8
[ 2] .note.gnu.build-id NOTE 00000000000002d8 000002d8 00000024 0 A 0 0 4
[ 3] .note.qt.metadata NOTE 00000000000002fc 000002fc 000001ac 0 A 0 0 4
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
...
NOTE 0x0002a8 0x00000000000002a8 0x00000000000002a8 0x000030 0x000030 R 0x8
NOTE 0x0002d8 0x00000000000002d8 0x00000000000002d8 0x0001d0 0x0001d0 R 0x4
The Qt metadata note is 4-byte aligned and can thus be found in the
second note section, which spans from 0x02d8 to 0x02d8+0xac=0x0384.
GNU readelf -n can even show it:
Displaying notes found in: .note.qt.metadata
Owner Data size Description
qt-project! 0x0000018f Unknown note type: (0x74510001)
description data: 01 06 03 81 bf ...... ff
I chose 0x7451 as the prefix for our notes, even though they're already
namespaced by the owner in the first place, because eu-readelf
mistakenly tries to interpret note 1 as a GNU ABI tag regardless of
owner. The owner name was chosen to be 12 bytes long, so the ELF note
header is 24 bytes in total. There's no space wasted because the payload
needs to be aligned to 32-bit anyway and I didn't want to use only 4
characters (header total size 16 bytes) so we'd skip the "GNU" note on
size, without string comparison. And I couldn't think of a 4-character
representative string ("QtP" ?).
[1] https://github.com/openwrt/video/issues/1
Fixes: QTBUG-96327
Change-Id: I2de1b4dfacd443148279fffd16a3987729346567
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
8c2969ea86